[PATCH v2 00/11] dm-pcache – persistent-memory cache for block devices

[PATCH v2 00/11] dm-pcache – persistent-memory cache for block devices

[Dongsheng Yang]

Hi Mikulas,
	This is V2 for dm-pcache, please take a look.

Code:
    https://github.com/DataTravelGuide/linux tags/pcache_v2

Changelogs

V2 from V1:
	- introduce req_alloc() and req_init() in backing_dev.c, then we
	  can do req_alloc() before holding spinlock and do req_init()
	  in subtree_walk().
	- introduce pre_alloc_key and pre_alloc_req in walk_ctx, that
	  means we can pre-allocate cache_key or backing_dev_request
	  before subtree walking.
	- use mempool_alloc() with NOIO for the allocation of cache_key
	  and backing_dev_req.
	- some coding style changes from comments of Jonathan.

V1 from RFC-V2:
	- use crc32c to replace crc32
	- only retry pcache_req when cache full, add pcache_req into defer_list,
	  and wait cache invalidation happen.
	- new format for pcache table, it is more easily extended with
	  new parameters later.
	- remove __packed.
	- use spin_lock_irq in req_complete_fn to replace
	  spin_lock_irqsave.
	- fix bug in backing_dev_bio_end with spin_lock_irqsave.
	- queue_work() inside spinlock.
	- introduce inline_bvecs in backing_dev_req.
	- use kmalloc_array for bvecs allocation.
	- calculate ->off with dm_target_offset() before use it.

Dongsheng Yang (11):
  dm-pcache: add pcache_internal.h
  dm-pcache: add backing device management
  dm-pcache: add cache device
  dm-pcache: add segment layer
  dm-pcache: add cache_segment
  dm-pcache: add cache_writeback
  dm-pcache: add cache_gc
  dm-pcache: add cache_key
  dm-pcache: add cache_req
  dm-pcache: add cache core
  dm-pcache: initial dm-pcache target

 .../admin-guide/device-mapper/dm-pcache.rst   | 201 ++++
 MAINTAINERS                                   |   8 +
 drivers/md/Kconfig                            |   2 +
 drivers/md/Makefile                           |   1 +
 drivers/md/dm-pcache/Kconfig                  |  17 +
 drivers/md/dm-pcache/Makefile                 |   3 +
 drivers/md/dm-pcache/backing_dev.c            | 345 +++++++
 drivers/md/dm-pcache/backing_dev.h            |  93 ++
 drivers/md/dm-pcache/cache.c                  | 432 +++++++++
 drivers/md/dm-pcache/cache.h                  | 616 ++++++++++++
 drivers/md/dm-pcache/cache_dev.c              | 299 ++++++
 drivers/md/dm-pcache/cache_dev.h              |  70 ++
 drivers/md/dm-pcache/cache_gc.c               | 170 ++++
 drivers/md/dm-pcache/cache_key.c              | 900 ++++++++++++++++++
 drivers/md/dm-pcache/cache_req.c              | 840 ++++++++++++++++
 drivers/md/dm-pcache/cache_segment.c          | 293 ++++++
 drivers/md/dm-pcache/cache_writeback.c        | 279 ++++++
 drivers/md/dm-pcache/dm_pcache.c              | 466 +++++++++
 drivers/md/dm-pcache/dm_pcache.h              |  65 ++
 drivers/md/dm-pcache/pcache_internal.h        | 117 +++
 drivers/md/dm-pcache/segment.c                |  61 ++
 drivers/md/dm-pcache/segment.h                |  73 ++
 22 files changed, 5351 insertions(+)
 create mode 100644 Documentation/admin-guide/device-mapper/dm-pcache.rst
 create mode 100644 drivers/md/dm-pcache/Kconfig
 create mode 100644 drivers/md/dm-pcache/Makefile
 create mode 100644 drivers/md/dm-pcache/backing_dev.c
 create mode 100644 drivers/md/dm-pcache/backing_dev.h
 create mode 100644 drivers/md/dm-pcache/cache.c
 create mode 100644 drivers/md/dm-pcache/cache.h
 create mode 100644 drivers/md/dm-pcache/cache_dev.c
 create mode 100644 drivers/md/dm-pcache/cache_dev.h
 create mode 100644 drivers/md/dm-pcache/cache_gc.c
 create mode 100644 drivers/md/dm-pcache/cache_key.c
 create mode 100644 drivers/md/dm-pcache/cache_req.c
 create mode 100644 drivers/md/dm-pcache/cache_segment.c
 create mode 100644 drivers/md/dm-pcache/cache_writeback.c
 create mode 100644 drivers/md/dm-pcache/dm_pcache.c
 create mode 100644 drivers/md/dm-pcache/dm_pcache.h
 create mode 100644 drivers/md/dm-pcache/pcache_internal.h
 create mode 100644 drivers/md/dm-pcache/segment.c
 create mode 100644 drivers/md/dm-pcache/segment.h

-- 
2.43.0

[PATCH v2 01/11] dm-pcache: add pcache_internal.h

[Dongsheng Yang]

Consolidate common PCACHE helpers into a new header so that subsequent
patches can include them without repeating boiler-plate.

- Logging macros with unified prefix and location info.
- Common constants (KB/MB helpers, metadata replica count, CRC seed).
- On-disk metadata header definition and CRC helper.
- Sequence-number comparison that handles wrap-around.
- pcache_meta_find_latest() to pick the newest valid metadata copy.

Signed-off-by: Dongsheng Yang <dongsheng.yang@linux.dev>
---
 drivers/md/dm-pcache/pcache_internal.h | 117 +++++++++++++++++++++++++
 1 file changed, 117 insertions(+)
 create mode 100644 drivers/md/dm-pcache/pcache_internal.h

diff --git a/drivers/md/dm-pcache/pcache_internal.h b/drivers/md/dm-pcache/pcache_internal.h
new file mode 100644
index 000000000000..d427e534727c
--- /dev/null
+++ b/drivers/md/dm-pcache/pcache_internal.h
@@ -0,0 +1,117 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+#ifndef _PCACHE_INTERNAL_H
+#define _PCACHE_INTERNAL_H
+
+#include <linux/delay.h>
+#include <linux/crc32c.h>
+
+#define pcache_err(fmt, ...)							\
+	pr_err("dm-pcache: %s:%u " fmt, __func__, __LINE__, ##__VA_ARGS__)
+#define pcache_info(fmt, ...)							\
+	pr_info("dm-pcache: %s:%u " fmt, __func__, __LINE__, ##__VA_ARGS__)
+#define pcache_debug(fmt, ...)							\
+	pr_debug("dm-pcache: %s:%u " fmt, __func__, __LINE__, ##__VA_ARGS__)
+
+#define PCACHE_KB			(1024ULL)
+#define PCACHE_MB			(1024 * PCACHE_KB)
+
+/* Maximum number of metadata indices */
+#define PCACHE_META_INDEX_MAX		2
+
+#define PCACHE_CRC_SEED			0x3B15A
+/*
+ * struct pcache_meta_header - PCACHE metadata header structure
+ * @crc: CRC checksum for validating metadata integrity.
+ * @seq: Sequence number to track metadata updates.
+ * @version: Metadata version.
+ * @res: Reserved space for future use.
+ */
+struct pcache_meta_header {
+	__u32 crc;
+	__u8  seq;
+	__u8  version;
+	__u16 res;
+};
+
+/*
+ * pcache_meta_crc - Calculate CRC for the given metadata header.
+ * @header: Pointer to the metadata header.
+ * @meta_size: Size of the metadata structure.
+ *
+ * Returns the CRC checksum calculated by excluding the CRC field itself.
+ */
+static inline u32 pcache_meta_crc(struct pcache_meta_header *header, u32 meta_size)
+{
+	return crc32c(PCACHE_CRC_SEED, (void *)header + 4, meta_size - 4);
+}
+
+/*
+ * pcache_meta_seq_after - Check if a sequence number is more recent, accounting for overflow.
+ * @seq1: First sequence number.
+ * @seq2: Second sequence number.
+ *
+ * Determines if @seq1 is more recent than @seq2 by calculating the signed
+ * difference between them. This approach allows handling sequence number
+ * overflow correctly because the difference wraps naturally, and any value
+ * greater than zero indicates that @seq1 is "after" @seq2. This method
+ * assumes 8-bit unsigned sequence numbers, where the difference wraps
+ * around if seq1 overflows past seq2.
+ *
+ * Returns:
+ *   - true if @seq1 is more recent than @seq2, indicating it comes "after"
+ *   - false otherwise.
+ */
+static inline bool pcache_meta_seq_after(u8 seq1, u8 seq2)
+{
+	return (s8)(seq1 - seq2) > 0;
+}
+
+/*
+ * pcache_meta_find_latest - Find the latest valid metadata.
+ * @header: Pointer to the metadata header.
+ * @meta_size: Size of each metadata block.
+ *
+ * Finds the latest valid metadata by checking sequence numbers. If a
+ * valid entry with the highest sequence number is found, its pointer
+ * is returned. Returns NULL if no valid metadata is found.
+ */
+static inline void __must_check *pcache_meta_find_latest(struct pcache_meta_header *header,
+					u32 meta_size, u32 meta_max_size,
+					void *meta_ret)
+{
+	struct pcache_meta_header *meta, *latest = NULL;
+	u32 i, seq_latest = 0;
+	void *meta_addr;
+
+	meta = meta_ret;
+
+	for (i = 0; i < PCACHE_META_INDEX_MAX; i++) {
+		meta_addr = (void *)header + (i * meta_max_size);
+		if (copy_mc_to_kernel(meta, meta_addr, meta_size)) {
+			pcache_err("hardware memory error when copy meta");
+			return ERR_PTR(-EIO);
+		}
+
+		/* Skip if CRC check fails, which means corrupted */
+		if (meta->crc != pcache_meta_crc(meta, meta_size))
+			continue;
+
+		/* Update latest if a more recent sequence is found */
+		if (!latest || pcache_meta_seq_after(meta->seq, seq_latest)) {
+			seq_latest = meta->seq;
+			latest = (void *)header + (i * meta_max_size);
+		}
+	}
+
+	if (!latest)
+		return NULL;
+
+	if (copy_mc_to_kernel(meta_ret, latest, meta_size)) {
+		pcache_err("hardware memory error");
+		return ERR_PTR(-EIO);
+	}
+
+	return latest;
+}
+
+#endif /* _PCACHE_INTERNAL_H */
-- 
2.43.0

[PATCH v2 02/11] dm-pcache: add backing device management

[Dongsheng Yang]

This patch introduces *backing_dev.{c,h}*, a self-contained layer that
handles all interaction with the *backing block device* where cache
write-back and cache-miss reads are serviced.  Isolating this logic
keeps the core dm-pcache code free of low-level bio plumbing.

* Device setup / teardown
  - Opens the target with `dm_get_device()`, stores `bdev`, file and
    size, and initialises a dedicated `bioset`.
  - Gracefully releases resources via `backing_dev_stop()`.

* Request object (`struct pcache_backing_dev_req`)
  - Two request flavours:
    - REQ-type – cloned from an upper `struct bio` issued to
      dm-pcache; trimmed and re-targeted to the backing LBA.
    - KMEM-type – maps an arbitrary kernel memory buffer
      into a freshly built.
  - Private completion callback (`end_req`) propagates status to the
    upper layer and handles resource recycling.

* Submission & completion path
  - Lock-protected submit queue + worker (`req_submit_work`) let pcache
    push many requests asynchronously, at the same time, allow caller
    to submit backing_dev_req in atomic context.
  - End-io handler moves finished requests to a completion list processed
    by `req_complete_work`, ensuring callbacks run in process context.
  - Direct-submit option for non-atomic context.

* Flush
  - `backing_dev_flush()` issues a flush to persist backing-device data.

Signed-off-by: Dongsheng Yang <dongsheng.yang@linux.dev>
---
 drivers/md/dm-pcache/backing_dev.c | 345 +++++++++++++++++++++++++++++
 drivers/md/dm-pcache/backing_dev.h |  93 ++++++++
 2 files changed, 438 insertions(+)
 create mode 100644 drivers/md/dm-pcache/backing_dev.c
 create mode 100644 drivers/md/dm-pcache/backing_dev.h

diff --git a/drivers/md/dm-pcache/backing_dev.c b/drivers/md/dm-pcache/backing_dev.c
new file mode 100644
index 000000000000..29b9988f372b
--- /dev/null
+++ b/drivers/md/dm-pcache/backing_dev.c
@@ -0,0 +1,345 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+#include <linux/blkdev.h>
+
+#include "../dm-core.h"
+#include "pcache_internal.h"
+#include "cache_dev.h"
+#include "backing_dev.h"
+#include "cache.h"
+#include "dm_pcache.h"
+
+static void backing_dev_exit(struct pcache_backing_dev *backing_dev)
+{
+	mempool_exit(&backing_dev->req_pool);
+	kmem_cache_destroy(backing_dev->backing_req_cache);
+}
+
+static void req_submit_fn(struct work_struct *work);
+static void req_complete_fn(struct work_struct *work);
+static int backing_dev_init(struct dm_pcache *pcache)
+{
+	struct pcache_backing_dev *backing_dev = &pcache->backing_dev;
+	int ret;
+
+	backing_dev->backing_req_cache = KMEM_CACHE(pcache_backing_dev_req, 0);
+	if (!backing_dev->backing_req_cache) {
+		ret = -ENOMEM;
+		goto err;
+	}
+
+	ret = mempool_init_slab_pool(&backing_dev->req_pool, 128, backing_dev->backing_req_cache);
+	if (ret)
+		goto cache_destroy;
+
+	INIT_LIST_HEAD(&backing_dev->submit_list);
+	INIT_LIST_HEAD(&backing_dev->complete_list);
+	spin_lock_init(&backing_dev->submit_lock);
+	spin_lock_init(&backing_dev->complete_lock);
+	INIT_WORK(&backing_dev->req_submit_work, req_submit_fn);
+	INIT_WORK(&backing_dev->req_complete_work, req_complete_fn);
+
+	return 0;
+cache_destroy:
+	kmem_cache_destroy(backing_dev->backing_req_cache);
+err:
+	return ret;
+}
+
+int backing_dev_start(struct dm_pcache *pcache)
+{
+	struct pcache_backing_dev *backing_dev = &pcache->backing_dev;
+	int ret;
+
+	ret = backing_dev_init(pcache);
+	if (ret)
+		return ret;
+
+	backing_dev->dev_size = bdev_nr_sectors(backing_dev->dm_dev->bdev);
+
+	return 0;
+}
+
+void backing_dev_stop(struct dm_pcache *pcache)
+{
+	struct pcache_backing_dev *backing_dev = &pcache->backing_dev;
+
+	flush_work(&backing_dev->req_submit_work);
+	flush_work(&backing_dev->req_complete_work);
+
+	/* There should be no inflight backing_dev_request */
+	BUG_ON(!list_empty(&backing_dev->submit_list));
+	BUG_ON(!list_empty(&backing_dev->complete_list));
+
+	backing_dev_exit(backing_dev);
+}
+
+/* pcache_backing_dev_req functions */
+void backing_dev_req_end(struct pcache_backing_dev_req *backing_req)
+{
+	struct pcache_backing_dev *backing_dev = backing_req->backing_dev;
+
+	if (backing_req->end_req)
+		backing_req->end_req(backing_req, backing_req->ret);
+
+	switch (backing_req->type) {
+	case BACKING_DEV_REQ_TYPE_REQ:
+		if (backing_req->req.upper_req)
+			pcache_req_put(backing_req->req.upper_req, backing_req->ret);
+		break;
+	case BACKING_DEV_REQ_TYPE_KMEM:
+		if (backing_req->kmem.bvecs != backing_req->kmem.inline_bvecs)
+			kfree(backing_req->kmem.bvecs);
+		break;
+	default:
+		BUG();
+	}
+
+	mempool_free(backing_req, &backing_dev->req_pool);
+}
+
+static void req_complete_fn(struct work_struct *work)
+{
+	struct pcache_backing_dev *backing_dev = container_of(work, struct pcache_backing_dev, req_complete_work);
+	struct pcache_backing_dev_req *backing_req;
+	LIST_HEAD(tmp_list);
+
+	spin_lock_irq(&backing_dev->complete_lock);
+	list_splice_init(&backing_dev->complete_list, &tmp_list);
+	spin_unlock_irq(&backing_dev->complete_lock);
+
+	while (!list_empty(&tmp_list)) {
+		backing_req = list_first_entry(&tmp_list,
+					    struct pcache_backing_dev_req, node);
+		list_del_init(&backing_req->node);
+		backing_dev_req_end(backing_req);
+	}
+}
+
+static void backing_dev_bio_end(struct bio *bio)
+{
+	struct pcache_backing_dev_req *backing_req = bio->bi_private;
+	struct pcache_backing_dev *backing_dev = backing_req->backing_dev;
+	unsigned long flags;
+
+	backing_req->ret = bio->bi_status;
+
+	spin_lock_irqsave(&backing_dev->complete_lock, flags);
+	list_move_tail(&backing_req->node, &backing_dev->complete_list);
+	queue_work(BACKING_DEV_TO_PCACHE(backing_dev)->task_wq, &backing_dev->req_complete_work);
+	spin_unlock_irqrestore(&backing_dev->complete_lock, flags);
+}
+
+static void req_submit_fn(struct work_struct *work)
+{
+	struct pcache_backing_dev *backing_dev = container_of(work, struct pcache_backing_dev, req_submit_work);
+	struct pcache_backing_dev_req *backing_req;
+	LIST_HEAD(tmp_list);
+
+	spin_lock(&backing_dev->submit_lock);
+	list_splice_init(&backing_dev->submit_list, &tmp_list);
+	spin_unlock(&backing_dev->submit_lock);
+
+	while (!list_empty(&tmp_list)) {
+		backing_req = list_first_entry(&tmp_list,
+					    struct pcache_backing_dev_req, node);
+		list_del_init(&backing_req->node);
+		submit_bio_noacct(&backing_req->bio);
+	}
+}
+
+void backing_dev_req_submit(struct pcache_backing_dev_req *backing_req, bool direct)
+{
+	struct pcache_backing_dev *backing_dev = backing_req->backing_dev;
+
+	if (direct) {
+		submit_bio_noacct(&backing_req->bio);
+		return;
+	}
+
+	spin_lock(&backing_dev->submit_lock);
+	list_add_tail(&backing_req->node, &backing_dev->submit_list);
+	queue_work(BACKING_DEV_TO_PCACHE(backing_dev)->task_wq, &backing_dev->req_submit_work);
+	spin_unlock(&backing_dev->submit_lock);
+}
+
+static void bio_map(struct bio *bio, void *base, size_t size)
+{
+	struct page *page;
+	unsigned int offset;
+	unsigned int len;
+
+	if (!is_vmalloc_addr(base)) {
+		page = virt_to_page(base);
+		offset = offset_in_page(base);
+
+		BUG_ON(!bio_add_page(bio, page, size, offset));
+		return;
+	}
+
+	flush_kernel_vmap_range(base, size);
+	while (size) {
+		page = vmalloc_to_page(base);
+		offset = offset_in_page(base);
+		len = min_t(size_t, PAGE_SIZE - offset, size);
+
+		BUG_ON(!bio_add_page(bio, page, len, offset));
+		size -= len;
+		base += len;
+	}
+}
+
+static struct pcache_backing_dev_req *req_type_req_alloc(struct pcache_backing_dev *backing_dev,
+							struct pcache_backing_dev_req_opts *opts)
+{
+	struct pcache_request *pcache_req = opts->req.upper_req;
+	struct pcache_backing_dev_req *backing_req;
+	struct bio *orig = pcache_req->bio;
+	int ret;
+
+	backing_req = mempool_alloc(&backing_dev->req_pool, GFP_NOIO);
+	if (!backing_req)
+		return NULL;
+
+	memset(backing_req, 0, sizeof(struct pcache_backing_dev_req));
+
+	ret = bio_init_clone(backing_dev->dm_dev->bdev, &backing_req->bio, orig, GFP_NOIO);
+	if (ret)
+		goto err_free_req;
+
+	backing_req->type = BACKING_DEV_REQ_TYPE_REQ;
+	backing_req->backing_dev = backing_dev;
+
+	return backing_req;
+
+err_free_req:
+	mempool_free(backing_req, &backing_dev->req_pool);
+	return NULL;
+}
+
+static u32 get_n_vecs(void *data, u32 len)
+{
+	if (!is_vmalloc_addr(data))
+		return 1;
+
+	return DIV_ROUND_UP(len, PAGE_SIZE);
+}
+
+static struct pcache_backing_dev_req *kmem_type_req_alloc(struct pcache_backing_dev *backing_dev,
+						struct pcache_backing_dev_req_opts *opts)
+{
+	struct pcache_backing_dev_req *backing_req;
+	u32 n_vecs = get_n_vecs(opts->kmem.data, opts->kmem.len);
+
+	backing_req = mempool_alloc(&backing_dev->req_pool, GFP_NOIO);
+	if (!backing_req)
+		return NULL;
+
+	memset(backing_req, 0, sizeof(struct pcache_backing_dev_req));
+
+	if (n_vecs > BACKING_DEV_REQ_INLINE_BVECS) {
+		backing_req->kmem.bvecs = kmalloc_array(n_vecs, sizeof(struct bio_vec), GFP_NOIO);
+		if (!backing_req->kmem.bvecs)
+			goto err_free_req;
+	} else {
+		backing_req->kmem.bvecs = backing_req->kmem.inline_bvecs;
+	}
+
+	backing_req->kmem.n_vecs = n_vecs;
+	backing_req->type = BACKING_DEV_REQ_TYPE_KMEM;
+	backing_req->backing_dev = backing_dev;
+
+	return backing_req;
+
+err_free_req:
+	mempool_free(backing_req, &backing_dev->req_pool);
+	return NULL;
+}
+
+struct pcache_backing_dev_req *backing_dev_req_alloc(struct pcache_backing_dev *backing_dev,
+						struct pcache_backing_dev_req_opts *opts)
+{
+	if (opts->type == BACKING_DEV_REQ_TYPE_REQ)
+		return req_type_req_alloc(backing_dev, opts);
+
+	if (opts->type == BACKING_DEV_REQ_TYPE_KMEM)
+		return kmem_type_req_alloc(backing_dev, opts);
+
+	return NULL;
+}
+
+static void req_type_req_init(struct pcache_backing_dev_req *backing_req,
+			struct pcache_backing_dev_req_opts *opts)
+{
+	struct pcache_request *pcache_req = opts->req.upper_req;
+	struct bio *clone;
+	u32 off = opts->req.req_off;
+	u32 len = opts->req.len;
+
+	clone = &backing_req->bio;
+	BUG_ON(off & SECTOR_MASK);
+	BUG_ON(len & SECTOR_MASK);
+	bio_trim(clone, off >> SECTOR_SHIFT, len >> SECTOR_SHIFT);
+
+	clone->bi_iter.bi_sector = (pcache_req->off + off) >> SECTOR_SHIFT;
+	clone->bi_private = backing_req;
+	clone->bi_end_io = backing_dev_bio_end;
+
+	INIT_LIST_HEAD(&backing_req->node);
+	backing_req->end_req     = opts->end_fn;
+
+	pcache_req_get(pcache_req);
+	backing_req->req.upper_req	= pcache_req;
+	backing_req->req.bio_off	= off;
+}
+
+static void kmem_type_req_init(struct pcache_backing_dev_req *backing_req,
+			struct pcache_backing_dev_req_opts *opts)
+{
+	struct pcache_backing_dev *backing_dev = backing_req->backing_dev;
+	struct bio *backing_bio;
+
+	bio_init(&backing_req->bio, backing_dev->dm_dev->bdev, backing_req->kmem.bvecs,
+			backing_req->kmem.n_vecs, opts->kmem.opf);
+
+	backing_bio = &backing_req->bio;
+	bio_map(backing_bio, opts->kmem.data, opts->kmem.len);
+
+	backing_bio->bi_iter.bi_sector = (opts->kmem.backing_off) >> SECTOR_SHIFT;
+	backing_bio->bi_private = backing_req;
+	backing_bio->bi_end_io = backing_dev_bio_end;
+
+	INIT_LIST_HEAD(&backing_req->node);
+	backing_req->end_req	= opts->end_fn;
+	backing_req->priv_data	= opts->priv_data;
+}
+
+void backing_dev_req_init(struct pcache_backing_dev_req *backing_req,
+			struct pcache_backing_dev_req_opts *opts)
+{
+	if (opts->type == BACKING_DEV_REQ_TYPE_REQ)
+		return req_type_req_init(backing_req, opts);
+
+	if (opts->type == BACKING_DEV_REQ_TYPE_KMEM)
+		return kmem_type_req_init(backing_req, opts);
+
+	BUG();
+}
+
+struct pcache_backing_dev_req *backing_dev_req_create(struct pcache_backing_dev *backing_dev,
+						struct pcache_backing_dev_req_opts *opts)
+{
+	struct pcache_backing_dev_req *backing_req;
+
+	backing_req = backing_dev_req_alloc(backing_dev, opts);
+	if (!backing_req)
+		return NULL;
+
+	backing_dev_req_init(backing_req, opts);
+
+	return backing_req;
+}
+
+void backing_dev_flush(struct pcache_backing_dev *backing_dev)
+{
+	blkdev_issue_flush(backing_dev->dm_dev->bdev);
+}
diff --git a/drivers/md/dm-pcache/backing_dev.h b/drivers/md/dm-pcache/backing_dev.h
new file mode 100644
index 000000000000..2115ebbff49e
--- /dev/null
+++ b/drivers/md/dm-pcache/backing_dev.h
@@ -0,0 +1,93 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+#ifndef _BACKING_DEV_H
+#define _BACKING_DEV_H
+
+#include <linux/device-mapper.h>
+
+#include "pcache_internal.h"
+
+struct pcache_backing_dev_req;
+typedef void (*backing_req_end_fn_t)(struct pcache_backing_dev_req *backing_req, int ret);
+
+#define BACKING_DEV_REQ_TYPE_REQ		1
+#define BACKING_DEV_REQ_TYPE_KMEM		2
+
+#define BACKING_DEV_REQ_INLINE_BVECS		4
+
+struct pcache_request;
+struct pcache_backing_dev_req {
+	u8				type;
+	struct bio			bio;
+	struct pcache_backing_dev	*backing_dev;
+
+	void				*priv_data;
+	backing_req_end_fn_t		end_req;
+
+	struct list_head		node;
+	int				ret;
+
+	union {
+		struct {
+			struct pcache_request		*upper_req;
+			u32				bio_off;
+		} req;
+		struct {
+			struct bio_vec	inline_bvecs[BACKING_DEV_REQ_INLINE_BVECS];
+			struct bio_vec	*bvecs;
+			u32		n_vecs;
+		} kmem;
+	};
+};
+
+struct pcache_backing_dev {
+	struct pcache_cache		*cache;
+
+	struct dm_dev			*dm_dev;
+	struct kmem_cache		*backing_req_cache;
+	mempool_t			req_pool;
+
+	struct list_head		submit_list;
+	spinlock_t			submit_lock;
+	struct work_struct		req_submit_work;
+
+	struct list_head		complete_list;
+	spinlock_t			complete_lock;
+	struct work_struct		req_complete_work;
+
+	u64				dev_size;
+};
+
+struct dm_pcache;
+int backing_dev_start(struct dm_pcache *pcache);
+void backing_dev_stop(struct dm_pcache *pcache);
+
+struct pcache_backing_dev_req_opts {
+	u32 type;
+	union {
+		struct {
+			struct pcache_request *upper_req;
+			u32 req_off;
+			u32 len;
+		} req;
+		struct {
+			void *data;
+			blk_opf_t opf;
+			u32 len;
+			u64 backing_off;
+		} kmem;
+	};
+
+	backing_req_end_fn_t	end_fn;
+	void			*priv_data;
+};
+
+void backing_dev_req_submit(struct pcache_backing_dev_req *backing_req, bool direct);
+void backing_dev_req_end(struct pcache_backing_dev_req *backing_req);
+struct pcache_backing_dev_req *backing_dev_req_create(struct pcache_backing_dev *backing_dev,
+						struct pcache_backing_dev_req_opts *opts);
+struct pcache_backing_dev_req *backing_dev_req_alloc(struct pcache_backing_dev *backing_dev,
+						struct pcache_backing_dev_req_opts *opts);
+void backing_dev_req_init(struct pcache_backing_dev_req *backing_req,
+			struct pcache_backing_dev_req_opts *opts);
+void backing_dev_flush(struct pcache_backing_dev *backing_dev);
+#endif /* _BACKING_DEV_H */
-- 
2.43.0

[PATCH v2 03/11] dm-pcache: add cache device

[Dongsheng Yang]

Add cache_dev.{c,h} to manage the persistent-memory device that stores
all pcache metadata and data segments.  Splitting this logic out keeps
the main dm-pcache code focused on policy while cache_dev handles the
low-level interaction with the DAX block device.

* DAX mapping
  - Opens the underlying device via dm_get_device().
  - Uses dax_direct_access() to obtain a direct linear mapping; falls
    back to vmap() when the range is fragmented.

* On-disk layout
  ┌─ 4 KB ─┐  super-block (SB)
  ├─ 4 KB ─┤  cache_info[0]
  ├─ 4 KB ─┤  cache_info[1]
  ├─ 4 KB ─┤  cache_ctrl
  └─ ...  ─┘  segments
  Constants and macros in the header expose offsets and sizes.

* Super-block handling
  - sb_read(), sb_validate(), sb_init() verify magic, CRC32 and host
    endianness (flag *PCACHE_SB_F_BIGENDIAN*).
  - Formatting zeroes the metadata replicas and initialises the segment
    bitmap when the SB is blank.

* Segment allocator
  - Bitmap protected by seg_lock; find_next_zero_bit() yields the next
    free 16 MB segment.

* Lifecycle helpers
  - cache_dev_start()/stop() encapsulate init/exit and are invoked by
    dm-pcache core.
  - Gracefully handles errors: CRC mismatch, wrong endianness, device
    too small (< 512 MB), or failed DAX mapping.

Signed-off-by: Dongsheng Yang <dongsheng.yang@linux.dev>
---
 drivers/md/dm-pcache/cache_dev.c | 299 +++++++++++++++++++++++++++++++
 drivers/md/dm-pcache/cache_dev.h |  70 ++++++++
 2 files changed, 369 insertions(+)
 create mode 100644 drivers/md/dm-pcache/cache_dev.c
 create mode 100644 drivers/md/dm-pcache/cache_dev.h

diff --git a/drivers/md/dm-pcache/cache_dev.c b/drivers/md/dm-pcache/cache_dev.c
new file mode 100644
index 000000000000..4dcebc9c167e
--- /dev/null
+++ b/drivers/md/dm-pcache/cache_dev.c
@@ -0,0 +1,299 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include <linux/blkdev.h>
+#include <linux/dax.h>
+#include <linux/vmalloc.h>
+#include <linux/pfn_t.h>
+#include <linux/parser.h>
+
+#include "cache_dev.h"
+#include "backing_dev.h"
+#include "cache.h"
+#include "dm_pcache.h"
+
+static void cache_dev_dax_exit(struct pcache_cache_dev *cache_dev)
+{
+	if (cache_dev->use_vmap)
+		vunmap(cache_dev->mapping);
+}
+
+static int build_vmap(struct dax_device *dax_dev, long total_pages, void **vaddr)
+{
+	struct page **pages;
+	long i = 0, chunk;
+	pfn_t pfn;
+	int ret;
+
+	pages = vmalloc_array(total_pages, sizeof(struct page *));
+	if (!pages)
+		return -ENOMEM;
+
+	do {
+		chunk = dax_direct_access(dax_dev, i, total_pages - i,
+					  DAX_ACCESS, NULL, &pfn);
+		if (chunk <= 0) {
+			ret = chunk ? chunk : -EINVAL;
+			goto out_free;
+		}
+
+		if (!pfn_t_has_page(pfn)) {
+			ret = -EOPNOTSUPP;
+			goto out_free;
+		}
+
+		while (chunk-- && i < total_pages) {
+			pages[i++] = pfn_t_to_page(pfn);
+			pfn.val++;
+			if (!(i & 15))
+				cond_resched();
+		}
+	} while (i < total_pages);
+
+	*vaddr = vmap(pages, total_pages, VM_MAP, PAGE_KERNEL);
+	if (!*vaddr)
+		ret = -ENOMEM;
+out_free:
+	vfree(pages);
+	return ret;
+}
+
+static int cache_dev_dax_init(struct pcache_cache_dev *cache_dev)
+{
+	struct dm_pcache	*pcache = CACHE_DEV_TO_PCACHE(cache_dev);
+	struct dax_device	*dax_dev;
+	long			total_pages, mapped_pages;
+	u64			bdev_size;
+	void			*vaddr;
+	int			ret;
+	int			id;
+	pfn_t			pfn;
+
+	dax_dev	= cache_dev->dm_dev->dax_dev;
+	/* total size check */
+	bdev_size = bdev_nr_bytes(cache_dev->dm_dev->bdev);
+	if (bdev_size < PCACHE_CACHE_DEV_SIZE_MIN) {
+		pcache_dev_err(pcache, "dax device is too small, required at least %llu",
+				PCACHE_CACHE_DEV_SIZE_MIN);
+		ret = -ENOSPC;
+		goto out;
+	}
+
+	total_pages = bdev_size >> PAGE_SHIFT;
+	/* attempt: direct-map the whole range */
+	id = dax_read_lock();
+	mapped_pages = dax_direct_access(dax_dev, 0, total_pages,
+					 DAX_ACCESS, &vaddr, &pfn);
+	if (mapped_pages < 0) {
+		pcache_dev_err(pcache, "dax_direct_access failed: %ld\n", mapped_pages);
+		ret = mapped_pages;
+		goto unlock;
+	}
+
+	if (!pfn_t_has_page(pfn)) {
+		ret = -EOPNOTSUPP;
+		goto unlock;
+	}
+
+	if (mapped_pages == total_pages) {
+		/* success: contiguous direct mapping */
+		cache_dev->mapping = vaddr;
+	} else {
+		/* need vmap fallback */
+		ret = build_vmap(dax_dev, total_pages, &vaddr);
+		if (ret) {
+			pcache_dev_err(pcache, "vmap fallback failed: %d\n", ret);
+			goto unlock;
+		}
+
+		cache_dev->mapping	= vaddr;
+		cache_dev->use_vmap	= true;
+	}
+	dax_read_unlock(id);
+
+	return 0;
+unlock:
+	dax_read_unlock(id);
+out:
+	return ret;
+}
+
+void cache_dev_zero_range(struct pcache_cache_dev *cache_dev, void *pos, u32 size)
+{
+	memset(pos, 0, size);
+	dax_flush(cache_dev->dm_dev->dax_dev, pos, size);
+}
+
+static int sb_read(struct pcache_cache_dev *cache_dev, struct pcache_sb *sb)
+{
+	struct pcache_sb *sb_addr = CACHE_DEV_SB(cache_dev);
+
+	if (copy_mc_to_kernel(sb, sb_addr, sizeof(struct pcache_sb)))
+		return -EIO;
+
+	return 0;
+}
+
+static void sb_write(struct pcache_cache_dev *cache_dev, struct pcache_sb *sb)
+{
+	struct pcache_sb *sb_addr = CACHE_DEV_SB(cache_dev);
+
+	memcpy_flushcache(sb_addr, sb, sizeof(struct pcache_sb));
+	pmem_wmb();
+}
+
+static int sb_init(struct pcache_cache_dev *cache_dev, struct pcache_sb *sb)
+{
+	struct dm_pcache *pcache = CACHE_DEV_TO_PCACHE(cache_dev);
+	u64 nr_segs;
+	u64 cache_dev_size;
+	u64 magic;
+	u32 flags = 0;
+
+	magic = le64_to_cpu(sb->magic);
+	if (magic)
+		return -EEXIST;
+
+	cache_dev_size = bdev_nr_bytes(file_bdev(cache_dev->dm_dev->bdev_file));
+	if (cache_dev_size < PCACHE_CACHE_DEV_SIZE_MIN) {
+		pcache_dev_err(pcache, "dax device is too small, required at least %llu",
+				PCACHE_CACHE_DEV_SIZE_MIN);
+		return -ENOSPC;
+	}
+
+	nr_segs = (cache_dev_size - PCACHE_SEGMENTS_OFF) / ((PCACHE_SEG_SIZE));
+
+#if defined(__BYTE_ORDER) ? (__BIG_ENDIAN == __BYTE_ORDER) : defined(__BIG_ENDIAN)
+	flags |= PCACHE_SB_F_BIGENDIAN;
+#endif
+	sb->flags = cpu_to_le32(flags);
+	sb->magic = cpu_to_le64(PCACHE_MAGIC);
+	sb->seg_num = cpu_to_le32(nr_segs);
+	sb->crc = cpu_to_le32(crc32c(PCACHE_CRC_SEED, (void *)(sb) + 4, sizeof(struct pcache_sb) - 4));
+
+	cache_dev_zero_range(cache_dev, CACHE_DEV_CACHE_INFO(cache_dev),
+			     PCACHE_CACHE_INFO_SIZE * PCACHE_META_INDEX_MAX +
+			     PCACHE_CACHE_CTRL_SIZE);
+
+	return 0;
+}
+
+static int sb_validate(struct pcache_cache_dev *cache_dev, struct pcache_sb *sb)
+{
+	struct dm_pcache *pcache = CACHE_DEV_TO_PCACHE(cache_dev);
+	u32 flags;
+	u32 crc;
+
+	if (le64_to_cpu(sb->magic) != PCACHE_MAGIC) {
+		pcache_dev_err(pcache, "unexpected magic: %llx\n",
+				le64_to_cpu(sb->magic));
+		return -EINVAL;
+	}
+
+	crc = crc32c(PCACHE_CRC_SEED, (void *)(sb) + 4, sizeof(struct pcache_sb) - 4);
+	if (crc != le32_to_cpu(sb->crc)) {
+		pcache_dev_err(pcache, "corrupted sb: %u, expected: %u\n", crc, le32_to_cpu(sb->crc));
+		return -EINVAL;
+	}
+
+	flags = le32_to_cpu(sb->flags);
+#if defined(__BYTE_ORDER) ? (__BIG_ENDIAN == __BYTE_ORDER) : defined(__BIG_ENDIAN)
+	if (!(flags & PCACHE_SB_F_BIGENDIAN)) {
+		pcache_dev_err(pcache, "cache_dev is not big endian\n");
+		return -EINVAL;
+	}
+#else
+	if (flags & PCACHE_SB_F_BIGENDIAN) {
+		pcache_dev_err(pcache, "cache_dev is big endian\n");
+		return -EINVAL;
+	}
+#endif
+	return 0;
+}
+
+static int cache_dev_init(struct pcache_cache_dev *cache_dev, u32 seg_num)
+{
+	cache_dev->seg_num = seg_num;
+	cache_dev->seg_bitmap = bitmap_zalloc(cache_dev->seg_num, GFP_KERNEL);
+	if (!cache_dev->seg_bitmap)
+		return -ENOMEM;
+
+	return 0;
+}
+
+static void cache_dev_exit(struct pcache_cache_dev *cache_dev)
+{
+	bitmap_free(cache_dev->seg_bitmap);
+}
+
+void cache_dev_stop(struct dm_pcache *pcache)
+{
+	struct pcache_cache_dev *cache_dev = &pcache->cache_dev;
+
+	cache_dev_exit(cache_dev);
+	cache_dev_dax_exit(cache_dev);
+}
+
+int cache_dev_start(struct dm_pcache *pcache)
+{
+	struct pcache_cache_dev *cache_dev = &pcache->cache_dev;
+	struct pcache_sb sb;
+	bool format = false;
+	int ret;
+
+	mutex_init(&cache_dev->seg_lock);
+
+	ret = cache_dev_dax_init(cache_dev);
+	if (ret) {
+		pcache_dev_err(pcache, "failed to init cache_dev %s via dax way: %d.",
+			       cache_dev->dm_dev->name, ret);
+		goto err;
+	}
+
+	ret = sb_read(cache_dev, &sb);
+	if (ret)
+		goto dax_release;
+
+	if (le64_to_cpu(sb.magic) == 0) {
+		format = true;
+		ret = sb_init(cache_dev, &sb);
+		if (ret < 0)
+			goto dax_release;
+	}
+
+	ret = sb_validate(cache_dev, &sb);
+	if (ret)
+		goto dax_release;
+
+	cache_dev->sb_flags = le32_to_cpu(sb.flags);
+	ret = cache_dev_init(cache_dev, sb.seg_num);
+	if (ret)
+		goto dax_release;
+
+	if (format)
+		sb_write(cache_dev, &sb);
+
+	return 0;
+
+dax_release:
+	cache_dev_dax_exit(cache_dev);
+err:
+	return ret;
+}
+
+int cache_dev_get_empty_segment_id(struct pcache_cache_dev *cache_dev, u32 *seg_id)
+{
+	int ret;
+
+	mutex_lock(&cache_dev->seg_lock);
+	*seg_id = find_next_zero_bit(cache_dev->seg_bitmap, cache_dev->seg_num, 0);
+	if (*seg_id == cache_dev->seg_num) {
+		ret = -ENOSPC;
+		goto unlock;
+	}
+
+	set_bit(*seg_id, cache_dev->seg_bitmap);
+	ret = 0;
+unlock:
+	mutex_unlock(&cache_dev->seg_lock);
+	return ret;
+}
diff --git a/drivers/md/dm-pcache/cache_dev.h b/drivers/md/dm-pcache/cache_dev.h
new file mode 100644
index 000000000000..6251eb4ebe96
--- /dev/null
+++ b/drivers/md/dm-pcache/cache_dev.h
@@ -0,0 +1,70 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+#ifndef _PCACHE_CACHE_DEV_H
+#define _PCACHE_CACHE_DEV_H
+
+#include <linux/device.h>
+#include <linux/device-mapper.h>
+
+#include "pcache_internal.h"
+
+#define PCACHE_MAGIC				0x65B05EFA96C596EFULL
+
+#define PCACHE_SB_OFF				(4 * PCACHE_KB)
+#define PCACHE_SB_SIZE				(4 * PCACHE_KB)
+
+#define PCACHE_CACHE_INFO_OFF			(PCACHE_SB_OFF + PCACHE_SB_SIZE)
+#define PCACHE_CACHE_INFO_SIZE			(4 * PCACHE_KB)
+
+#define PCACHE_CACHE_CTRL_OFF			(PCACHE_CACHE_INFO_OFF + (PCACHE_CACHE_INFO_SIZE * PCACHE_META_INDEX_MAX))
+#define PCACHE_CACHE_CTRL_SIZE			(4 * PCACHE_KB)
+
+#define PCACHE_SEGMENTS_OFF			(PCACHE_CACHE_CTRL_OFF + PCACHE_CACHE_CTRL_SIZE)
+#define PCACHE_SEG_INFO_SIZE			(4 * PCACHE_KB)
+
+#define PCACHE_CACHE_DEV_SIZE_MIN		(512 * PCACHE_MB)	/* 512 MB */
+#define PCACHE_SEG_SIZE				(16 * PCACHE_MB)	/* Size of each PCACHE segment (16 MB) */
+
+#define CACHE_DEV_SB(cache_dev)			((struct pcache_sb *)(cache_dev->mapping + PCACHE_SB_OFF))
+#define CACHE_DEV_CACHE_INFO(cache_dev)		((void *)cache_dev->mapping + PCACHE_CACHE_INFO_OFF)
+#define CACHE_DEV_CACHE_CTRL(cache_dev)		((void *)cache_dev->mapping + PCACHE_CACHE_CTRL_OFF)
+#define CACHE_DEV_SEGMENTS(cache_dev)		((void *)cache_dev->mapping + PCACHE_SEGMENTS_OFF)
+#define CACHE_DEV_SEGMENT(cache_dev, id)	((void *)CACHE_DEV_SEGMENTS(cache_dev) + (u64)id * PCACHE_SEG_SIZE)
+
+/*
+ * PCACHE SB flags configured during formatting
+ *
+ * The PCACHE_SB_F_xxx flags define registration requirements based on cache_dev
+ * formatting. For a machine to register a cache_dev:
+ * - PCACHE_SB_F_BIGENDIAN: Requires a big-endian machine.
+ */
+#define PCACHE_SB_F_BIGENDIAN			BIT(0)
+
+struct pcache_sb {
+	__le32 crc;
+	__le32 flags;
+	__le64 magic;
+
+	__le32 seg_num;
+};
+
+struct pcache_cache_dev {
+	u32				sb_flags;
+	u32				seg_num;
+	void				*mapping;
+	bool				use_vmap;
+
+	struct dm_dev			*dm_dev;
+
+	struct mutex			seg_lock;
+	unsigned long			*seg_bitmap;
+};
+
+struct dm_pcache;
+int cache_dev_start(struct dm_pcache *pcache);
+void cache_dev_stop(struct dm_pcache *pcache);
+
+void cache_dev_zero_range(struct pcache_cache_dev *cache_dev, void *pos, u32 size);
+
+int cache_dev_get_empty_segment_id(struct pcache_cache_dev *cache_dev, u32 *seg_id);
+
+#endif /* _PCACHE_CACHE_DEV_H */
-- 
2.43.0

[PATCH v2 04/11] dm-pcache: add segment layer

[Dongsheng Yang]

Introduce segment.{c,h}, an internal abstraction that encapsulates
everything related to a single pcache *segment* (the fixed-size
allocation unit stored on the cache-device).

* On-disk metadata (`struct pcache_segment_info`)
  - Embedded `struct pcache_meta_header` for CRC/sequence handling.
  - `flags` field encodes a “has-next” bit and a 4-bit *type* class
    (`CACHE_DATA` added as the first type).

* Initialisation
  - `pcache_segment_init()` populates the in-memory
    `struct pcache_segment` from a given segment id, data offset and
    metadata pointer, computing the usable `data_size` and virtual
    address within the DAX mapping.

* IO helpers
  - `segment_copy_to_bio()` / `segment_copy_from_bio()` move data
    between pmem and a bio, using `_copy_mc_to_iter()` and
    `_copy_from_iter_flushcache()` to tolerate hw memory errors and
    ensure durability.
  - `segment_pos_advance()` advances an internal offset while staying
    inside the segment’s data area.

These helpers allow upper layers (cache key management, write-back
logic, GC, etc.) to treat a segment as a contiguous byte array without
knowing about DAX mappings or persistence details.

Signed-off-by: Dongsheng Yang <dongsheng.yang@linux.dev>
---
 drivers/md/dm-pcache/segment.c | 61 ++++++++++++++++++++++++++++
 drivers/md/dm-pcache/segment.h | 73 ++++++++++++++++++++++++++++++++++
 2 files changed, 134 insertions(+)
 create mode 100644 drivers/md/dm-pcache/segment.c
 create mode 100644 drivers/md/dm-pcache/segment.h

diff --git a/drivers/md/dm-pcache/segment.c b/drivers/md/dm-pcache/segment.c
new file mode 100644
index 000000000000..7e9818701445
--- /dev/null
+++ b/drivers/md/dm-pcache/segment.c
@@ -0,0 +1,61 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+#include <linux/dax.h>
+
+#include "pcache_internal.h"
+#include "cache_dev.h"
+#include "segment.h"
+
+int segment_copy_to_bio(struct pcache_segment *segment,
+		u32 data_off, u32 data_len, struct bio *bio, u32 bio_off)
+{
+	struct iov_iter iter;
+	size_t copied;
+	void *src;
+
+	iov_iter_bvec(&iter, ITER_DEST, &bio->bi_io_vec[bio->bi_iter.bi_idx],
+			bio_segments(bio), bio->bi_iter.bi_size);
+	iter.iov_offset = bio->bi_iter.bi_bvec_done;
+	if (bio_off)
+		iov_iter_advance(&iter, bio_off);
+
+	src = segment->data + data_off;
+	copied = _copy_mc_to_iter(src, data_len, &iter);
+	if (copied != data_len)
+		return -EIO;
+
+	return 0;
+}
+
+int segment_copy_from_bio(struct pcache_segment *segment,
+		u32 data_off, u32 data_len, struct bio *bio, u32 bio_off)
+{
+	struct iov_iter iter;
+	size_t copied;
+	void *dst;
+
+	iov_iter_bvec(&iter, ITER_SOURCE, &bio->bi_io_vec[bio->bi_iter.bi_idx],
+			bio_segments(bio), bio->bi_iter.bi_size);
+	iter.iov_offset = bio->bi_iter.bi_bvec_done;
+	if (bio_off)
+		iov_iter_advance(&iter, bio_off);
+
+	dst = segment->data + data_off;
+	copied = _copy_from_iter_flushcache(dst, data_len, &iter);
+	if (copied != data_len)
+		return -EIO;
+	pmem_wmb();
+
+	return 0;
+}
+
+void pcache_segment_init(struct pcache_cache_dev *cache_dev, struct pcache_segment *segment,
+		      struct pcache_segment_init_options *options)
+{
+	segment->seg_info = options->seg_info;
+	segment_info_set_type(segment->seg_info, options->type);
+
+	segment->cache_dev = cache_dev;
+	segment->seg_id = options->seg_id;
+	segment->data_size = PCACHE_SEG_SIZE - options->data_off;
+	segment->data = CACHE_DEV_SEGMENT(cache_dev, options->seg_id) + options->data_off;
+}
diff --git a/drivers/md/dm-pcache/segment.h b/drivers/md/dm-pcache/segment.h
new file mode 100644
index 000000000000..cab53504f1d7
--- /dev/null
+++ b/drivers/md/dm-pcache/segment.h
@@ -0,0 +1,73 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+#ifndef _PCACHE_SEGMENT_H
+#define _PCACHE_SEGMENT_H
+
+#include <linux/bio.h>
+
+#include "pcache_internal.h"
+
+struct pcache_segment_info {
+	struct pcache_meta_header	header;
+	__u32			flags;
+	__u32			next_seg;
+};
+
+#define PCACHE_SEG_INFO_FLAGS_HAS_NEXT		BIT(0)
+
+#define PCACHE_SEG_INFO_FLAGS_TYPE_MASK         GENMASK(4, 1)
+#define PCACHE_SEGMENT_TYPE_CACHE_DATA		1
+
+static inline bool segment_info_has_next(struct pcache_segment_info *seg_info)
+{
+	return (seg_info->flags & PCACHE_SEG_INFO_FLAGS_HAS_NEXT);
+}
+
+static inline void segment_info_set_type(struct pcache_segment_info *seg_info, u8 type)
+{
+	seg_info->flags &= ~PCACHE_SEG_INFO_FLAGS_TYPE_MASK;
+	seg_info->flags |= FIELD_PREP(PCACHE_SEG_INFO_FLAGS_TYPE_MASK, type);
+}
+
+static inline u8 segment_info_get_type(struct pcache_segment_info *seg_info)
+{
+	return FIELD_GET(PCACHE_SEG_INFO_FLAGS_TYPE_MASK, seg_info->flags);
+}
+
+struct pcache_segment_pos {
+	struct pcache_segment	*segment;	/* Segment associated with the position */
+	u32			off;		/* Offset within the segment */
+};
+
+struct pcache_segment_init_options {
+	u8			type;
+	u32			seg_id;
+	u32			data_off;
+
+	struct pcache_segment_info	*seg_info;
+};
+
+struct pcache_segment {
+	struct pcache_cache_dev	*cache_dev;
+
+	void			*data;
+	u32			data_size;
+	u32			seg_id;
+
+	struct pcache_segment_info	*seg_info;
+};
+
+int segment_copy_to_bio(struct pcache_segment *segment,
+		      u32 data_off, u32 data_len, struct bio *bio, u32 bio_off);
+int segment_copy_from_bio(struct pcache_segment *segment,
+			u32 data_off, u32 data_len, struct bio *bio, u32 bio_off);
+
+static inline void segment_pos_advance(struct pcache_segment_pos *seg_pos, u32 len)
+{
+	BUG_ON(seg_pos->off + len > seg_pos->segment->data_size);
+
+	seg_pos->off += len;
+}
+
+void pcache_segment_init(struct pcache_cache_dev *cache_dev, struct pcache_segment *segment,
+		      struct pcache_segment_init_options *options);
+#endif /* _PCACHE_SEGMENT_H */
-- 
2.43.0

[PATCH v2 05/11] dm-pcache: add cache_segment

[Dongsheng Yang]

Introduce *cache_segment.c*, the in-memory/on-disk glue that lets a
`struct pcache_cache` manage its array of data segments.

* Metadata handling
  - Loads the most-recent replica of both the segment-info block
    (`struct pcache_segment_info`) and per-segment generation counter
    (`struct pcache_cache_seg_gen`) using `pcache_meta_find_latest()`.
  - Updates those structures atomically with CRC + sequence rollover,
    writing alternately to the two metadata slots inside each segment.

* Segment initialisation (`cache_seg_init`)
  - Builds a `struct pcache_segment` pointing to the segment’s data
    area, sets up locks, generation counters, and, when formatting a new
    cache, zeroes the on-segment kset header.

* Linked-list of segments
  - `cache_seg_set_next_seg()` stores the *next* segment id in
    `seg_info->next_seg` and sets the HAS_NEXT flag, allowing a cache to
    span multiple segments. This is important to allow other type of
    segment added in future.

* Runtime life-cycle
  - Reference counting (`cache_seg_get/put`) with invalidate-on-last-put
    that clears the bitmap slot and schedules cleanup work.
  - Generation bump (`cache_seg_gen_increase`) persists a new generation
    record whenever the segment is modified.

* Allocator
  - `get_cache_segment()` uses a bitmap and per-cache hint to pick the
    next free segment, retrying with micro-delays when none are
    immediately available.

Signed-off-by: Dongsheng Yang <dongsheng.yang@linux.dev>
---
 drivers/md/dm-pcache/cache_segment.c | 293 +++++++++++++++++++++++++++
 1 file changed, 293 insertions(+)
 create mode 100644 drivers/md/dm-pcache/cache_segment.c

diff --git a/drivers/md/dm-pcache/cache_segment.c b/drivers/md/dm-pcache/cache_segment.c
new file mode 100644
index 000000000000..4c1f037f1fd3
--- /dev/null
+++ b/drivers/md/dm-pcache/cache_segment.c
@@ -0,0 +1,293 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include "cache_dev.h"
+#include "cache.h"
+#include "backing_dev.h"
+#include "dm_pcache.h"
+
+static inline struct pcache_segment_info *get_seg_info_addr(struct pcache_cache_segment *cache_seg)
+{
+	struct pcache_segment_info *seg_info_addr;
+	u32 seg_id = cache_seg->segment.seg_id;
+	void *seg_addr;
+
+	seg_addr = CACHE_DEV_SEGMENT(cache_seg->cache->cache_dev, seg_id);
+	seg_info_addr = seg_addr + PCACHE_SEG_INFO_SIZE * cache_seg->info_index;
+
+	return seg_info_addr;
+}
+
+static void cache_seg_info_write(struct pcache_cache_segment *cache_seg)
+{
+	struct pcache_segment_info *seg_info_addr;
+	struct pcache_segment_info *seg_info = &cache_seg->cache_seg_info;
+
+	mutex_lock(&cache_seg->info_lock);
+	seg_info->header.seq++;
+	seg_info->header.crc = pcache_meta_crc(&seg_info->header, sizeof(struct pcache_segment_info));
+
+	seg_info_addr = get_seg_info_addr(cache_seg);
+	memcpy_flushcache(seg_info_addr, seg_info, sizeof(struct pcache_segment_info));
+	pmem_wmb();
+
+	cache_seg->info_index = (cache_seg->info_index + 1) % PCACHE_META_INDEX_MAX;
+	mutex_unlock(&cache_seg->info_lock);
+}
+
+static int cache_seg_info_load(struct pcache_cache_segment *cache_seg)
+{
+	struct pcache_segment_info *cache_seg_info_addr_base, *cache_seg_info_addr;
+	struct pcache_cache_dev *cache_dev = cache_seg->cache->cache_dev;
+	struct dm_pcache *pcache = CACHE_DEV_TO_PCACHE(cache_dev);
+	u32 seg_id = cache_seg->segment.seg_id;
+	int ret = 0;
+
+	cache_seg_info_addr_base = CACHE_DEV_SEGMENT(cache_dev, seg_id);
+
+	mutex_lock(&cache_seg->info_lock);
+	cache_seg_info_addr = pcache_meta_find_latest(&cache_seg_info_addr_base->header,
+						sizeof(struct pcache_segment_info),
+						PCACHE_SEG_INFO_SIZE,
+						&cache_seg->cache_seg_info);
+	if (IS_ERR(cache_seg_info_addr)) {
+		ret = PTR_ERR(cache_seg_info_addr);
+		goto out;
+	} else if (!cache_seg_info_addr) {
+		ret = -EIO;
+		goto out;
+	}
+	cache_seg->info_index = cache_seg_info_addr - cache_seg_info_addr_base;
+out:
+	mutex_unlock(&cache_seg->info_lock);
+
+	if (ret)
+		pcache_dev_err(pcache, "can't read segment info of segment: %u, ret: %d\n",
+			      cache_seg->segment.seg_id, ret);
+	return ret;
+}
+
+static int cache_seg_ctrl_load(struct pcache_cache_segment *cache_seg)
+{
+	struct pcache_cache_seg_ctrl *cache_seg_ctrl = cache_seg->cache_seg_ctrl;
+	struct pcache_cache_seg_gen cache_seg_gen, *cache_seg_gen_addr;
+	int ret = 0;
+
+	mutex_lock(&cache_seg->ctrl_lock);
+	cache_seg_gen_addr = pcache_meta_find_latest(&cache_seg_ctrl->gen->header,
+					     sizeof(struct pcache_cache_seg_gen),
+					     sizeof(struct pcache_cache_seg_gen),
+					     &cache_seg_gen);
+	if (IS_ERR(cache_seg_gen_addr)) {
+		ret = PTR_ERR(cache_seg_gen_addr);
+		goto out;
+	}
+
+	if (!cache_seg_gen_addr) {
+		cache_seg->gen = 0;
+		cache_seg->gen_seq = 0;
+		cache_seg->gen_index = 0;
+		goto out;
+	}
+
+	cache_seg->gen = cache_seg_gen.gen;
+	cache_seg->gen_seq = cache_seg_gen.header.seq;
+	cache_seg->gen_index = (cache_seg_gen_addr - cache_seg_ctrl->gen);
+out:
+	mutex_unlock(&cache_seg->ctrl_lock);
+
+	return ret;
+}
+
+static inline struct pcache_cache_seg_gen *get_cache_seg_gen_addr(struct pcache_cache_segment *cache_seg)
+{
+	struct pcache_cache_seg_ctrl *cache_seg_ctrl = cache_seg->cache_seg_ctrl;
+
+	return (cache_seg_ctrl->gen + cache_seg->gen_index);
+}
+
+static void cache_seg_ctrl_write(struct pcache_cache_segment *cache_seg)
+{
+	struct pcache_cache_seg_gen cache_seg_gen;
+
+	mutex_lock(&cache_seg->ctrl_lock);
+	cache_seg_gen.gen = cache_seg->gen;
+	cache_seg_gen.header.seq = ++cache_seg->gen_seq;
+	cache_seg_gen.header.crc = pcache_meta_crc(&cache_seg_gen.header,
+						 sizeof(struct pcache_cache_seg_gen));
+
+	memcpy_flushcache(get_cache_seg_gen_addr(cache_seg), &cache_seg_gen, sizeof(struct pcache_cache_seg_gen));
+	pmem_wmb();
+
+	cache_seg->gen_index = (cache_seg->gen_index + 1) % PCACHE_META_INDEX_MAX;
+	mutex_unlock(&cache_seg->ctrl_lock);
+}
+
+static void cache_seg_ctrl_init(struct pcache_cache_segment *cache_seg)
+{
+	cache_seg->gen = 0;
+	cache_seg->gen_seq = 0;
+	cache_seg->gen_index = 0;
+	cache_seg_ctrl_write(cache_seg);
+}
+
+static int cache_seg_meta_load(struct pcache_cache_segment *cache_seg)
+{
+	int ret;
+
+	ret = cache_seg_info_load(cache_seg);
+	if (ret)
+		goto err;
+
+	ret = cache_seg_ctrl_load(cache_seg);
+	if (ret)
+		goto err;
+
+	return 0;
+err:
+	return ret;
+}
+
+/**
+ * cache_seg_set_next_seg - Sets the ID of the next segment
+ * @cache_seg: Pointer to the cache segment structure.
+ * @seg_id: The segment ID to set as the next segment.
+ *
+ * A pcache_cache allocates multiple cache segments, which are linked together
+ * through next_seg. When loading a pcache_cache, the first cache segment can
+ * be found using cache->seg_id, which allows access to all the cache segments.
+ */
+void cache_seg_set_next_seg(struct pcache_cache_segment *cache_seg, u32 seg_id)
+{
+	cache_seg->cache_seg_info.flags |= PCACHE_SEG_INFO_FLAGS_HAS_NEXT;
+	cache_seg->cache_seg_info.next_seg = seg_id;
+	cache_seg_info_write(cache_seg);
+}
+
+int cache_seg_init(struct pcache_cache *cache, u32 seg_id, u32 cache_seg_id,
+		   bool new_cache)
+{
+	struct pcache_cache_dev *cache_dev = cache->cache_dev;
+	struct pcache_cache_segment *cache_seg = &cache->segments[cache_seg_id];
+	struct pcache_segment_init_options seg_options = { 0 };
+	struct pcache_segment *segment = &cache_seg->segment;
+	int ret;
+
+	cache_seg->cache = cache;
+	cache_seg->cache_seg_id = cache_seg_id;
+	spin_lock_init(&cache_seg->gen_lock);
+	atomic_set(&cache_seg->refs, 0);
+	mutex_init(&cache_seg->info_lock);
+	mutex_init(&cache_seg->ctrl_lock);
+
+	/* init pcache_segment */
+	seg_options.type = PCACHE_SEGMENT_TYPE_CACHE_DATA;
+	seg_options.data_off = PCACHE_CACHE_SEG_CTRL_OFF + PCACHE_CACHE_SEG_CTRL_SIZE;
+	seg_options.seg_id = seg_id;
+	seg_options.seg_info = &cache_seg->cache_seg_info;
+	pcache_segment_init(cache_dev, segment, &seg_options);
+
+	cache_seg->cache_seg_ctrl = CACHE_DEV_SEGMENT(cache_dev, seg_id) + PCACHE_CACHE_SEG_CTRL_OFF;
+
+	if (new_cache) {
+		cache_dev_zero_range(cache_dev, CACHE_DEV_SEGMENT(cache_dev, seg_id),
+				     PCACHE_SEG_INFO_SIZE * PCACHE_META_INDEX_MAX +
+				     PCACHE_CACHE_SEG_CTRL_SIZE);
+
+		cache_seg_ctrl_init(cache_seg);
+
+		cache_seg->info_index = 0;
+		cache_seg_info_write(cache_seg);
+
+		/* clear outdated kset in segment */
+		memcpy_flushcache(segment->data, &pcache_empty_kset, sizeof(struct pcache_cache_kset_onmedia));
+		pmem_wmb();
+	} else {
+		ret = cache_seg_meta_load(cache_seg);
+		if (ret)
+			goto err;
+	}
+
+	return 0;
+err:
+	return ret;
+}
+
+/**
+ * get_cache_segment - Retrieves a free cache segment from the cache.
+ * @cache: Pointer to the cache structure.
+ *
+ * This function attempts to find a free cache segment that can be used.
+ * It locks the segment map and checks for the next available segment ID.
+ * If a free segment is found, it initializes it and returns a pointer to the
+ * cache segment structure. Returns NULL if no segments are available.
+ */
+struct pcache_cache_segment *get_cache_segment(struct pcache_cache *cache)
+{
+	struct pcache_cache_segment *cache_seg;
+	u32 seg_id;
+
+	spin_lock(&cache->seg_map_lock);
+again:
+	seg_id = find_next_zero_bit(cache->seg_map, cache->n_segs, cache->last_cache_seg);
+	if (seg_id == cache->n_segs) {
+		/* reset the hint of ->last_cache_seg and retry */
+		if (cache->last_cache_seg) {
+			cache->last_cache_seg = 0;
+			goto again;
+		}
+		cache->cache_full = true;
+		spin_unlock(&cache->seg_map_lock);
+		return NULL;
+	}
+
+	/*
+	 * found an available cache_seg, mark it used in seg_map
+	 * and update the search hint ->last_cache_seg
+	 */
+	__set_bit(seg_id, cache->seg_map);
+	cache->last_cache_seg = seg_id;
+	spin_unlock(&cache->seg_map_lock);
+
+	cache_seg = &cache->segments[seg_id];
+	cache_seg->cache_seg_id = seg_id;
+
+	return cache_seg;
+}
+
+static void cache_seg_gen_increase(struct pcache_cache_segment *cache_seg)
+{
+	spin_lock(&cache_seg->gen_lock);
+	cache_seg->gen++;
+	spin_unlock(&cache_seg->gen_lock);
+
+	cache_seg_ctrl_write(cache_seg);
+}
+
+void cache_seg_get(struct pcache_cache_segment *cache_seg)
+{
+	atomic_inc(&cache_seg->refs);
+}
+
+static void cache_seg_invalidate(struct pcache_cache_segment *cache_seg)
+{
+	struct pcache_cache *cache;
+
+	cache = cache_seg->cache;
+	cache_seg_gen_increase(cache_seg);
+
+	spin_lock(&cache->seg_map_lock);
+	if (cache->cache_full)
+		cache->cache_full = false;
+	clear_bit(cache_seg->cache_seg_id, cache->seg_map);
+	spin_unlock(&cache->seg_map_lock);
+
+	pcache_defer_reqs_kick(CACHE_TO_PCACHE(cache));
+	/* clean_work will clean the bad key in key_tree*/
+	queue_work(cache_get_wq(cache), &cache->clean_work);
+}
+
+void cache_seg_put(struct pcache_cache_segment *cache_seg)
+{
+	if (atomic_dec_and_test(&cache_seg->refs))
+		cache_seg_invalidate(cache_seg);
+}
-- 
2.43.0

[PATCH v2 06/11] dm-pcache: add cache_writeback

[Dongsheng Yang]

Introduce cache_writeback.c, which implements the asynchronous write-back
path for pcache.  The new file is responsible for detecting dirty data,
organising it into an in-memory tree, issuing bios to the backing block
device, and advancing the cache’s *dirty tail* pointer once data has
been safely persisted.

* Dirty-state detection
  - `__is_cache_clean()` reads the kset header at `dirty_tail`, checks
    magic and CRC, and thus decides whether there is anything to flush.

* Write-back scheduler
  - `cache_writeback_work` is queued on the cache task-workqueue and
    re-arms itself at `PCACHE_CACHE_WRITEBACK_INTERVAL`.
  - Uses an internal spin-protected `writeback_key_tree` to batch keys
    belonging to the same stripe before IO.

* Key processing
  - `cache_kset_insert_tree()` decodes each key inside the on-media
    kset, allocates an in-memory key object, and inserts it into the
    writeback_key_tree.
  - `cache_key_writeback()` builds a *KMEM-type* backing request that
    maps the persistent-memory range directly into a WRITE bio and
    submits it with `submit_bio_noacct()`.
  - After all keys from the writeback_key_tree have been flushed,
    `backing_dev_flush()` issues a single FLUSH to ensure durability.

* Tail advancement
  - Once a kset is written back, `cache_pos_advance()` moves
    `cache->dirty_tail` by the exact on-disk size and the new position is
    persisted via `cache_encode_dirty_tail()`.
  - When the `PCACHE_KSET_FLAGS_LAST` flag is seen, the write-back
    engine switches to the next segment indicated by `next_cache_seg_id`.

Signed-off-by: Dongsheng Yang <dongsheng.yang@linux.dev>
---
 drivers/md/dm-pcache/cache_writeback.c | 279 +++++++++++++++++++++++++
 1 file changed, 279 insertions(+)
 create mode 100644 drivers/md/dm-pcache/cache_writeback.c

diff --git a/drivers/md/dm-pcache/cache_writeback.c b/drivers/md/dm-pcache/cache_writeback.c
new file mode 100644
index 000000000000..970536187daa
--- /dev/null
+++ b/drivers/md/dm-pcache/cache_writeback.c
@@ -0,0 +1,279 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include <linux/bio.h>
+
+#include "cache.h"
+#include "backing_dev.h"
+#include "cache_dev.h"
+#include "dm_pcache.h"
+
+static void writeback_ctx_end(struct pcache_cache *cache, int ret)
+{
+	if (ret && !cache->writeback_ctx.ret) {
+		pcache_dev_err(CACHE_TO_PCACHE(cache), "writeback error: %d", ret);
+		cache->writeback_ctx.ret = ret;
+	}
+
+	if (!atomic_dec_and_test(&cache->writeback_ctx.pending))
+		return;
+
+	if (!cache->writeback_ctx.ret) {
+		backing_dev_flush(cache->backing_dev);
+
+		mutex_lock(&cache->dirty_tail_lock);
+		cache_pos_advance(&cache->dirty_tail, cache->writeback_ctx.advance);
+		cache_encode_dirty_tail(cache);
+		mutex_unlock(&cache->dirty_tail_lock);
+	}
+	queue_delayed_work(cache_get_wq(cache), &cache->writeback_work, 0);
+}
+
+static void writeback_end_req(struct pcache_backing_dev_req *backing_req, int ret)
+{
+	struct pcache_cache *cache = backing_req->priv_data;
+
+	mutex_lock(&cache->writeback_lock);
+	writeback_ctx_end(cache, ret);
+	mutex_unlock(&cache->writeback_lock);
+}
+
+static inline bool is_cache_clean(struct pcache_cache *cache, struct pcache_cache_pos *dirty_tail)
+{
+	struct dm_pcache *pcache = CACHE_TO_PCACHE(cache);
+	struct pcache_cache_kset_onmedia *kset_onmedia;
+	u32 to_copy;
+	void *addr;
+	int ret;
+
+	addr = cache_pos_addr(dirty_tail);
+	kset_onmedia = (struct pcache_cache_kset_onmedia *)cache->wb_kset_onmedia_buf;
+
+	to_copy = min(PCACHE_KSET_ONMEDIA_SIZE_MAX, PCACHE_SEG_SIZE - dirty_tail->seg_off);
+	ret = copy_mc_to_kernel(kset_onmedia, addr, to_copy);
+	if (ret) {
+		pcache_dev_err(pcache, "error to read kset: %d", ret);
+		return true;
+	}
+
+	/* Check if the magic number matches the expected value */
+	if (kset_onmedia->magic != PCACHE_KSET_MAGIC) {
+		pcache_dev_debug(pcache, "dirty_tail: %u:%u magic: %llx, not expected: %llx\n",
+				dirty_tail->cache_seg->cache_seg_id, dirty_tail->seg_off,
+				kset_onmedia->magic, PCACHE_KSET_MAGIC);
+		return true;
+	}
+
+	/* Verify the CRC checksum for data integrity */
+	if (kset_onmedia->crc != cache_kset_crc(kset_onmedia)) {
+		pcache_dev_debug(pcache, "dirty_tail: %u:%u crc: %x, not expected: %x\n",
+				dirty_tail->cache_seg->cache_seg_id, dirty_tail->seg_off,
+				cache_kset_crc(kset_onmedia), kset_onmedia->crc);
+		return true;
+	}
+
+	return false;
+}
+
+void cache_writeback_exit(struct pcache_cache *cache)
+{
+	cancel_delayed_work_sync(&cache->writeback_work);
+	cache_tree_exit(&cache->writeback_key_tree);
+}
+
+int cache_writeback_init(struct pcache_cache *cache)
+{
+	int ret;
+
+	ret = cache_tree_init(cache, &cache->writeback_key_tree, 1);
+	if (ret)
+		goto err;
+
+	atomic_set(&cache->writeback_ctx.pending, 0);
+
+	/* Queue delayed work to start writeback handling */
+	queue_delayed_work(cache_get_wq(cache), &cache->writeback_work, 0);
+
+	return 0;
+err:
+	return ret;
+}
+
+static int cache_key_writeback(struct pcache_cache *cache, struct pcache_cache_key *key)
+{
+	struct pcache_backing_dev_req *writeback_req;
+	struct pcache_backing_dev_req_opts writeback_req_opts = { 0 };
+	struct pcache_cache_pos *pos;
+	void *addr;
+	u32 seg_remain;
+	u64 off;
+
+	if (cache_key_clean(key))
+		return 0;
+
+	pos = &key->cache_pos;
+
+	seg_remain = cache_seg_remain(pos);
+	BUG_ON(seg_remain < key->len);
+
+	addr = cache_pos_addr(pos);
+	off = key->off;
+
+	writeback_req_opts.type = BACKING_DEV_REQ_TYPE_KMEM;
+	writeback_req_opts.end_fn = writeback_end_req;
+	writeback_req_opts.priv_data = cache;
+
+	writeback_req_opts.kmem.data = addr;
+	writeback_req_opts.kmem.opf = REQ_OP_WRITE;
+	writeback_req_opts.kmem.len = key->len;
+	writeback_req_opts.kmem.backing_off = off;
+
+	writeback_req = backing_dev_req_create(cache->backing_dev, &writeback_req_opts);
+	if (!writeback_req)
+		return -EIO;
+
+	atomic_inc(&cache->writeback_ctx.pending);
+	backing_dev_req_submit(writeback_req, true);
+
+	return 0;
+}
+
+static int cache_wb_tree_writeback(struct pcache_cache *cache, u32 advance)
+{
+	struct dm_pcache *pcache = CACHE_TO_PCACHE(cache);
+	struct pcache_cache_tree *cache_tree = &cache->writeback_key_tree;
+	struct pcache_cache_subtree *cache_subtree;
+	struct rb_node *node;
+	struct pcache_cache_key *key;
+	int ret = 0;
+	u32 i;
+
+	cache->writeback_ctx.ret = 0;
+	cache->writeback_ctx.advance = advance;
+	atomic_set(&cache->writeback_ctx.pending, 1);
+
+	for (i = 0; i < cache_tree->n_subtrees; i++) {
+		cache_subtree = &cache_tree->subtrees[i];
+
+		node = rb_first(&cache_subtree->root);
+		while (node) {
+			key = CACHE_KEY(node);
+			node = rb_next(node);
+
+			ret = cache_key_writeback(cache, key);
+			if (ret) {
+				pcache_dev_err(pcache, "writeback error: %d\n", ret);
+				goto release;
+			}
+
+			cache_key_delete(key);
+		}
+	}
+release:
+	writeback_ctx_end(cache, ret);
+
+	return ret;
+}
+
+static int cache_kset_insert_tree(struct pcache_cache *cache, struct pcache_cache_kset_onmedia *kset_onmedia)
+{
+	struct pcache_cache_key_onmedia *key_onmedia;
+	struct pcache_cache_subtree *cache_subtree;
+	struct pcache_cache_key *key;
+	int ret;
+	u32 i;
+
+	/* Iterate through all keys in the kset and write each back to storage */
+	for (i = 0; i < kset_onmedia->key_num; i++) {
+		key_onmedia = &kset_onmedia->data[i];
+
+		key = cache_key_alloc(&cache->writeback_key_tree);
+		if (!key)
+			return -ENOMEM;
+
+		ret = cache_key_decode(cache, key_onmedia, key);
+		if (ret) {
+			cache_key_put(key);
+			return ret;
+		}
+
+		cache_subtree = get_subtree(&cache->writeback_key_tree, key->off);
+		spin_lock(&cache_subtree->tree_lock);
+		ret = cache_key_insert(&cache->writeback_key_tree, key, true);
+		spin_unlock(&cache_subtree->tree_lock);
+		if (ret) {
+			cache_key_put(key);
+			return ret;
+		}
+	}
+
+	return 0;
+}
+
+static void last_kset_writeback(struct pcache_cache *cache,
+		struct pcache_cache_kset_onmedia *last_kset_onmedia)
+{
+	struct dm_pcache *pcache = CACHE_TO_PCACHE(cache);
+	struct pcache_cache_segment *next_seg;
+
+	pcache_dev_debug(pcache, "last kset, next: %u\n", last_kset_onmedia->next_cache_seg_id);
+
+	next_seg = &cache->segments[last_kset_onmedia->next_cache_seg_id];
+
+	mutex_lock(&cache->dirty_tail_lock);
+	cache->dirty_tail.cache_seg = next_seg;
+	cache->dirty_tail.seg_off = 0;
+	cache_encode_dirty_tail(cache);
+	mutex_unlock(&cache->dirty_tail_lock);
+}
+
+void cache_writeback_fn(struct work_struct *work)
+{
+	struct pcache_cache *cache = container_of(work, struct pcache_cache, writeback_work.work);
+	struct dm_pcache *pcache = CACHE_TO_PCACHE(cache);
+	struct pcache_cache_pos dirty_tail;
+	struct pcache_cache_kset_onmedia *kset_onmedia;
+	u32 delay;
+	int ret;
+
+	mutex_lock(&cache->writeback_lock);
+	if (atomic_read(&cache->writeback_ctx.pending))
+		goto unlock;
+
+	if (pcache_is_stopping(pcache))
+		goto unlock;
+
+	kset_onmedia = (struct pcache_cache_kset_onmedia *)cache->wb_kset_onmedia_buf;
+
+	mutex_lock(&cache->dirty_tail_lock);
+	cache_pos_copy(&dirty_tail, &cache->dirty_tail);
+	mutex_unlock(&cache->dirty_tail_lock);
+
+	if (is_cache_clean(cache, &dirty_tail)) {
+		delay = PCACHE_CACHE_WRITEBACK_INTERVAL;
+		goto queue_work;
+	}
+
+	if (kset_onmedia->flags & PCACHE_KSET_FLAGS_LAST) {
+		last_kset_writeback(cache, kset_onmedia);
+		delay = 0;
+		goto queue_work;
+	}
+
+	ret = cache_kset_insert_tree(cache, kset_onmedia);
+	if (ret) {
+		delay = PCACHE_CACHE_WRITEBACK_INTERVAL;
+		goto queue_work;
+	}
+
+	ret = cache_wb_tree_writeback(cache, get_kset_onmedia_size(kset_onmedia));
+	if (ret) {
+		delay = PCACHE_CACHE_WRITEBACK_INTERVAL;
+		goto queue_work;
+	}
+
+	delay = 0;
+queue_work:
+	queue_delayed_work(cache_get_wq(cache), &cache->writeback_work, delay);
+unlock:
+	mutex_unlock(&cache->writeback_lock);
+}
-- 
2.43.0

[PATCH v2 07/11] dm-pcache: add cache_gc

[Dongsheng Yang]

Introduce cache_gc.c, a self-contained engine that reclaims cache
segments whose data have already been flushed to the backing device.
Running in the cache workqueue, the GC keeps segment usage below the
user-configurable *cache_gc_percent* threshold.

* need_gc() – decides when to trigger GC by checking:
  - *dirty_tail* vs *key_tail* position,
  - kset integrity (magic + CRC),
  - bitmap utilisation against the gc-percent threshold.

* Per-key reclamation
  - Decodes each key in the target kset (`cache_key_decode()`).
  - Drops the segment reference with `cache_seg_put()`, allowing the
    segment to be invalidated once all keys are gone.
  - When the reference count hits zero the segment is cleared from
    `seg_map`, making it immediately reusable by the allocator.

* Scheduling
  - `pcache_cache_gc_fn()` loops until no more work is needed, then
    re-queues itself after *PCACHE_CACHE_GC_INTERVAL*.

Signed-off-by: Dongsheng Yang <dongsheng.yang@linux.dev>
---
 drivers/md/dm-pcache/cache_gc.c | 170 ++++++++++++++++++++++++++++++++
 1 file changed, 170 insertions(+)
 create mode 100644 drivers/md/dm-pcache/cache_gc.c

diff --git a/drivers/md/dm-pcache/cache_gc.c b/drivers/md/dm-pcache/cache_gc.c
new file mode 100644
index 000000000000..a122d95c8f46
--- /dev/null
+++ b/drivers/md/dm-pcache/cache_gc.c
@@ -0,0 +1,170 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+#include "cache.h"
+#include "backing_dev.h"
+#include "cache_dev.h"
+#include "dm_pcache.h"
+
+/**
+ * cache_key_gc - Releases the reference of a cache key segment.
+ * @cache: Pointer to the pcache_cache structure.
+ * @key: Pointer to the cache key to be garbage collected.
+ *
+ * This function decrements the reference count of the cache segment
+ * associated with the given key. If the reference count drops to zero,
+ * the segment may be invalidated and reused.
+ */
+static void cache_key_gc(struct pcache_cache *cache, struct pcache_cache_key *key)
+{
+	cache_seg_put(key->cache_pos.cache_seg);
+}
+
+static bool need_gc(struct pcache_cache *cache, struct pcache_cache_pos *dirty_tail, struct pcache_cache_pos *key_tail)
+{
+	struct dm_pcache *pcache = CACHE_TO_PCACHE(cache);
+	struct pcache_cache_kset_onmedia *kset_onmedia;
+	void *dirty_addr, *key_addr;
+	u32 segs_used, segs_gc_threshold, to_copy;
+	int ret;
+
+	dirty_addr = cache_pos_addr(dirty_tail);
+	key_addr = cache_pos_addr(key_tail);
+	if (dirty_addr == key_addr) {
+		pcache_dev_debug(pcache, "key tail is equal to dirty tail: %u:%u\n",
+				dirty_tail->cache_seg->cache_seg_id,
+				dirty_tail->seg_off);
+		return false;
+	}
+
+	kset_onmedia = (struct pcache_cache_kset_onmedia *)cache->gc_kset_onmedia_buf;
+
+	to_copy = min(PCACHE_KSET_ONMEDIA_SIZE_MAX, PCACHE_SEG_SIZE - key_tail->seg_off);
+	ret = copy_mc_to_kernel(kset_onmedia, key_addr, to_copy);
+	if (ret) {
+		pcache_dev_err(pcache, "error to read kset: %d", ret);
+		return false;
+	}
+
+	/* Check if kset_onmedia is corrupted */
+	if (kset_onmedia->magic != PCACHE_KSET_MAGIC) {
+		pcache_dev_debug(pcache, "gc error: magic is not as expected. key_tail: %u:%u magic: %llx, expected: %llx\n",
+					key_tail->cache_seg->cache_seg_id, key_tail->seg_off,
+					kset_onmedia->magic, PCACHE_KSET_MAGIC);
+		return false;
+	}
+
+	/* Verify the CRC of the kset_onmedia */
+	if (kset_onmedia->crc != cache_kset_crc(kset_onmedia)) {
+		pcache_dev_debug(pcache, "gc error: crc is not as expected. crc: %x, expected: %x\n",
+					cache_kset_crc(kset_onmedia), kset_onmedia->crc);
+		return false;
+	}
+
+	segs_used = bitmap_weight(cache->seg_map, cache->n_segs);
+	segs_gc_threshold = cache->n_segs * pcache_cache_get_gc_percent(cache) / 100;
+	if (segs_used < segs_gc_threshold) {
+		pcache_dev_debug(pcache, "segs_used: %u, segs_gc_threshold: %u\n", segs_used, segs_gc_threshold);
+		return false;
+	}
+
+	return true;
+}
+
+/**
+ * last_kset_gc - Advances the garbage collection for the last kset.
+ * @cache: Pointer to the pcache_cache structure.
+ * @kset_onmedia: Pointer to the kset_onmedia structure for the last kset.
+ */
+static void last_kset_gc(struct pcache_cache *cache, struct pcache_cache_kset_onmedia *kset_onmedia)
+{
+	struct dm_pcache *pcache = CACHE_TO_PCACHE(cache);
+	struct pcache_cache_segment *cur_seg, *next_seg;
+
+	cur_seg = cache->key_tail.cache_seg;
+
+	next_seg = &cache->segments[kset_onmedia->next_cache_seg_id];
+
+	mutex_lock(&cache->key_tail_lock);
+	cache->key_tail.cache_seg = next_seg;
+	cache->key_tail.seg_off = 0;
+	cache_encode_key_tail(cache);
+	mutex_unlock(&cache->key_tail_lock);
+
+	pcache_dev_debug(pcache, "gc advance kset seg: %u\n", cur_seg->cache_seg_id);
+
+	spin_lock(&cache->seg_map_lock);
+	clear_bit(cur_seg->cache_seg_id, cache->seg_map);
+	spin_unlock(&cache->seg_map_lock);
+}
+
+void pcache_cache_gc_fn(struct work_struct *work)
+{
+	struct pcache_cache *cache = container_of(work, struct pcache_cache, gc_work.work);
+	struct dm_pcache *pcache = CACHE_TO_PCACHE(cache);
+	struct pcache_cache_pos dirty_tail, key_tail;
+	struct pcache_cache_kset_onmedia *kset_onmedia;
+	struct pcache_cache_key_onmedia *key_onmedia;
+	struct pcache_cache_key *key;
+	int ret;
+	int i;
+
+	kset_onmedia = (struct pcache_cache_kset_onmedia *)cache->gc_kset_onmedia_buf;
+
+	while (true) {
+		if (pcache_is_stopping(pcache) || atomic_read(&cache->gc_errors))
+			return;
+
+		/* Get new tail positions */
+		mutex_lock(&cache->dirty_tail_lock);
+		cache_pos_copy(&dirty_tail, &cache->dirty_tail);
+		mutex_unlock(&cache->dirty_tail_lock);
+
+		mutex_lock(&cache->key_tail_lock);
+		cache_pos_copy(&key_tail, &cache->key_tail);
+		mutex_unlock(&cache->key_tail_lock);
+
+		if (!need_gc(cache, &dirty_tail, &key_tail))
+			break;
+
+		if (kset_onmedia->flags & PCACHE_KSET_FLAGS_LAST) {
+			/* Don't move to the next segment if dirty_tail has not moved */
+			if (dirty_tail.cache_seg == key_tail.cache_seg)
+				break;
+
+			last_kset_gc(cache, kset_onmedia);
+			continue;
+		}
+
+		for (i = 0; i < kset_onmedia->key_num; i++) {
+			struct pcache_cache_key key_tmp = { 0 };
+
+			key_onmedia = &kset_onmedia->data[i];
+
+			key = &key_tmp;
+			cache_key_init(&cache->req_key_tree, key);
+
+			ret = cache_key_decode(cache, key_onmedia, key);
+			if (ret) {
+				/* return without re-arm gc work, and prevent future
+				 * gc, because we can't retry the partial-gc-ed kset
+				 */
+				atomic_inc(&cache->gc_errors);
+				pcache_dev_err(pcache, "failed to decode cache key in gc\n");
+				return;
+			}
+
+			cache_key_gc(cache, key);
+		}
+
+		pcache_dev_debug(pcache, "gc advance: %u:%u %u\n",
+			key_tail.cache_seg->cache_seg_id,
+			key_tail.seg_off,
+			get_kset_onmedia_size(kset_onmedia));
+
+		mutex_lock(&cache->key_tail_lock);
+		cache_pos_advance(&cache->key_tail, get_kset_onmedia_size(kset_onmedia));
+		cache_encode_key_tail(cache);
+		mutex_unlock(&cache->key_tail_lock);
+	}
+
+	queue_delayed_work(cache_get_wq(cache), &cache->gc_work, PCACHE_CACHE_GC_INTERVAL);
+}
-- 
2.43.0

[PATCH v2 08/11] dm-pcache: add cache_key

[Dongsheng Yang]

Add *cache_key.c* which becomes the heart of dm-pcache’s
in-memory index and on-media key-set (“kset”) format.

* Key objects (`struct pcache_cache_key`)
  - Slab-backed allocator & ref-count helpers
  - `cache_key_encode()/decode()` translate between in-memory keys and
    their on-disk representation, validating CRC when
    *cache_data_crc* is enabled.

* Kset construction & persistence
  - Per-kset buffer lives in `struct pcache_cache_kset`; keys are
    appended until full or *force_close* triggers an immediate flush.
  - `cache_kset_close()` writes the kset to the *key_head* segment,
    automatically chaining a *LAST* kset header when rolling over to a
    freshly allocated segment.

* Red-black tree with striping
  - Cache space is divided into *subtrees* to reduce lock
    contention; each subtree owns its own RB-root + spinlock.
  - Complex overlap-resolution logic (`cache_insert_fixup()`) ensures
    newly inserted keys never leave overlapping stale ranges behind
    (head/tail/contain/contained cases handled).

* Replay on start-up
  - `cache_replay()` walks from *key_tail* to *key_head*, re-hydrates
    keys, validates CRC/magic, seamlessly
    skipping placeholder “empty” keys left by read-misses.

* Background maintenance
  - `clean_work` lazily prunes invalidated keys after GC.
  - `kset_flush_work` background thread to close a kset.

With this patch dm-pcache can persistently track cached extents, rebuild
its index after crash, and guarantee non-overlapping key space – paving
the way for functional read/write caching.

Signed-off-by: Dongsheng Yang <dongsheng.yang@linux.dev>
---
 drivers/md/dm-pcache/cache_key.c | 900 +++++++++++++++++++++++++++++++
 1 file changed, 900 insertions(+)
 create mode 100644 drivers/md/dm-pcache/cache_key.c

diff --git a/drivers/md/dm-pcache/cache_key.c b/drivers/md/dm-pcache/cache_key.c
new file mode 100644
index 000000000000..1b0a1ebe92f2
--- /dev/null
+++ b/drivers/md/dm-pcache/cache_key.c
@@ -0,0 +1,900 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+#include "cache.h"
+#include "backing_dev.h"
+#include "cache_dev.h"
+#include "dm_pcache.h"
+
+struct pcache_cache_kset_onmedia pcache_empty_kset = { 0 };
+
+void cache_key_init(struct pcache_cache_tree *cache_tree, struct pcache_cache_key *key)
+{
+	kref_init(&key->ref);
+	key->cache_tree = cache_tree;
+	INIT_LIST_HEAD(&key->list_node);
+	RB_CLEAR_NODE(&key->rb_node);
+}
+
+struct pcache_cache_key *cache_key_alloc(struct pcache_cache_tree *cache_tree)
+{
+	struct pcache_cache_key *key;
+
+	key = mempool_alloc(&cache_tree->key_pool, GFP_NOIO);
+	if (!key)
+		return NULL;
+
+	memset(key, 0, sizeof(struct pcache_cache_key));
+	cache_key_init(cache_tree, key);
+
+	return key;
+}
+
+/**
+ * cache_key_get - Increment the reference count of a cache key.
+ * @key: Pointer to the pcache_cache_key structure.
+ *
+ * This function increments the reference count of the specified cache key,
+ * ensuring that it is not freed while still in use.
+ */
+void cache_key_get(struct pcache_cache_key *key)
+{
+	kref_get(&key->ref);
+}
+
+/**
+ * cache_key_destroy - Free a cache key structure when its reference count drops to zero.
+ * @ref: Pointer to the kref structure.
+ *
+ * This function is called when the reference count of the cache key reaches zero.
+ * It frees the allocated cache key back to the slab cache.
+ */
+static void cache_key_destroy(struct kref *ref)
+{
+	struct pcache_cache_key *key = container_of(ref, struct pcache_cache_key, ref);
+	struct pcache_cache_tree *cache_tree = key->cache_tree;
+
+	mempool_free(key, &cache_tree->key_pool);
+}
+
+void cache_key_put(struct pcache_cache_key *key)
+{
+	kref_put(&key->ref, cache_key_destroy);
+}
+
+void cache_pos_advance(struct pcache_cache_pos *pos, u32 len)
+{
+	/* Ensure enough space remains in the current segment */
+	BUG_ON(cache_seg_remain(pos) < len);
+
+	pos->seg_off += len;
+}
+
+static void cache_key_encode(struct pcache_cache *cache,
+			     struct pcache_cache_key_onmedia *key_onmedia,
+			     struct pcache_cache_key *key)
+{
+	key_onmedia->off = key->off;
+	key_onmedia->len = key->len;
+
+	key_onmedia->cache_seg_id = key->cache_pos.cache_seg->cache_seg_id;
+	key_onmedia->cache_seg_off = key->cache_pos.seg_off;
+
+	key_onmedia->seg_gen = key->seg_gen;
+	key_onmedia->flags = key->flags;
+
+	if (cache_data_crc_on(cache))
+		key_onmedia->data_crc = cache_key_data_crc(key);
+}
+
+int cache_key_decode(struct pcache_cache *cache,
+			struct pcache_cache_key_onmedia *key_onmedia,
+			struct pcache_cache_key *key)
+{
+	struct dm_pcache *pcache = CACHE_TO_PCACHE(cache);
+
+	key->off = key_onmedia->off;
+	key->len = key_onmedia->len;
+
+	key->cache_pos.cache_seg = &cache->segments[key_onmedia->cache_seg_id];
+	key->cache_pos.seg_off = key_onmedia->cache_seg_off;
+
+	key->seg_gen = key_onmedia->seg_gen;
+	key->flags = key_onmedia->flags;
+
+	if (cache_data_crc_on(cache) &&
+			key_onmedia->data_crc != cache_key_data_crc(key)) {
+		pcache_dev_err(pcache, "key: %llu:%u seg %u:%u data_crc error: %x, expected: %x\n",
+				key->off, key->len, key->cache_pos.cache_seg->cache_seg_id,
+				key->cache_pos.seg_off, cache_key_data_crc(key), key_onmedia->data_crc);
+		return -EIO;
+	}
+
+	return 0;
+}
+
+static void append_last_kset(struct pcache_cache *cache, u32 next_seg)
+{
+	struct pcache_cache_kset_onmedia kset_onmedia = { 0 };
+
+	kset_onmedia.flags |= PCACHE_KSET_FLAGS_LAST;
+	kset_onmedia.next_cache_seg_id = next_seg;
+	kset_onmedia.magic = PCACHE_KSET_MAGIC;
+	kset_onmedia.crc = cache_kset_crc(&kset_onmedia);
+
+	memcpy_flushcache(get_key_head_addr(cache), &kset_onmedia, sizeof(struct pcache_cache_kset_onmedia));
+	pmem_wmb();
+	cache_pos_advance(&cache->key_head, sizeof(struct pcache_cache_kset_onmedia));
+}
+
+int cache_kset_close(struct pcache_cache *cache, struct pcache_cache_kset *kset)
+{
+	struct pcache_cache_kset_onmedia *kset_onmedia;
+	u32 kset_onmedia_size;
+	int ret;
+
+	kset_onmedia = &kset->kset_onmedia;
+
+	if (!kset_onmedia->key_num)
+		return 0;
+
+	kset_onmedia_size = struct_size(kset_onmedia, data, kset_onmedia->key_num);
+
+	spin_lock(&cache->key_head_lock);
+again:
+	/* Reserve space for the last kset */
+	if (cache_seg_remain(&cache->key_head) < kset_onmedia_size + sizeof(struct pcache_cache_kset_onmedia)) {
+		struct pcache_cache_segment *next_seg;
+
+		next_seg = get_cache_segment(cache);
+		if (!next_seg) {
+			ret = -EBUSY;
+			goto out;
+		}
+
+		/* clear outdated kset in next seg */
+		memcpy_flushcache(next_seg->segment.data, &pcache_empty_kset,
+					sizeof(struct pcache_cache_kset_onmedia));
+		append_last_kset(cache, next_seg->cache_seg_id);
+		cache->key_head.cache_seg = next_seg;
+		cache->key_head.seg_off = 0;
+		goto again;
+	}
+
+	kset_onmedia->magic = PCACHE_KSET_MAGIC;
+	kset_onmedia->crc = cache_kset_crc(kset_onmedia);
+
+	/* clear outdated kset after current kset */
+	memcpy_flushcache(get_key_head_addr(cache) + kset_onmedia_size, &pcache_empty_kset,
+				sizeof(struct pcache_cache_kset_onmedia));
+	/* write current kset into segment */
+	memcpy_flushcache(get_key_head_addr(cache), kset_onmedia, kset_onmedia_size);
+	pmem_wmb();
+
+	/* reset kset_onmedia */
+	memset(kset_onmedia, 0, sizeof(struct pcache_cache_kset_onmedia));
+	cache_pos_advance(&cache->key_head, kset_onmedia_size);
+
+	ret = 0;
+out:
+	spin_unlock(&cache->key_head_lock);
+
+	return ret;
+}
+
+/**
+ * cache_key_append - Append a cache key to the related kset.
+ * @cache: Pointer to the pcache_cache structure.
+ * @key: Pointer to the cache key structure to append.
+ * @force_close: Need to close current kset if true.
+ *
+ * This function appends a cache key to the appropriate kset. If the kset
+ * is full, it closes the kset. If not, it queues a flush work to write
+ * the kset to media.
+ *
+ * Returns 0 on success, or a negative error code on failure.
+ */
+int cache_key_append(struct pcache_cache *cache, struct pcache_cache_key *key, bool force_close)
+{
+	struct pcache_cache_kset *kset;
+	struct pcache_cache_kset_onmedia *kset_onmedia;
+	struct pcache_cache_key_onmedia *key_onmedia;
+	u32 kset_id = get_kset_id(cache, key->off);
+	int ret = 0;
+
+	kset = get_kset(cache, kset_id);
+	kset_onmedia = &kset->kset_onmedia;
+
+	spin_lock(&kset->kset_lock);
+	key_onmedia = &kset_onmedia->data[kset_onmedia->key_num];
+	cache_key_encode(cache, key_onmedia, key);
+
+	/* Check if the current kset has reached the maximum number of keys */
+	if (++kset_onmedia->key_num == PCACHE_KSET_KEYS_MAX || force_close) {
+		/* If full, close the kset */
+		ret = cache_kset_close(cache, kset);
+		if (ret) {
+			kset_onmedia->key_num--;
+			goto out;
+		}
+	} else {
+		/* If not full, queue a delayed work to flush the kset */
+		queue_delayed_work(cache_get_wq(cache), &kset->flush_work, 1 * HZ);
+	}
+out:
+	spin_unlock(&kset->kset_lock);
+
+	return ret;
+}
+
+/**
+ * cache_subtree_walk - Traverse the cache tree.
+ * @ctx: Pointer to the context structure for traversal.
+ *
+ * This function traverses the cache tree starting from the specified node.
+ * It calls the appropriate callback functions based on the relationships
+ * between the keys in the cache tree.
+ *
+ * Returns 0 on success, or a negative error code on failure.
+ */
+int cache_subtree_walk(struct pcache_cache_subtree_walk_ctx *ctx)
+{
+	struct pcache_cache_key *key_tmp, *key;
+	struct rb_node *node_tmp;
+	int ret = SUBTREE_WALK_RET_OK;
+
+	key = ctx->key;
+	node_tmp = ctx->start_node;
+
+	while (node_tmp) {
+		if (ctx->walk_done && ctx->walk_done(ctx))
+			break;
+
+		key_tmp = CACHE_KEY(node_tmp);
+		/*
+		 * If key_tmp ends before the start of key, continue to the next node.
+		 * |----------|
+		 *              |=====|
+		 */
+		if (cache_key_lend(key_tmp) <= cache_key_lstart(key)) {
+			if (ctx->after) {
+				ret = ctx->after(key, key_tmp, ctx);
+				if (ret)
+					goto out;
+			}
+			goto next;
+		}
+
+		/*
+		 * If key_tmp starts after the end of key, stop traversing.
+		 *	  |--------|
+		 * |====|
+		 */
+		if (cache_key_lstart(key_tmp) >= cache_key_lend(key)) {
+			if (ctx->before) {
+				ret = ctx->before(key, key_tmp, ctx);
+				if (ret)
+					goto out;
+			}
+			break;
+		}
+
+		/* Handle overlapping keys */
+		if (cache_key_lstart(key_tmp) >= cache_key_lstart(key)) {
+			/*
+			 * If key_tmp encompasses key.
+			 *     |----------------|	key_tmp
+			 * |===========|		key
+			 */
+			if (cache_key_lend(key_tmp) >= cache_key_lend(key)) {
+				if (ctx->overlap_tail) {
+					ret = ctx->overlap_tail(key, key_tmp, ctx);
+					if (ret)
+						goto out;
+				}
+				break;
+			}
+
+			/*
+			 * If key_tmp is contained within key.
+			 *    |----|		key_tmp
+			 * |==========|		key
+			 */
+			if (ctx->overlap_contain) {
+				ret = ctx->overlap_contain(key, key_tmp, ctx);
+				if (ret)
+					goto out;
+			}
+
+			goto next;
+		}
+
+		/*
+		 * If key_tmp starts before key ends but ends after key.
+		 * |-----------|	key_tmp
+		 *   |====|		key
+		 */
+		if (cache_key_lend(key_tmp) > cache_key_lend(key)) {
+			if (ctx->overlap_contained) {
+				ret = ctx->overlap_contained(key, key_tmp, ctx);
+				if (ret)
+					goto out;
+			}
+			break;
+		}
+
+		/*
+		 * If key_tmp starts before key and ends within key.
+		 * |--------|		key_tmp
+		 *   |==========|	key
+		 */
+		if (ctx->overlap_head) {
+			ret = ctx->overlap_head(key, key_tmp, ctx);
+			if (ret)
+				goto out;
+		}
+next:
+		node_tmp = rb_next(node_tmp);
+	}
+
+out:
+	if (ctx->walk_finally)
+		ret = ctx->walk_finally(ctx, ret);
+
+	return ret;
+}
+
+/**
+ * cache_subtree_search - Search for a key in the cache tree.
+ * @cache_subtree: Pointer to the cache tree structure.
+ * @key: Pointer to the cache key to search for.
+ * @parentp: Pointer to store the parent node of the found node.
+ * @newp: Pointer to store the location where the new node should be inserted.
+ * @delete_key_list: List to collect invalid keys for deletion.
+ *
+ * This function searches the cache tree for a specific key and returns
+ * the node that is the predecessor of the key, or first node if the key is
+ * less than all keys in the tree. If any invalid keys are found during
+ * the search, they are added to the delete_key_list for later cleanup.
+ *
+ * Returns a pointer to the previous node.
+ */
+struct rb_node *cache_subtree_search(struct pcache_cache_subtree *cache_subtree, struct pcache_cache_key *key,
+				  struct rb_node **parentp, struct rb_node ***newp,
+				  struct list_head *delete_key_list)
+{
+	struct rb_node **new, *parent = NULL;
+	struct pcache_cache_key *key_tmp;
+	struct rb_node *prev_node = NULL;
+
+	new = &(cache_subtree->root.rb_node);
+	while (*new) {
+		key_tmp = container_of(*new, struct pcache_cache_key, rb_node);
+		if (cache_key_invalid(key_tmp))
+			list_add(&key_tmp->list_node, delete_key_list);
+
+		parent = *new;
+		if (key_tmp->off >= key->off) {
+			new = &((*new)->rb_left);
+		} else {
+			prev_node = *new;
+			new = &((*new)->rb_right);
+		}
+	}
+
+	if (!prev_node)
+		prev_node = rb_first(&cache_subtree->root);
+
+	if (parentp)
+		*parentp = parent;
+
+	if (newp)
+		*newp = new;
+
+	return prev_node;
+}
+
+/**
+ * fixup_overlap_tail - Adjust the key when it overlaps at the tail.
+ * @key: Pointer to the new cache key being inserted.
+ * @key_tmp: Pointer to the existing key that overlaps.
+ * @ctx: Pointer to the context for walking the cache tree.
+ *
+ * This function modifies the existing key (key_tmp) when there is an
+ * overlap at the tail with the new key. If the modified key becomes
+ * empty, it is deleted.
+ */
+static int fixup_overlap_tail(struct pcache_cache_key *key,
+			       struct pcache_cache_key *key_tmp,
+			       struct pcache_cache_subtree_walk_ctx *ctx)
+{
+	/*
+	 *     |----------------|	key_tmp
+	 * |===========|		key
+	 */
+	BUG_ON(cache_key_empty(key));
+	if (cache_key_empty(key_tmp)) {
+		cache_key_delete(key_tmp);
+		return SUBTREE_WALK_RET_RESEARCH;
+	}
+
+	cache_key_cutfront(key_tmp, cache_key_lend(key) - cache_key_lstart(key_tmp));
+	if (key_tmp->len == 0) {
+		cache_key_delete(key_tmp);
+		return SUBTREE_WALK_RET_RESEARCH;
+	}
+
+	return SUBTREE_WALK_RET_OK;
+}
+
+/**
+ * fixup_overlap_contain - Handle case where new key completely contains an existing key.
+ * @key: Pointer to the new cache key being inserted.
+ * @key_tmp: Pointer to the existing key that is being contained.
+ * @ctx: Pointer to the context for walking the cache tree.
+ *
+ * This function deletes the existing key (key_tmp) when the new key
+ * completely contains it. It returns SUBTREE_WALK_RET_RESEARCH to indicate that the
+ * tree structure may have changed, necessitating a re-insertion of
+ * the new key.
+ */
+static int fixup_overlap_contain(struct pcache_cache_key *key,
+				  struct pcache_cache_key *key_tmp,
+				  struct pcache_cache_subtree_walk_ctx *ctx)
+{
+	/*
+	 *    |----|			key_tmp
+	 * |==========|			key
+	 */
+	BUG_ON(cache_key_empty(key));
+	cache_key_delete(key_tmp);
+
+	return SUBTREE_WALK_RET_RESEARCH;
+}
+
+/**
+ * fixup_overlap_contained - Handle overlap when a new key is contained in an existing key.
+ * @key: The new cache key being inserted.
+ * @key_tmp: The existing cache key that overlaps with the new key.
+ * @ctx: Context for the cache tree walk.
+ *
+ * This function adjusts the existing key if the new key is contained
+ * within it. If the existing key is empty, it indicates a placeholder key
+ * that was inserted during a miss read. This placeholder will later be
+ * updated with real data from the backing_dev, making it no longer an empty key.
+ *
+ * If we delete key or insert a key, the structure of the entire cache tree may change,
+ * requiring a full research of the tree to find a new insertion point.
+ */
+static int fixup_overlap_contained(struct pcache_cache_key *key,
+	struct pcache_cache_key *key_tmp, struct pcache_cache_subtree_walk_ctx *ctx)
+{
+	struct pcache_cache_tree *cache_tree = ctx->cache_tree;
+
+	/*
+	 * |-----------|		key_tmp
+	 *   |====|			key
+	 */
+	BUG_ON(cache_key_empty(key));
+	if (cache_key_empty(key_tmp)) {
+		/* If key_tmp is empty, don't split it;
+		 * it's a placeholder key for miss reads that will be updated later.
+		 */
+		cache_key_cutback(key_tmp, cache_key_lend(key_tmp) - cache_key_lstart(key));
+		if (key_tmp->len == 0) {
+			cache_key_delete(key_tmp);
+			return SUBTREE_WALK_RET_RESEARCH;
+		}
+	} else {
+		struct pcache_cache_key *key_fixup;
+		bool need_research = false;
+
+		if (!ctx->pre_alloc_key)
+			return SUBTREE_WALK_RET_NEED_KEY;
+
+		key_fixup = ctx->pre_alloc_key;
+		ctx->pre_alloc_key = NULL;
+
+		cache_key_copy(key_fixup, key_tmp);
+
+		/* Split key_tmp based on the new key's range */
+		cache_key_cutback(key_tmp, cache_key_lend(key_tmp) - cache_key_lstart(key));
+		if (key_tmp->len == 0) {
+			cache_key_delete(key_tmp);
+			need_research = true;
+		}
+
+		/* Create a new portion for key_fixup */
+		cache_key_cutfront(key_fixup, cache_key_lend(key) - cache_key_lstart(key_tmp));
+		if (key_fixup->len == 0) {
+			cache_key_put(key_fixup);
+		} else {
+			/* Insert the new key into the cache */
+			ctx->ret = cache_key_insert(cache_tree, key_fixup, false);
+			if (ctx->ret)
+				return SUBTREE_WALK_RET_ERR;
+			need_research = true;
+		}
+
+		if (need_research)
+			return SUBTREE_WALK_RET_RESEARCH;
+	}
+
+	return SUBTREE_WALK_RET_OK;
+}
+
+/**
+ * fixup_overlap_head - Handle overlap when a new key overlaps with the head of an existing key.
+ * @key: The new cache key being inserted.
+ * @key_tmp: The existing cache key that overlaps with the new key.
+ * @ctx: Context for the cache tree walk.
+ *
+ * This function adjusts the existing key if the new key overlaps
+ * with the beginning of it. If the resulting key length is zero
+ * after the adjustment, the key is deleted. This indicates that
+ * the key no longer holds valid data and requires the tree to be
+ * re-researched for a new insertion point.
+ */
+static int fixup_overlap_head(struct pcache_cache_key *key,
+	struct pcache_cache_key *key_tmp, struct pcache_cache_subtree_walk_ctx *ctx)
+{
+	/*
+	 * |--------|		key_tmp
+	 *   |==========|	key
+	 */
+	BUG_ON(cache_key_empty(key));
+	/* Adjust key_tmp by cutting back based on the new key's start */
+	cache_key_cutback(key_tmp, cache_key_lend(key_tmp) - cache_key_lstart(key));
+	if (key_tmp->len == 0) {
+		/* If the adjusted key_tmp length is zero, delete it */
+		cache_key_delete(key_tmp);
+		return SUBTREE_WALK_RET_RESEARCH;
+	}
+
+	return SUBTREE_WALK_RET_OK;
+}
+
+/**
+ * cache_key_insert - Insert a new cache key into the cache tree.
+ * @cache_tree: Pointer to the cache_tree structure.
+ * @key: The cache key to insert.
+ * @fixup: Indicates if this is a new key being inserted.
+ *
+ * This function searches for the appropriate location to insert
+ * a new cache key into the cache tree. It handles key overlaps
+ * and ensures any invalid keys are removed before insertion.
+ *
+ * Returns 0 on success or a negative error code on failure.
+ */
+int cache_key_insert(struct pcache_cache_tree *cache_tree, struct pcache_cache_key *key, bool fixup)
+{
+	struct pcache_cache_subtree_walk_ctx walk_ctx = { 0 };
+	struct rb_node **new, *parent = NULL;
+	struct pcache_cache_subtree *cache_subtree;
+	struct pcache_cache_key *key_tmp = NULL, *key_next;
+	struct rb_node *prev_node = NULL;
+	LIST_HEAD(delete_key_list);
+	int ret;
+
+	cache_subtree = get_subtree(cache_tree, key->off);
+	key->cache_subtree = cache_subtree;
+search:
+	prev_node = cache_subtree_search(cache_subtree, key, &parent, &new, &delete_key_list);
+	if (!list_empty(&delete_key_list)) {
+		/* Remove invalid keys from the delete list */
+		list_for_each_entry_safe(key_tmp, key_next, &delete_key_list, list_node) {
+			list_del_init(&key_tmp->list_node);
+			cache_key_delete(key_tmp);
+		}
+		goto search;
+	}
+
+	if (fixup) {
+		/* Set up the context with the cache, start node, and new key */
+		walk_ctx.cache_tree = cache_tree;
+		walk_ctx.start_node = prev_node;
+		walk_ctx.key = key;
+
+		/* Assign overlap handling functions for different scenarios */
+		walk_ctx.overlap_tail = fixup_overlap_tail;
+		walk_ctx.overlap_head = fixup_overlap_head;
+		walk_ctx.overlap_contain = fixup_overlap_contain;
+		walk_ctx.overlap_contained = fixup_overlap_contained;
+
+		ret = cache_subtree_walk(&walk_ctx);
+		switch (ret) {
+		case SUBTREE_WALK_RET_OK:
+			break;
+		case SUBTREE_WALK_RET_RESEARCH:
+			goto search;
+		case SUBTREE_WALK_RET_NEED_KEY:
+			spin_unlock(&cache_subtree->tree_lock);
+			walk_ctx.pre_alloc_key = cache_key_alloc(cache_tree);
+			spin_lock(&cache_subtree->tree_lock);
+
+			if (!walk_ctx.pre_alloc_key) {
+				ret = -ENOMEM;
+				goto out;
+			}
+			goto search;
+		case SUBTREE_WALK_RET_ERR:
+			ret = walk_ctx.ret;
+			goto out;
+		default:
+			BUG();
+		}
+	}
+
+	/* Link and insert the new key into the red-black tree */
+	rb_link_node(&key->rb_node, parent, new);
+	rb_insert_color(&key->rb_node, &cache_subtree->root);
+	ret = 0;
+out:
+	if (walk_ctx.pre_alloc_key)
+		cache_key_put(walk_ctx.pre_alloc_key);
+
+	return ret;
+}
+
+/**
+ * clean_fn - Cleanup function to remove invalid keys from the cache tree.
+ * @work: Pointer to the work_struct associated with the cleanup.
+ *
+ * This function cleans up invalid keys from the cache tree in the background
+ * after a cache segment has been invalidated during cache garbage collection.
+ * It processes a maximum of PCACHE_CLEAN_KEYS_MAX keys per iteration and holds
+ * the tree lock to ensure thread safety.
+ */
+void clean_fn(struct work_struct *work)
+{
+	struct pcache_cache *cache = container_of(work, struct pcache_cache, clean_work);
+	struct pcache_cache_subtree *cache_subtree;
+	struct rb_node *node;
+	struct pcache_cache_key *key;
+	int i, count;
+
+	for (i = 0; i < cache->req_key_tree.n_subtrees; i++) {
+		cache_subtree = &cache->req_key_tree.subtrees[i];
+
+again:
+		if (pcache_is_stopping(CACHE_TO_PCACHE(cache)))
+			return;
+
+		/* Delete up to PCACHE_CLEAN_KEYS_MAX keys in one iteration */
+		count = 0;
+		spin_lock(&cache_subtree->tree_lock);
+		node = rb_first(&cache_subtree->root);
+		while (node) {
+			key = CACHE_KEY(node);
+			node = rb_next(node);
+			if (cache_key_invalid(key)) {
+				count++;
+				cache_key_delete(key);
+			}
+
+			if (count >= PCACHE_CLEAN_KEYS_MAX) {
+				/* Unlock and pause before continuing cleanup */
+				spin_unlock(&cache_subtree->tree_lock);
+				usleep_range(1000, 2000);
+				goto again;
+			}
+		}
+		spin_unlock(&cache_subtree->tree_lock);
+	}
+}
+
+/*
+ * kset_flush_fn - Flush work for a cache kset.
+ *
+ * This function is called when a kset flush work is queued from
+ * cache_key_append(). If the kset is full, it will be closed
+ * immediately. If not, the flush work will be queued for later closure.
+ *
+ * If cache_kset_close detects that a new segment is required to store
+ * the kset and there are no available segments, it will return an error.
+ * In this scenario, a retry will be attempted.
+ */
+void kset_flush_fn(struct work_struct *work)
+{
+	struct pcache_cache_kset *kset = container_of(work, struct pcache_cache_kset, flush_work.work);
+	struct pcache_cache *cache = kset->cache;
+	int ret;
+
+	if (pcache_is_stopping(CACHE_TO_PCACHE(cache)))
+		return;
+
+	spin_lock(&kset->kset_lock);
+	ret = cache_kset_close(cache, kset);
+	spin_unlock(&kset->kset_lock);
+
+	if (ret) {
+		/* Failed to flush kset, schedule a retry. */
+		queue_delayed_work(cache_get_wq(cache), &kset->flush_work, msecs_to_jiffies(100));
+	}
+}
+
+static int kset_replay(struct pcache_cache *cache, struct pcache_cache_kset_onmedia *kset_onmedia)
+{
+	struct pcache_cache_key_onmedia *key_onmedia;
+	struct pcache_cache_subtree *cache_subtree;
+	struct pcache_cache_key *key;
+	int ret;
+	int i;
+
+	for (i = 0; i < kset_onmedia->key_num; i++) {
+		key_onmedia = &kset_onmedia->data[i];
+
+		key = cache_key_alloc(&cache->req_key_tree);
+		if (!key) {
+			ret = -ENOMEM;
+			goto err;
+		}
+
+		ret = cache_key_decode(cache, key_onmedia, key);
+		if (ret) {
+			cache_key_put(key);
+			goto err;
+		}
+
+		/* Mark the segment as used in the segment map. */
+		set_bit(key->cache_pos.cache_seg->cache_seg_id, cache->seg_map);
+
+		/* Check if the segment generation is valid for insertion. */
+		if (key->seg_gen < key->cache_pos.cache_seg->gen) {
+			cache_key_put(key);
+		} else {
+			cache_subtree = get_subtree(&cache->req_key_tree, key->off);
+			spin_lock(&cache_subtree->tree_lock);
+			ret = cache_key_insert(&cache->req_key_tree, key, true);
+			spin_unlock(&cache_subtree->tree_lock);
+			if (ret) {
+				cache_key_put(key);
+				goto err;
+			}
+		}
+
+		cache_seg_get(key->cache_pos.cache_seg);
+	}
+
+	return 0;
+err:
+	return ret;
+}
+
+int cache_replay(struct pcache_cache *cache)
+{
+	struct dm_pcache *pcache = CACHE_TO_PCACHE(cache);
+	struct pcache_cache_pos pos_tail;
+	struct pcache_cache_pos *pos;
+	struct pcache_cache_kset_onmedia *kset_onmedia;
+	u32 to_copy, count = 0;
+	int ret = 0;
+
+	kset_onmedia = kzalloc(PCACHE_KSET_ONMEDIA_SIZE_MAX, GFP_KERNEL);
+	if (!kset_onmedia)
+		return -ENOMEM;
+
+	cache_pos_copy(&pos_tail, &cache->key_tail);
+	pos = &pos_tail;
+
+	/* Mark the segment as used in the segment map. */
+	set_bit(pos->cache_seg->cache_seg_id, cache->seg_map);
+
+	while (true) {
+		to_copy = min(PCACHE_KSET_ONMEDIA_SIZE_MAX, PCACHE_SEG_SIZE - pos->seg_off);
+		ret = copy_mc_to_kernel(kset_onmedia, cache_pos_addr(pos), to_copy);
+		if (ret) {
+			ret = -EIO;
+			goto out;
+		}
+
+		if (kset_onmedia->magic != PCACHE_KSET_MAGIC ||
+				kset_onmedia->crc != cache_kset_crc(kset_onmedia)) {
+			break;
+		}
+
+		/* Process the last kset and prepare for the next segment. */
+		if (kset_onmedia->flags & PCACHE_KSET_FLAGS_LAST) {
+			struct pcache_cache_segment *next_seg;
+
+			pcache_dev_debug(pcache, "last kset replay, next: %u\n", kset_onmedia->next_cache_seg_id);
+
+			next_seg = &cache->segments[kset_onmedia->next_cache_seg_id];
+
+			pos->cache_seg = next_seg;
+			pos->seg_off = 0;
+
+			set_bit(pos->cache_seg->cache_seg_id, cache->seg_map);
+			continue;
+		}
+
+		/* Replay the kset and check for errors. */
+		ret = kset_replay(cache, kset_onmedia);
+		if (ret)
+			goto out;
+
+		/* Advance the position after processing the kset. */
+		cache_pos_advance(pos, get_kset_onmedia_size(kset_onmedia));
+		if (++count > 512) {
+			cond_resched();
+			count = 0;
+		}
+	}
+
+	/* Update the key_head position after replaying. */
+	spin_lock(&cache->key_head_lock);
+	cache_pos_copy(&cache->key_head, pos);
+	spin_unlock(&cache->key_head_lock);
+out:
+	kfree(kset_onmedia);
+	return ret;
+}
+
+int cache_tree_init(struct pcache_cache *cache, struct pcache_cache_tree *cache_tree, u32 n_subtrees)
+{
+	int ret;
+	u32 i;
+
+	cache_tree->cache = cache;
+	cache_tree->n_subtrees = n_subtrees;
+
+	cache_tree->key_cache = KMEM_CACHE(pcache_cache_key, 0);
+	if (!cache_tree->key_cache) {
+		ret = -ENOMEM;
+		goto err;
+	}
+
+	ret = mempool_init_slab_pool(&cache_tree->key_pool, 1024, cache_tree->key_cache);
+	if (ret)
+		goto destroy_key_cache;
+
+	/*
+	 * Allocate and initialize the subtrees array.
+	 * Each element is a cache tree structure that contains
+	 * an RB tree root and a spinlock for protecting its contents.
+	 */
+	cache_tree->subtrees = kvcalloc(cache_tree->n_subtrees, sizeof(struct pcache_cache_subtree), GFP_KERNEL);
+	if (!cache_tree->subtrees) {
+		ret = -ENOMEM;
+		goto key_pool_exit;
+	}
+
+	for (i = 0; i < cache_tree->n_subtrees; i++) {
+		struct pcache_cache_subtree *cache_subtree = &cache_tree->subtrees[i];
+
+		cache_subtree->root = RB_ROOT;
+		spin_lock_init(&cache_subtree->tree_lock);
+	}
+
+	return 0;
+
+key_pool_exit:
+	mempool_exit(&cache_tree->key_pool);
+destroy_key_cache:
+	kmem_cache_destroy(cache_tree->key_cache);
+err:
+	return ret;
+}
+
+void cache_tree_exit(struct pcache_cache_tree *cache_tree)
+{
+	struct pcache_cache_subtree *cache_subtree;
+	struct rb_node *node;
+	struct pcache_cache_key *key;
+	u32 i;
+
+	for (i = 0; i < cache_tree->n_subtrees; i++) {
+		cache_subtree = &cache_tree->subtrees[i];
+
+		spin_lock(&cache_subtree->tree_lock);
+		node = rb_first(&cache_subtree->root);
+		while (node) {
+			key = CACHE_KEY(node);
+			node = rb_next(node);
+
+			cache_key_delete(key);
+		}
+		spin_unlock(&cache_subtree->tree_lock);
+	}
+	kvfree(cache_tree->subtrees);
+	mempool_exit(&cache_tree->key_pool);
+	kmem_cache_destroy(cache_tree->key_cache);
+}
-- 
2.43.0

[PATCH v2 09/11] dm-pcache: add cache_req

[Dongsheng Yang]

Introduce cache_req.c, the high-level engine that
drives I/O requests through dm-pcache. It decides whether data is served
from the cache or fetched from the backing device, allocates new cache
space on writes, and flushes dirty ksets when required.

* Read path
  - Traverses the striped RB-trees to locate cached extents.
  - Generates backing READ requests for gaps and inserts placeholder
    “empty” keys to avoid duplicate fetches.
  - Copies valid data directly from pmem into the caller’s bio; CRC and
    generation checks guard against stale segments.

* Write path
  - Allocates space in the current data segment via cache_data_alloc().
  - Copies data from the bio into pmem, then inserts or updates keys,
    splitting or trimming overlapped ranges as needed.
  - Adds each new key to the active kset; forces kset close when FUA is
    requested or the kset is full.

* Miss handling
  - create_cache_miss_req() builds a backing READ, optionally attaching
    an empty key.
  - miss_read_end_req() replaces the placeholder with real data once the
    READ completes, or deletes it on error.

* Flush support
  - cache_flush() iterates over all ksets and forces them to close,
    ensuring data durability when REQ_PREFLUSH is received.

Signed-off-by: Dongsheng Yang <dongsheng.yang@linux.dev>
---
 drivers/md/dm-pcache/cache_req.c | 840 +++++++++++++++++++++++++++++++
 1 file changed, 840 insertions(+)
 create mode 100644 drivers/md/dm-pcache/cache_req.c

diff --git a/drivers/md/dm-pcache/cache_req.c b/drivers/md/dm-pcache/cache_req.c
new file mode 100644
index 000000000000..3309a593cacc
--- /dev/null
+++ b/drivers/md/dm-pcache/cache_req.c
@@ -0,0 +1,840 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include "cache.h"
+#include "backing_dev.h"
+#include "cache_dev.h"
+#include "dm_pcache.h"
+
+static int cache_data_head_init(struct pcache_cache *cache)
+{
+	struct pcache_cache_segment *next_seg;
+	struct pcache_cache_data_head *data_head;
+
+	data_head = get_data_head(cache);
+	next_seg = get_cache_segment(cache);
+	if (!next_seg)
+		return -EBUSY;
+
+	cache_seg_get(next_seg);
+	data_head->head_pos.cache_seg = next_seg;
+	data_head->head_pos.seg_off = 0;
+
+	return 0;
+}
+
+/**
+ * cache_data_alloc - Allocate data for a cache key.
+ * @cache: Pointer to the cache structure.
+ * @key: Pointer to the cache key to allocate data for.
+ *
+ * This function tries to allocate space from the cache segment specified by the
+ * data head. If the remaining space in the segment is insufficient to allocate
+ * the requested length for the cache key, it will allocate whatever is available
+ * and adjust the key's length accordingly. This function does not allocate
+ * space that crosses segment boundaries.
+ */
+static int cache_data_alloc(struct pcache_cache *cache, struct pcache_cache_key *key)
+{
+	struct pcache_cache_data_head *data_head;
+	struct pcache_cache_pos *head_pos;
+	struct pcache_cache_segment *cache_seg;
+	u32 seg_remain;
+	u32 allocated = 0, to_alloc;
+	int ret = 0;
+
+	preempt_disable();
+	data_head = get_data_head(cache);
+again:
+	if (!data_head->head_pos.cache_seg) {
+		seg_remain = 0;
+	} else {
+		cache_pos_copy(&key->cache_pos, &data_head->head_pos);
+		key->seg_gen = key->cache_pos.cache_seg->gen;
+
+		head_pos = &data_head->head_pos;
+		cache_seg = head_pos->cache_seg;
+		seg_remain = cache_seg_remain(head_pos);
+		to_alloc = key->len - allocated;
+	}
+
+	if (seg_remain > to_alloc) {
+		/* If remaining space in segment is sufficient for the cache key, allocate it. */
+		cache_pos_advance(head_pos, to_alloc);
+		allocated += to_alloc;
+		cache_seg_get(cache_seg);
+	} else if (seg_remain) {
+		/* If remaining space is not enough, allocate the remaining space and adjust the cache key length. */
+		cache_pos_advance(head_pos, seg_remain);
+		key->len = seg_remain;
+
+		/* Get for key: obtain a reference to the cache segment for the key. */
+		cache_seg_get(cache_seg);
+		/* Put for head_pos->cache_seg: release the reference for the current head's segment. */
+		cache_seg_put(head_pos->cache_seg);
+		head_pos->cache_seg = NULL;
+	} else {
+		/* Initialize a new data head if no segment is available. */
+		ret = cache_data_head_init(cache);
+		if (ret)
+			goto out;
+
+		goto again;
+	}
+
+out:
+	preempt_enable();
+
+	return ret;
+}
+
+static int cache_copy_from_req_bio(struct pcache_cache *cache, struct pcache_cache_key *key,
+				struct pcache_request *pcache_req, u32 bio_off)
+{
+	struct pcache_cache_pos *pos = &key->cache_pos;
+	struct pcache_segment *segment;
+
+	segment = &pos->cache_seg->segment;
+
+	return segment_copy_from_bio(segment, pos->seg_off, key->len, pcache_req->bio, bio_off);
+}
+
+static int cache_copy_to_req_bio(struct pcache_cache *cache, struct pcache_request *pcache_req,
+			    u32 bio_off, u32 len, struct pcache_cache_pos *pos, u64 key_gen)
+{
+	struct pcache_cache_segment *cache_seg = pos->cache_seg;
+	struct pcache_segment *segment = &cache_seg->segment;
+	int ret;
+
+	spin_lock(&cache_seg->gen_lock);
+	if (key_gen < cache_seg->gen) {
+		spin_unlock(&cache_seg->gen_lock);
+		return -EINVAL;
+	}
+
+	ret = segment_copy_to_bio(segment, pos->seg_off, len, pcache_req->bio, bio_off);
+	spin_unlock(&cache_seg->gen_lock);
+
+	return ret;
+}
+
+/**
+ * miss_read_end_req - Handle the end of a miss read request.
+ * @backing_req: Pointer to the request structure.
+ * @read_ret: Return value of read.
+ *
+ * This function is called when a backing request to read data from
+ * the backing_dev is completed. If the key associated with the request
+ * is empty (a placeholder), it allocates cache space for the key,
+ * copies the data read from the bio into the cache, and updates
+ * the key's status. If the key has been overwritten by a write
+ * request during this process, it will be deleted from the cache
+ * tree and no further action will be taken.
+ */
+static void miss_read_end_req(struct pcache_backing_dev_req *backing_req, int read_ret)
+{
+	void *priv_data = backing_req->priv_data;
+	struct pcache_request *pcache_req = backing_req->req.upper_req;
+	struct pcache_cache *cache = backing_req->backing_dev->cache;
+	int ret;
+
+	if (priv_data) {
+		struct pcache_cache_key *key;
+		struct pcache_cache_subtree *cache_subtree;
+
+		key = (struct pcache_cache_key *)priv_data;
+		cache_subtree = key->cache_subtree;
+
+		/* if this key was deleted from cache_subtree by a write, key->flags should be cleared,
+		 * so if cache_key_empty() return true, this key is still in cache_subtree
+		 */
+		spin_lock(&cache_subtree->tree_lock);
+		if (cache_key_empty(key)) {
+			/* Check if the backing request was successful. */
+			if (read_ret) {
+				cache_key_delete(key);
+				goto unlock;
+			}
+
+			/* Allocate cache space for the key and copy data from the backing_dev. */
+			ret = cache_data_alloc(cache, key);
+			if (ret) {
+				cache_key_delete(key);
+				goto unlock;
+			}
+
+			ret = cache_copy_from_req_bio(cache, key, pcache_req, backing_req->req.bio_off);
+			if (ret) {
+				cache_seg_put(key->cache_pos.cache_seg);
+				cache_key_delete(key);
+				goto unlock;
+			}
+			key->flags &= ~PCACHE_CACHE_KEY_FLAGS_EMPTY;
+			key->flags |= PCACHE_CACHE_KEY_FLAGS_CLEAN;
+
+			/* Append the key to the cache. */
+			ret = cache_key_append(cache, key, false);
+			if (ret) {
+				cache_seg_put(key->cache_pos.cache_seg);
+				cache_key_delete(key);
+				goto unlock;
+			}
+		}
+unlock:
+		spin_unlock(&cache_subtree->tree_lock);
+		cache_key_put(key);
+	}
+}
+
+/**
+ * submit_cache_miss_req - Submit a backing request when cache data is missing
+ * @cache: The cache context that manages cache operations
+ * @backing_req: The cache request containing information about the read request
+ *
+ * This function is used to handle cases where a cache read request cannot locate
+ * the required data in the cache. When such a miss occurs during `cache_subtree_walk`,
+ * it triggers a backing read request to fetch data from the backing storage.
+ *
+ * If `pcache_req->priv_data` is set, it points to a `pcache_cache_key`, representing
+ * a new cache key to be inserted into the cache. The function calls `cache_key_insert`
+ * to attempt adding the key. On insertion failure, it releases the key reference and
+ * clears `priv_data` to avoid further processing.
+ */
+static void submit_cache_miss_req(struct pcache_cache *cache, struct pcache_backing_dev_req *backing_req)
+{
+	int ret;
+
+	if (backing_req->priv_data) {
+		struct pcache_cache_key *key;
+
+		/* Attempt to insert the key into the cache if priv_data is set */
+		key = (struct pcache_cache_key *)backing_req->priv_data;
+		ret = cache_key_insert(&cache->req_key_tree, key, true);
+		if (ret) {
+			/* Release the key if insertion fails */
+			cache_key_put(key);
+			backing_req->priv_data = NULL;
+		}
+	}
+	backing_dev_req_submit(backing_req, false);
+}
+
+static struct pcache_backing_dev_req *cache_miss_req_alloc(struct pcache_cache *cache, struct pcache_request *parent)
+{
+	struct pcache_backing_dev *backing_dev = cache->backing_dev;
+	struct pcache_backing_dev_req *backing_req;
+	struct pcache_cache_key *key = NULL;
+	struct pcache_backing_dev_req_opts req_opts = { 0 };
+
+	req_opts.type = BACKING_DEV_REQ_TYPE_REQ;
+	req_opts.req.upper_req = parent;
+
+	backing_req = backing_dev_req_alloc(backing_dev, &req_opts);
+	if (!backing_req)
+		goto out;
+
+	key = cache_key_alloc(&cache->req_key_tree);
+	if (!key) {
+		backing_req->ret = -ENOMEM;
+		goto end_req;
+	}
+
+	cache_key_get(key);
+	backing_req->priv_data = key;
+
+	return backing_req;
+end_req:
+	backing_dev_req_end(backing_req);
+out:
+	return NULL;
+}
+
+static void cache_miss_req_init(struct pcache_cache *cache,
+				struct pcache_backing_dev_req *backing_req,
+				struct pcache_request *parent,
+				u32 off, u32 len, bool insert_key)
+{
+	struct pcache_cache_key *key;
+	struct pcache_backing_dev_req_opts req_opts = { 0 };
+
+	req_opts.type = BACKING_DEV_REQ_TYPE_REQ;
+	req_opts.req.upper_req = parent;
+	req_opts.req.req_off = off;
+	req_opts.req.len = len;
+	req_opts.end_fn = miss_read_end_req;
+
+	backing_dev_req_init(backing_req, &req_opts);
+
+	if (insert_key) {
+		key = backing_req->priv_data;
+		key->off = parent->off + off;
+		key->len = len;
+		key->flags |= PCACHE_CACHE_KEY_FLAGS_EMPTY;
+	} else {
+		key = backing_req->priv_data;
+		backing_req->priv_data = NULL;
+		cache_key_put(key);
+	}
+}
+
+/*
+ * In the process of walking the cache tree to locate cached data, this
+ * function handles the situation where the requested data range lies
+ * entirely before an existing cache node (`key_tmp`). This outcome
+ * signifies that the target data is absent from the cache (cache miss).
+ *
+ * To fulfill this portion of the read request, the function creates a
+ * backing request (`backing_req`) for the missing data range represented
+ * by `key`. It then appends this request to the submission list in the
+ * `ctx`, which will later be processed to retrieve the data from backing
+ * storage. After setting up the backing request, `req_done` in `ctx` is
+ * updated to reflect the length of the handled range, and the range
+ * in `key` is adjusted by trimming off the portion that is now handled.
+ *
+ * The scenario handled here:
+ *
+ *	  |--------|			  key_tmp (existing cached range)
+ * |====|					   key (requested range, preceding key_tmp)
+ *
+ * Since `key` is before `key_tmp`, it signifies that the requested data
+ * range is missing in the cache (cache miss) and needs retrieval from
+ * backing storage.
+ */
+static int read_before(struct pcache_cache_key *key, struct pcache_cache_key *key_tmp,
+		struct pcache_cache_subtree_walk_ctx *ctx)
+{
+	struct pcache_backing_dev_req *backing_req;
+	struct pcache_cache *cache = ctx->cache_tree->cache;
+
+	/*
+	 * In this scenario, `key` represents a range that precedes `key_tmp`,
+	 * meaning the requested data range is missing from the cache tree
+	 * and must be retrieved from the backing_dev.
+	 */
+	if (!ctx->pre_alloc_req)
+		return SUBTREE_WALK_RET_NEED_REQ;
+
+	backing_req = ctx->pre_alloc_req;
+	ctx->pre_alloc_req = NULL;
+
+	cache_miss_req_init(cache, backing_req, ctx->pcache_req, ctx->req_done, key->len, true);
+
+	list_add(&backing_req->node, ctx->submit_req_list);
+	ctx->req_done += key->len;
+	cache_key_cutfront(key, key->len);
+
+	return SUBTREE_WALK_RET_OK;
+}
+
+/*
+ * During cache_subtree_walk, this function manages a scenario where part of the
+ * requested data range overlaps with an existing cache node (`key_tmp`).
+ *
+ *	 |----------------|  key_tmp (existing cached range)
+ * |===========|		   key (requested range, overlapping the tail of key_tmp)
+ */
+static int read_overlap_tail(struct pcache_cache_key *key, struct pcache_cache_key *key_tmp,
+		struct pcache_cache_subtree_walk_ctx *ctx)
+{
+	struct pcache_cache *cache = ctx->cache_tree->cache;
+	struct pcache_backing_dev_req *backing_req;
+	u32 io_len;
+	int ret;
+
+	/*
+	 * Calculate the length of the non-overlapping portion of `key`
+	 * before `key_tmp`, representing the data missing in the cache.
+	 */
+	io_len = cache_key_lstart(key_tmp) - cache_key_lstart(key);
+	if (io_len) {
+		if (!ctx->pre_alloc_req)
+			return SUBTREE_WALK_RET_NEED_REQ;
+
+		backing_req = ctx->pre_alloc_req;
+		ctx->pre_alloc_req = NULL;
+
+		cache_miss_req_init(cache, backing_req, ctx->pcache_req, ctx->req_done, io_len, true);
+
+		list_add(&backing_req->node, ctx->submit_req_list);
+		ctx->req_done += io_len;
+		cache_key_cutfront(key, io_len);
+	}
+
+	/*
+	 * Handle the overlapping portion by calculating the length of
+	 * the remaining data in `key` that coincides with `key_tmp`.
+	 */
+	io_len = cache_key_lend(key) - cache_key_lstart(key_tmp);
+	if (cache_key_empty(key_tmp)) {
+		if (!ctx->pre_alloc_req)
+			return SUBTREE_WALK_RET_NEED_REQ;
+
+		backing_req = ctx->pre_alloc_req;
+		ctx->pre_alloc_req = NULL;
+
+		cache_miss_req_init(cache, backing_req, ctx->pcache_req, ctx->req_done, io_len, false);
+		submit_cache_miss_req(cache, backing_req);
+	} else {
+		ret = cache_copy_to_req_bio(ctx->cache_tree->cache, ctx->pcache_req, ctx->req_done,
+					io_len, &key_tmp->cache_pos, key_tmp->seg_gen);
+		if (ret) {
+			if (ret == -EINVAL) {
+				cache_key_delete(key_tmp);
+				return SUBTREE_WALK_RET_RESEARCH;
+			}
+
+			ctx->ret = ret;
+			return SUBTREE_WALK_RET_ERR;
+		}
+	}
+
+	ctx->req_done += io_len;
+	cache_key_cutfront(key, io_len);
+
+	return SUBTREE_WALK_RET_OK;
+}
+
+/*
+ *    |----|          key_tmp (existing cached range)
+ * |==========|       key (requested range)
+ */
+static int read_overlap_contain(struct pcache_cache_key *key, struct pcache_cache_key *key_tmp,
+		struct pcache_cache_subtree_walk_ctx *ctx)
+{
+	struct pcache_cache *cache = ctx->cache_tree->cache;
+	struct pcache_backing_dev_req *backing_req;
+	u32 io_len;
+	int ret;
+
+	/*
+	 * Calculate the non-overlapping part of `key` before `key_tmp`
+	 * to identify the missing data length.
+	 */
+	io_len = cache_key_lstart(key_tmp) - cache_key_lstart(key);
+	if (io_len) {
+		if (!ctx->pre_alloc_req)
+			return SUBTREE_WALK_RET_NEED_REQ;
+
+		backing_req = ctx->pre_alloc_req;
+		ctx->pre_alloc_req = NULL;
+
+		cache_miss_req_init(cache, backing_req, ctx->pcache_req, ctx->req_done, io_len, true);
+
+		list_add(&backing_req->node, ctx->submit_req_list);
+
+		ctx->req_done += io_len;
+		cache_key_cutfront(key, io_len);
+	}
+
+	/*
+	 * Handle the overlapping portion between `key` and `key_tmp`.
+	 */
+	io_len = key_tmp->len;
+	if (cache_key_empty(key_tmp)) {
+		if (!ctx->pre_alloc_req)
+			return SUBTREE_WALK_RET_NEED_REQ;
+
+		backing_req = ctx->pre_alloc_req;
+		ctx->pre_alloc_req = NULL;
+
+		cache_miss_req_init(cache, backing_req, ctx->pcache_req, ctx->req_done, io_len, false);
+		submit_cache_miss_req(cache, backing_req);
+	} else {
+		ret = cache_copy_to_req_bio(ctx->cache_tree->cache, ctx->pcache_req, ctx->req_done,
+					io_len, &key_tmp->cache_pos, key_tmp->seg_gen);
+		if (ret) {
+			if (ret == -EINVAL) {
+				cache_key_delete(key_tmp);
+				return SUBTREE_WALK_RET_RESEARCH;
+			}
+
+			ctx->ret = ret;
+			return SUBTREE_WALK_RET_ERR;
+		}
+	}
+
+	ctx->req_done += io_len;
+	cache_key_cutfront(key, io_len);
+
+	return SUBTREE_WALK_RET_OK;
+}
+
+/*
+ *	 |-----------|		key_tmp (existing cached range)
+ *	   |====|			key (requested range, fully within key_tmp)
+ *
+ * If `key_tmp` contains valid cached data, this function copies the relevant
+ * portion to the request's bio. Otherwise, it sends a backing request to
+ * fetch the required data range.
+ */
+static int read_overlap_contained(struct pcache_cache_key *key, struct pcache_cache_key *key_tmp,
+		struct pcache_cache_subtree_walk_ctx *ctx)
+{
+	struct pcache_cache *cache = ctx->cache_tree->cache;
+	struct pcache_backing_dev_req *backing_req;
+	struct pcache_cache_pos pos;
+	int ret;
+
+	/*
+	 * Check if `key_tmp` is empty, indicating a miss. If so, initiate
+	 * a backing request to fetch the required data for `key`.
+	 */
+	if (cache_key_empty(key_tmp)) {
+		if (!ctx->pre_alloc_req)
+			return SUBTREE_WALK_RET_NEED_REQ;
+
+		backing_req = ctx->pre_alloc_req;
+		ctx->pre_alloc_req = NULL;
+
+		cache_miss_req_init(cache, backing_req, ctx->pcache_req, ctx->req_done, key->len, false);
+		submit_cache_miss_req(cache, backing_req);
+	} else {
+		cache_pos_copy(&pos, &key_tmp->cache_pos);
+		cache_pos_advance(&pos, cache_key_lstart(key) - cache_key_lstart(key_tmp));
+
+		ret = cache_copy_to_req_bio(ctx->cache_tree->cache, ctx->pcache_req, ctx->req_done,
+					key->len, &pos, key_tmp->seg_gen);
+		if (ret) {
+			if (ret == -EINVAL) {
+				cache_key_delete(key_tmp);
+				return SUBTREE_WALK_RET_RESEARCH;
+			}
+
+			ctx->ret = ret;
+			return SUBTREE_WALK_RET_ERR;
+		}
+	}
+
+	ctx->req_done += key->len;
+	cache_key_cutfront(key, key->len);
+
+	return SUBTREE_WALK_RET_OK;
+}
+
+/*
+ *	 |--------|		  key_tmp (existing cached range)
+ *	   |==========|	  key (requested range, overlapping the head of key_tmp)
+ */
+static int read_overlap_head(struct pcache_cache_key *key, struct pcache_cache_key *key_tmp,
+		struct pcache_cache_subtree_walk_ctx *ctx)
+{
+	struct pcache_cache *cache = ctx->cache_tree->cache;
+	struct pcache_backing_dev_req *backing_req;
+	struct pcache_cache_pos pos;
+	u32 io_len;
+	int ret;
+
+	io_len = cache_key_lend(key_tmp) - cache_key_lstart(key);
+
+	if (cache_key_empty(key_tmp)) {
+		if (!ctx->pre_alloc_req)
+			return SUBTREE_WALK_RET_NEED_REQ;
+
+		backing_req = ctx->pre_alloc_req;
+		ctx->pre_alloc_req = NULL;
+
+		cache_miss_req_init(cache, backing_req, ctx->pcache_req, ctx->req_done, io_len, false);
+		submit_cache_miss_req(cache, backing_req);
+	} else {
+		cache_pos_copy(&pos, &key_tmp->cache_pos);
+		cache_pos_advance(&pos, cache_key_lstart(key) - cache_key_lstart(key_tmp));
+
+		ret = cache_copy_to_req_bio(ctx->cache_tree->cache, ctx->pcache_req, ctx->req_done,
+					io_len, &pos, key_tmp->seg_gen);
+		if (ret) {
+			if (ret == -EINVAL) {
+				cache_key_delete(key_tmp);
+				return SUBTREE_WALK_RET_RESEARCH;
+			}
+
+			ctx->ret = ret;
+			return SUBTREE_WALK_RET_ERR;
+		}
+	}
+
+	ctx->req_done += io_len;
+	cache_key_cutfront(key, io_len);
+
+	return SUBTREE_WALK_RET_OK;
+}
+
+/**
+ * read_walk_finally - Finalizes the cache read tree walk by submitting any
+ *					 remaining backing requests
+ * @ctx:	Context structure holding information about the cache,
+ *		read request, and submission list
+ * @ret:	the return value after this walk.
+ *
+ * This function is called at the end of the `cache_subtree_walk` during a
+ * cache read operation. It completes the walk by checking if any data
+ * requested by `key` was not found in the cache tree, and if so, it sends
+ * a backing request to retrieve that data. Then, it iterates through the
+ * submission list of backing requests created during the walk, removing
+ * each request from the list and submitting it.
+ *
+ * The scenario managed here includes:
+ * - Sending a backing request for the remaining length of `key` if it was
+ *   not fulfilled by existing cache entries.
+ * - Iterating through `ctx->submit_req_list` to submit each backing request
+ *   enqueued during the walk.
+ *
+ * This ensures all necessary backing requests for cache misses are submitted
+ * to the backing storage to retrieve any data that could not be found in
+ * the cache.
+ */
+static int read_walk_finally(struct pcache_cache_subtree_walk_ctx *ctx, int ret)
+{
+	struct pcache_cache *cache = ctx->cache_tree->cache;
+	struct pcache_backing_dev_req *backing_req, *next_req;
+	struct pcache_cache_key *key = ctx->key;
+
+	list_for_each_entry_safe(backing_req, next_req, ctx->submit_req_list, node) {
+		list_del_init(&backing_req->node);
+		submit_cache_miss_req(ctx->cache_tree->cache, backing_req);
+	}
+
+	if (ret != SUBTREE_WALK_RET_OK)
+		return ret;
+
+	if (key->len) {
+		if (!ctx->pre_alloc_req)
+			return SUBTREE_WALK_RET_NEED_REQ;
+
+		backing_req = ctx->pre_alloc_req;
+		ctx->pre_alloc_req = NULL;
+
+		cache_miss_req_init(cache, backing_req, ctx->pcache_req, ctx->req_done, key->len, true);
+		submit_cache_miss_req(cache, backing_req);
+		ctx->req_done += key->len;
+	}
+
+	return SUBTREE_WALK_RET_OK;
+}
+
+/*
+ * This function is used within `cache_subtree_walk` to determine whether the
+ * read operation has covered the requested data length. It compares the
+ * amount of data processed (`ctx->req_done`) with the total data length
+ * specified in the original request (`ctx->pcache_req->data_len`).
+ *
+ * If `req_done` meets or exceeds the required data length, the function
+ * returns `true`, indicating the walk is complete. Otherwise, it returns `false`,
+ * signaling that additional data processing is needed to fulfill the request.
+ */
+static bool read_walk_done(struct pcache_cache_subtree_walk_ctx *ctx)
+{
+	return (ctx->req_done >= ctx->pcache_req->data_len);
+}
+
+/**
+ * cache_read - Process a read request by traversing the cache tree
+ * @cache:	 Cache structure holding cache trees and related configurations
+ * @pcache_req:   Request structure with information about the data to read
+ *
+ * This function attempts to fulfill a read request by traversing the cache tree(s)
+ * to locate cached data for the requested range. If parts of the data are missing
+ * in the cache, backing requests are generated to retrieve the required segments.
+ *
+ * The function operates by initializing a key for the requested data range and
+ * preparing a context (`walk_ctx`) to manage the cache tree traversal. The context
+ * includes pointers to functions (e.g., `read_before`, `read_overlap_tail`) that handle
+ * specific conditions encountered during the traversal. The `walk_finally` and `walk_done`
+ * functions manage the end stages of the traversal, while the `delete_key_list` and
+ * `submit_req_list` lists track any keys to be deleted or requests to be submitted.
+ *
+ * The function first calculates the requested range and checks if it fits within the
+ * current cache tree (based on the tree's size limits). It then locks the cache tree
+ * and performs a search to locate any matching keys. If there are outdated keys,
+ * these are deleted, and the search is restarted to ensure accurate data retrieval.
+ *
+ * If the requested range spans multiple cache trees, the function moves on to the
+ * next tree once the current range has been processed. This continues until the
+ * entire requested data length has been handled.
+ */
+static int cache_read(struct pcache_cache *cache, struct pcache_request *pcache_req)
+{
+	struct pcache_cache_key key_data = { .off = pcache_req->off, .len = pcache_req->data_len };
+	struct pcache_cache_subtree *cache_subtree;
+	struct pcache_cache_key *key_tmp = NULL, *key_next;
+	struct rb_node *prev_node = NULL;
+	struct pcache_cache_key *key = &key_data;
+	struct pcache_cache_subtree_walk_ctx walk_ctx = { 0 };
+	struct pcache_backing_dev_req *backing_req, *next_req;
+	LIST_HEAD(delete_key_list);
+	LIST_HEAD(submit_req_list);
+	int ret;
+
+	walk_ctx.cache_tree = &cache->req_key_tree;
+	walk_ctx.req_done = 0;
+	walk_ctx.pcache_req = pcache_req;
+	walk_ctx.before = read_before;
+	walk_ctx.overlap_tail = read_overlap_tail;
+	walk_ctx.overlap_head = read_overlap_head;
+	walk_ctx.overlap_contain = read_overlap_contain;
+	walk_ctx.overlap_contained = read_overlap_contained;
+	walk_ctx.walk_finally = read_walk_finally;
+	walk_ctx.walk_done = read_walk_done;
+	walk_ctx.delete_key_list = &delete_key_list;
+	walk_ctx.submit_req_list = &submit_req_list;
+
+next:
+	key->off = pcache_req->off + walk_ctx.req_done;
+	key->len = pcache_req->data_len - walk_ctx.req_done;
+	if (key->len > PCACHE_CACHE_SUBTREE_SIZE - (key->off & PCACHE_CACHE_SUBTREE_SIZE_MASK))
+		key->len = PCACHE_CACHE_SUBTREE_SIZE - (key->off & PCACHE_CACHE_SUBTREE_SIZE_MASK);
+
+	cache_subtree = get_subtree(&cache->req_key_tree, key->off);
+	spin_lock(&cache_subtree->tree_lock);
+search:
+	prev_node = cache_subtree_search(cache_subtree, key, NULL, NULL, &delete_key_list);
+	if (!list_empty(&delete_key_list)) {
+		list_for_each_entry_safe(key_tmp, key_next, &delete_key_list, list_node) {
+			list_del_init(&key_tmp->list_node);
+			cache_key_delete(key_tmp);
+		}
+		goto search;
+	}
+
+	walk_ctx.start_node = prev_node;
+	walk_ctx.key = key;
+
+	ret = cache_subtree_walk(&walk_ctx);
+	if (ret == SUBTREE_WALK_RET_RESEARCH)
+		goto search;
+	spin_unlock(&cache_subtree->tree_lock);
+
+	if (ret == SUBTREE_WALK_RET_ERR) {
+		ret = walk_ctx.ret;
+		goto out;
+	}
+
+	if (ret == SUBTREE_WALK_RET_NEED_REQ) {
+		walk_ctx.pre_alloc_req = cache_miss_req_alloc(cache, pcache_req);
+		if (!walk_ctx.pre_alloc_req) {
+			ret = -ENOMEM;
+			goto out;
+		}
+	}
+
+	if (walk_ctx.req_done < pcache_req->data_len)
+		goto next;
+	ret = 0;
+out:
+	if (walk_ctx.pre_alloc_req)
+		backing_dev_req_end(walk_ctx.pre_alloc_req);
+
+	list_for_each_entry_safe(backing_req, next_req, &submit_req_list, node) {
+		list_del_init(&backing_req->node);
+		backing_dev_req_end(backing_req);
+	}
+
+	return ret;
+}
+
+static int cache_write(struct pcache_cache *cache, struct pcache_request *pcache_req)
+{
+	struct pcache_cache_subtree *cache_subtree;
+	struct pcache_cache_key *key;
+	u64 offset = pcache_req->off;
+	u32 length = pcache_req->data_len;
+	u32 io_done = 0;
+	int ret;
+
+	while (true) {
+		if (io_done >= length)
+			break;
+
+		key = cache_key_alloc(&cache->req_key_tree);
+		if (!key) {
+			ret = -ENOMEM;
+			goto err;
+		}
+
+		key->off = offset + io_done;
+		key->len = length - io_done;
+		if (key->len > PCACHE_CACHE_SUBTREE_SIZE - (key->off & PCACHE_CACHE_SUBTREE_SIZE_MASK))
+			key->len = PCACHE_CACHE_SUBTREE_SIZE - (key->off & PCACHE_CACHE_SUBTREE_SIZE_MASK);
+
+		ret = cache_data_alloc(cache, key);
+		if (ret) {
+			cache_key_put(key);
+			goto err;
+		}
+
+		ret = cache_copy_from_req_bio(cache, key, pcache_req, io_done);
+		if (ret) {
+			cache_seg_put(key->cache_pos.cache_seg);
+			cache_key_put(key);
+			goto err;
+		}
+
+		cache_subtree = get_subtree(&cache->req_key_tree, key->off);
+		spin_lock(&cache_subtree->tree_lock);
+		ret = cache_key_insert(&cache->req_key_tree, key, true);
+		if (ret) {
+			cache_seg_put(key->cache_pos.cache_seg);
+			cache_key_put(key);
+			goto unlock;
+		}
+
+		ret = cache_key_append(cache, key, pcache_req->bio->bi_opf & REQ_FUA);
+		if (ret) {
+			cache_seg_put(key->cache_pos.cache_seg);
+			cache_key_delete(key);
+			goto unlock;
+		}
+
+		io_done += key->len;
+		spin_unlock(&cache_subtree->tree_lock);
+	}
+
+	return 0;
+unlock:
+	spin_unlock(&cache_subtree->tree_lock);
+err:
+	return ret;
+}
+
+/**
+ * cache_flush - Flush all ksets to persist any pending cache data
+ * @cache: Pointer to the cache structure
+ *
+ * This function iterates through all ksets associated with the provided `cache`
+ * and ensures that any data marked for persistence is written to media. For each
+ * kset, it acquires the kset lock, then invokes `cache_kset_close`, which handles
+ * the persistence logic for that kset.
+ *
+ * If `cache_kset_close` encounters an error, the function exits immediately with
+ * the respective error code, preventing the flush operation from proceeding to
+ * subsequent ksets.
+ */
+int cache_flush(struct pcache_cache *cache)
+{
+	struct pcache_cache_kset *kset;
+	u32 i, ret;
+
+	for (i = 0; i < cache->n_ksets; i++) {
+		kset = get_kset(cache, i);
+
+		spin_lock(&kset->kset_lock);
+		ret = cache_kset_close(cache, kset);
+		spin_unlock(&kset->kset_lock);
+
+		if (ret)
+			return ret;
+	}
+
+	return 0;
+}
+
+int pcache_cache_handle_req(struct pcache_cache *cache, struct pcache_request *pcache_req)
+{
+	struct bio *bio = pcache_req->bio;
+
+	if (unlikely(bio->bi_opf & REQ_PREFLUSH))
+		return cache_flush(cache);
+
+	if (bio_data_dir(bio) == READ)
+		return cache_read(cache, pcache_req);
+
+	return cache_write(cache, pcache_req);
+}
-- 
2.43.0

[PATCH v2 10/11] dm-pcache: add cache core

[Dongsheng Yang]

Add cache.c and cache.h that introduce the top-level
“struct pcache_cache”. This object glues together the backing block
device, the persistent-memory cache device, segment array, RB-tree
indexes, and the background workers for write-back and garbage
collection.

* Persistent metadata
  - pcache_cache_info tracks options such as cache mode, data-crc flag
    and GC threshold, written atomically with CRC+sequence.
  - key_tail and dirty_tail positions are double-buffered and recovered
    at mount time.

* Segment management
  - kvcalloc()’d array of pcache_cache_segment objects, bitmap for fast
    allocation, refcounts and generation numbers so GC can invalidate
    old extents safely.
  - First segment hosts a pcache_cache_ctrl block shared by all
    threads.

* Request path hooks
  - pcache_cache_handle_req() dispatches READ, WRITE and FLUSH bios to
    the engines added in earlier patches.
  - Per-CPU data_heads support lock-free allocation of space for new
    writes.

* Background workers
  - Delayed work items for write-back (5 s) and GC (5 s).
  - clean_work removes stale keys after segments are reclaimed.

* Lifecycle helpers
  - pcache_cache_start()/stop() bring the cache online, replay keys,
    start workers, and flush everything on shutdown.

With this piece in place dm-pcache has a fully initialised cache object
capable of serving I/O and maintaining its on-disk structures.

Signed-off-by: Dongsheng Yang <dongsheng.yang@linux.dev>
---
 drivers/md/dm-pcache/cache.c | 432 ++++++++++++++++++++++++
 drivers/md/dm-pcache/cache.h | 616 +++++++++++++++++++++++++++++++++++
 2 files changed, 1048 insertions(+)
 create mode 100644 drivers/md/dm-pcache/cache.c
 create mode 100644 drivers/md/dm-pcache/cache.h

diff --git a/drivers/md/dm-pcache/cache.c b/drivers/md/dm-pcache/cache.c
new file mode 100644
index 000000000000..d649f6b2c935
--- /dev/null
+++ b/drivers/md/dm-pcache/cache.c
@@ -0,0 +1,432 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+#include <linux/blk_types.h>
+
+#include "cache.h"
+#include "cache_dev.h"
+#include "backing_dev.h"
+#include "dm_pcache.h"
+
+static inline struct pcache_cache_info *get_cache_info_addr(struct pcache_cache *cache)
+{
+	return cache->cache_info_addr + cache->info_index;
+}
+
+static void cache_info_write(struct pcache_cache *cache)
+{
+	struct pcache_cache_info *cache_info = &cache->cache_info;
+
+	cache_info->header.seq++;
+	cache_info->header.crc = pcache_meta_crc(&cache_info->header,
+						sizeof(struct pcache_cache_info));
+
+	memcpy_flushcache(get_cache_info_addr(cache), cache_info,
+			sizeof(struct pcache_cache_info));
+
+	cache->info_index = (cache->info_index + 1) % PCACHE_META_INDEX_MAX;
+}
+
+static void cache_info_init_default(struct pcache_cache *cache);
+static int cache_info_init(struct pcache_cache *cache, struct pcache_cache_options *opts)
+{
+	struct dm_pcache *pcache = CACHE_TO_PCACHE(cache);
+	struct pcache_cache_info *cache_info_addr;
+
+	cache_info_addr = pcache_meta_find_latest(&cache->cache_info_addr->header,
+						sizeof(struct pcache_cache_info),
+						PCACHE_CACHE_INFO_SIZE,
+						&cache->cache_info);
+	if (IS_ERR(cache_info_addr))
+		return PTR_ERR(cache_info_addr);
+
+	if (cache_info_addr) {
+		if (opts->data_crc !=
+				(cache->cache_info.flags & PCACHE_CACHE_FLAGS_DATA_CRC)) {
+			pcache_dev_err(pcache, "invalid option for data_crc: %s, expected: %s",
+					opts->data_crc ? "true" : "false",
+					cache->cache_info.flags & PCACHE_CACHE_FLAGS_DATA_CRC ? "true" : "false");
+			return -EINVAL;
+		}
+
+		return 0;
+	}
+
+	/* init cache_info for new cache */
+	cache_info_init_default(cache);
+
+	cache->cache_info.flags &= ~PCACHE_CACHE_FLAGS_CACHE_MODE_MASK;
+	cache->cache_info.flags |= FIELD_PREP(PCACHE_CACHE_FLAGS_CACHE_MODE_MASK, opts->cache_mode);
+
+	if (opts->data_crc)
+		cache->cache_info.flags |= PCACHE_CACHE_FLAGS_DATA_CRC;
+
+	return 0;
+}
+
+static void cache_info_set_gc_percent(struct pcache_cache_info *cache_info, u8 percent)
+{
+	cache_info->flags &= ~PCACHE_CACHE_FLAGS_GC_PERCENT_MASK;
+	cache_info->flags |= FIELD_PREP(PCACHE_CACHE_FLAGS_GC_PERCENT_MASK, percent);
+}
+
+int pcache_cache_set_gc_percent(struct pcache_cache *cache, u8 percent)
+{
+	if (percent > PCACHE_CACHE_GC_PERCENT_MAX || percent < PCACHE_CACHE_GC_PERCENT_MIN)
+		return -EINVAL;
+
+	mutex_lock(&cache->cache_info_lock);
+	cache_info_set_gc_percent(&cache->cache_info, percent);
+
+	cache_info_write(cache);
+	mutex_unlock(&cache->cache_info_lock);
+
+	return 0;
+}
+
+void cache_pos_encode(struct pcache_cache *cache,
+			     struct pcache_cache_pos_onmedia *pos_onmedia_base,
+			     struct pcache_cache_pos *pos, u64 seq, u32 *index)
+{
+	struct pcache_cache_pos_onmedia pos_onmedia;
+	struct pcache_cache_pos_onmedia *pos_onmedia_addr = pos_onmedia_base + *index;
+
+	pos_onmedia.cache_seg_id = pos->cache_seg->cache_seg_id;
+	pos_onmedia.seg_off = pos->seg_off;
+	pos_onmedia.header.seq = seq;
+	pos_onmedia.header.crc = cache_pos_onmedia_crc(&pos_onmedia);
+
+	memcpy_flushcache(pos_onmedia_addr, &pos_onmedia, sizeof(struct pcache_cache_pos_onmedia));
+	pmem_wmb();
+
+	*index = (*index + 1) % PCACHE_META_INDEX_MAX;
+}
+
+int cache_pos_decode(struct pcache_cache *cache,
+			    struct pcache_cache_pos_onmedia *pos_onmedia,
+			    struct pcache_cache_pos *pos, u64 *seq, u32 *index)
+{
+	struct pcache_cache_pos_onmedia latest, *latest_addr;
+
+	latest_addr = pcache_meta_find_latest(&pos_onmedia->header,
+					sizeof(struct pcache_cache_pos_onmedia),
+					sizeof(struct pcache_cache_pos_onmedia),
+					&latest);
+	if (IS_ERR(latest_addr))
+		return PTR_ERR(latest_addr);
+
+	if (!latest_addr)
+		return -EIO;
+
+	pos->cache_seg = &cache->segments[latest.cache_seg_id];
+	pos->seg_off = latest.seg_off;
+	*seq = latest.header.seq;
+	*index = (latest_addr - pos_onmedia);
+
+	return 0;
+}
+
+static inline void cache_info_set_seg_id(struct pcache_cache *cache, u32 seg_id)
+{
+	cache->cache_info.seg_id = seg_id;
+}
+
+static int cache_init(struct dm_pcache *pcache)
+{
+	struct pcache_cache *cache = &pcache->cache;
+	struct pcache_backing_dev *backing_dev = &pcache->backing_dev;
+	struct pcache_cache_dev *cache_dev = &pcache->cache_dev;
+	int ret;
+
+	cache->segments = kvcalloc(cache_dev->seg_num, sizeof(struct pcache_cache_segment), GFP_KERNEL);
+	if (!cache->segments) {
+		ret = -ENOMEM;
+		goto err;
+	}
+
+	cache->seg_map = bitmap_zalloc(cache_dev->seg_num, GFP_KERNEL);
+	if (!cache->seg_map) {
+		ret = -ENOMEM;
+		goto free_segments;
+	}
+
+	cache->backing_dev = backing_dev;
+	cache->cache_dev = &pcache->cache_dev;
+	cache->n_segs = cache_dev->seg_num;
+	atomic_set(&cache->gc_errors, 0);
+	spin_lock_init(&cache->seg_map_lock);
+	spin_lock_init(&cache->key_head_lock);
+
+	mutex_init(&cache->cache_info_lock);
+	mutex_init(&cache->key_tail_lock);
+	mutex_init(&cache->dirty_tail_lock);
+	mutex_init(&cache->writeback_lock);
+
+	INIT_DELAYED_WORK(&cache->writeback_work, cache_writeback_fn);
+	INIT_DELAYED_WORK(&cache->gc_work, pcache_cache_gc_fn);
+	INIT_WORK(&cache->clean_work, clean_fn);
+
+	return 0;
+
+free_segments:
+	kvfree(cache->segments);
+err:
+	return ret;
+}
+
+static void cache_exit(struct pcache_cache *cache)
+{
+	bitmap_free(cache->seg_map);
+	kvfree(cache->segments);
+}
+
+static void cache_info_init_default(struct pcache_cache *cache)
+{
+	struct pcache_cache_info *cache_info = &cache->cache_info;
+
+	cache_info->header.seq = 0;
+	cache_info->n_segs = cache->cache_dev->seg_num;
+	cache_info_set_gc_percent(cache_info, PCACHE_CACHE_GC_PERCENT_DEFAULT);
+}
+
+static int cache_tail_init(struct pcache_cache *cache)
+{
+	struct dm_pcache *pcache = CACHE_TO_PCACHE(cache);
+	bool new_cache = !(cache->cache_info.flags & PCACHE_CACHE_FLAGS_INIT_DONE);
+
+	if (new_cache) {
+		set_bit(0, cache->seg_map);
+
+		cache->key_head.cache_seg = &cache->segments[0];
+		cache->key_head.seg_off = 0;
+		cache_pos_copy(&cache->key_tail, &cache->key_head);
+		cache_pos_copy(&cache->dirty_tail, &cache->key_head);
+
+		cache_encode_dirty_tail(cache);
+		cache_encode_key_tail(cache);
+	} else {
+		if (cache_decode_key_tail(cache) || cache_decode_dirty_tail(cache)) {
+			pcache_dev_err(pcache, "Corrupted key tail or dirty tail.\n");
+			return -EIO;
+		}
+	}
+
+	return 0;
+}
+
+static int get_seg_id(struct pcache_cache *cache,
+		      struct pcache_cache_segment *prev_cache_seg,
+		      bool new_cache, u32 *seg_id)
+{
+	struct dm_pcache *pcache = CACHE_TO_PCACHE(cache);
+	struct pcache_cache_dev *cache_dev = cache->cache_dev;
+	int ret;
+
+	if (new_cache) {
+		ret = cache_dev_get_empty_segment_id(cache_dev, seg_id);
+		if (ret) {
+			pcache_dev_err(pcache, "no available segment\n");
+			goto err;
+		}
+
+		if (prev_cache_seg)
+			cache_seg_set_next_seg(prev_cache_seg, *seg_id);
+		else
+			cache_info_set_seg_id(cache, *seg_id);
+	} else {
+		if (prev_cache_seg) {
+			struct pcache_segment_info *prev_seg_info;
+
+			prev_seg_info = &prev_cache_seg->cache_seg_info;
+			if (!segment_info_has_next(prev_seg_info)) {
+				ret = -EFAULT;
+				goto err;
+			}
+			*seg_id = prev_cache_seg->cache_seg_info.next_seg;
+		} else {
+			*seg_id = cache->cache_info.seg_id;
+		}
+	}
+	return 0;
+err:
+	return ret;
+}
+
+static int cache_segs_init(struct pcache_cache *cache)
+{
+	struct pcache_cache_segment *prev_cache_seg = NULL;
+	struct pcache_cache_info *cache_info = &cache->cache_info;
+	bool new_cache = !(cache->cache_info.flags & PCACHE_CACHE_FLAGS_INIT_DONE);
+	u32 seg_id;
+	int ret;
+	u32 i;
+
+	for (i = 0; i < cache_info->n_segs; i++) {
+		ret = get_seg_id(cache, prev_cache_seg, new_cache, &seg_id);
+		if (ret)
+			goto err;
+
+		ret = cache_seg_init(cache, seg_id, i, new_cache);
+		if (ret)
+			goto err;
+
+		prev_cache_seg = &cache->segments[i];
+	}
+	return 0;
+err:
+	return ret;
+}
+
+static int cache_init_req_keys(struct pcache_cache *cache, u32 n_paral)
+{
+	struct dm_pcache *pcache = CACHE_TO_PCACHE(cache);
+	u32 n_subtrees;
+	int ret;
+	u32 i, cpu;
+
+	/* Calculate number of cache trees based on the device size */
+	n_subtrees = DIV_ROUND_UP(cache->dev_size << SECTOR_SHIFT, PCACHE_CACHE_SUBTREE_SIZE);
+	ret = cache_tree_init(cache, &cache->req_key_tree, n_subtrees);
+	if (ret)
+		goto err;
+
+	cache->n_ksets = n_paral;
+	cache->ksets = kcalloc(cache->n_ksets, PCACHE_KSET_SIZE, GFP_KERNEL);
+	if (!cache->ksets) {
+		ret = -ENOMEM;
+		goto req_tree_exit;
+	}
+
+	/*
+	 * Initialize each kset with a spinlock and delayed work for flushing.
+	 * Each kset is associated with one queue to ensure independent handling
+	 * of cache keys across multiple queues, maximizing multiqueue concurrency.
+	 */
+	for (i = 0; i < cache->n_ksets; i++) {
+		struct pcache_cache_kset *kset = get_kset(cache, i);
+
+		kset->cache = cache;
+		spin_lock_init(&kset->kset_lock);
+		INIT_DELAYED_WORK(&kset->flush_work, kset_flush_fn);
+	}
+
+	cache->data_heads = alloc_percpu(struct pcache_cache_data_head);
+	if (!cache->data_heads) {
+		ret = -ENOMEM;
+		goto free_kset;
+	}
+
+	for_each_possible_cpu(cpu) {
+		struct pcache_cache_data_head *h =
+			per_cpu_ptr(cache->data_heads, cpu);
+		h->head_pos.cache_seg = NULL;
+	}
+
+	/*
+	 * Replay persisted cache keys using cache_replay.
+	 * This function loads and replays cache keys from previously stored
+	 * ksets, allowing the cache to restore its state after a restart.
+	 */
+	ret = cache_replay(cache);
+	if (ret) {
+		pcache_dev_err(pcache, "failed to replay keys\n");
+		goto free_heads;
+	}
+
+	return 0;
+
+free_heads:
+	free_percpu(cache->data_heads);
+free_kset:
+	kfree(cache->ksets);
+req_tree_exit:
+	cache_tree_exit(&cache->req_key_tree);
+err:
+	return ret;
+}
+
+static void cache_destroy_req_keys(struct pcache_cache *cache)
+{
+	u32 i;
+
+	for (i = 0; i < cache->n_ksets; i++) {
+		struct pcache_cache_kset *kset = get_kset(cache, i);
+
+		cancel_delayed_work_sync(&kset->flush_work);
+	}
+
+	free_percpu(cache->data_heads);
+	kfree(cache->ksets);
+	cache_tree_exit(&cache->req_key_tree);
+}
+
+int pcache_cache_start(struct dm_pcache *pcache)
+{
+	struct pcache_backing_dev *backing_dev = &pcache->backing_dev;
+	struct pcache_cache *cache = &pcache->cache;
+	struct pcache_cache_options *opts = &pcache->opts;
+	int ret;
+
+	ret = cache_init(pcache);
+	if (ret)
+		return ret;
+
+	cache->cache_info_addr = CACHE_DEV_CACHE_INFO(cache->cache_dev);
+	cache->cache_ctrl = CACHE_DEV_CACHE_CTRL(cache->cache_dev);
+	backing_dev->cache = cache;
+	cache->dev_size = backing_dev->dev_size;
+
+	ret = cache_info_init(cache, opts);
+	if (ret)
+		goto cache_exit;
+
+	ret = cache_segs_init(cache);
+	if (ret)
+		goto cache_exit;
+
+	ret = cache_tail_init(cache);
+	if (ret)
+		goto cache_exit;
+
+	ret = cache_init_req_keys(cache, num_online_cpus());
+	if (ret)
+		goto cache_exit;
+
+	ret = cache_writeback_init(cache);
+	if (ret)
+		goto destroy_keys;
+
+	cache->cache_info.flags |= PCACHE_CACHE_FLAGS_INIT_DONE;
+	cache_info_write(cache);
+	queue_delayed_work(cache_get_wq(cache), &cache->gc_work, 0);
+
+	return 0;
+
+destroy_keys:
+	cache_destroy_req_keys(cache);
+cache_exit:
+	cache_exit(cache);
+
+	return ret;
+}
+
+void pcache_cache_stop(struct dm_pcache *pcache)
+{
+	struct pcache_cache *cache = &pcache->cache;
+
+	cache_flush(cache);
+
+	cancel_delayed_work_sync(&cache->gc_work);
+	flush_work(&cache->clean_work);
+	cache_writeback_exit(cache);
+
+	if (cache->req_key_tree.n_subtrees)
+		cache_destroy_req_keys(cache);
+
+	cache_exit(cache);
+}
+
+struct workqueue_struct *cache_get_wq(struct pcache_cache *cache)
+{
+	struct dm_pcache *pcache = CACHE_TO_PCACHE(cache);
+
+	return pcache->task_wq;
+}
diff --git a/drivers/md/dm-pcache/cache.h b/drivers/md/dm-pcache/cache.h
new file mode 100644
index 000000000000..5f5e05724f4a
--- /dev/null
+++ b/drivers/md/dm-pcache/cache.h
@@ -0,0 +1,616 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+#ifndef _PCACHE_CACHE_H
+#define _PCACHE_CACHE_H
+
+#include "segment.h"
+
+/* Garbage collection thresholds */
+#define PCACHE_CACHE_GC_PERCENT_MIN       0                   /* Minimum GC percentage */
+#define PCACHE_CACHE_GC_PERCENT_MAX       90                  /* Maximum GC percentage */
+#define PCACHE_CACHE_GC_PERCENT_DEFAULT   70                  /* Default GC percentage */
+
+#define PCACHE_CACHE_SUBTREE_SIZE		(4 * 1024 * 1024)   /* 4MB total tree size */
+#define PCACHE_CACHE_SUBTREE_SIZE_MASK		0x3FFFFF            /* Mask for tree size */
+#define PCACHE_CACHE_SUBTREE_SIZE_SHIFT		22                  /* Bit shift for tree size */
+
+/* Maximum number of keys per key set */
+#define PCACHE_KSET_KEYS_MAX		128
+#define PCACHE_CACHE_SEGS_MAX		(1024 * 1024)	/* maximum cache size for each device is 16T */
+#define PCACHE_KSET_ONMEDIA_SIZE_MAX	struct_size_t(struct pcache_cache_kset_onmedia, data, PCACHE_KSET_KEYS_MAX)
+#define PCACHE_KSET_SIZE		(sizeof(struct pcache_cache_kset) + sizeof(struct pcache_cache_key_onmedia) * PCACHE_KSET_KEYS_MAX)
+
+/* Maximum number of keys to clean in one round of clean_work */
+#define PCACHE_CLEAN_KEYS_MAX             10
+
+/* Writeback and garbage collection intervals in jiffies */
+#define PCACHE_CACHE_WRITEBACK_INTERVAL   (5 * HZ)
+#define PCACHE_CACHE_GC_INTERVAL          (5 * HZ)
+
+/* Macro to get the cache key structure from an rb_node pointer */
+#define CACHE_KEY(node)                (container_of(node, struct pcache_cache_key, rb_node))
+
+struct pcache_cache_pos_onmedia {
+	struct pcache_meta_header header;
+	__u32 cache_seg_id;
+	__u32 seg_off;
+};
+
+/* Offset and size definitions for cache segment control */
+#define PCACHE_CACHE_SEG_CTRL_OFF     (PCACHE_SEG_INFO_SIZE * PCACHE_META_INDEX_MAX)
+#define PCACHE_CACHE_SEG_CTRL_SIZE    (4 * PCACHE_KB)
+
+struct pcache_cache_seg_gen {
+	struct pcache_meta_header header;
+	__u64 gen;
+};
+
+/* Control structure for cache segments */
+struct pcache_cache_seg_ctrl {
+	struct pcache_cache_seg_gen gen[PCACHE_META_INDEX_MAX];
+	__u64	res[64];
+};
+
+#define PCACHE_CACHE_FLAGS_DATA_CRC			BIT(0)
+#define PCACHE_CACHE_FLAGS_INIT_DONE			BIT(1)
+
+#define PCACHE_CACHE_FLAGS_CACHE_MODE_MASK		GENMASK(5, 2)
+#define PCACHE_CACHE_MODE_WRITEBACK			0
+#define PCACHE_CACHE_MODE_WRITETHROUGH			1
+#define PCACHE_CACHE_MODE_WRITEAROUND			2
+#define PCACHE_CACHE_MODE_WRITEONLY			3
+
+#define PCACHE_CACHE_FLAGS_GC_PERCENT_MASK		GENMASK(12, 6)
+
+struct pcache_cache_info {
+	struct pcache_meta_header header;
+	__u32 seg_id;
+	__u32 n_segs;
+	__u32 flags;
+	__u32 reserved;
+};
+
+struct pcache_cache_pos {
+	struct pcache_cache_segment *cache_seg;
+	u32 seg_off;
+};
+
+struct pcache_cache_segment {
+	struct pcache_cache	*cache;
+	u32			cache_seg_id;   /* Index in cache->segments */
+	struct pcache_segment	segment;
+	atomic_t		refs;
+
+	struct pcache_segment_info cache_seg_info;
+	struct mutex		info_lock;
+	u32			info_index;
+
+	spinlock_t		gen_lock;
+	u64			gen;
+	u64			gen_seq;
+	u32			gen_index;
+
+	struct pcache_cache_seg_ctrl *cache_seg_ctrl;
+	struct mutex		ctrl_lock;
+};
+
+/* rbtree for cache entries */
+struct pcache_cache_subtree {
+	struct rb_root root;
+	spinlock_t tree_lock;
+};
+
+struct pcache_cache_tree {
+	struct pcache_cache		*cache;
+	u32				n_subtrees;
+	struct kmem_cache		*key_cache;
+	mempool_t			key_pool;
+	struct pcache_cache_subtree	*subtrees;
+};
+
+struct pcache_cache_key {
+	struct pcache_cache_tree	*cache_tree;
+	struct pcache_cache_subtree	*cache_subtree;
+	struct kref			ref;
+	struct rb_node			rb_node;
+	struct list_head		list_node;
+	u64				off;
+	u32				len;
+	u32				flags;
+	struct pcache_cache_pos		cache_pos;
+	u64				seg_gen;
+};
+
+#define PCACHE_CACHE_KEY_FLAGS_EMPTY		BIT(0)
+#define PCACHE_CACHE_KEY_FLAGS_CLEAN		BIT(1)
+
+struct pcache_cache_key_onmedia {
+	__u64 off;
+	__u32 len;
+	__u32 flags;
+	__u32 cache_seg_id;
+	__u32 cache_seg_off;
+	__u64 seg_gen;
+	__u32 data_crc;
+	__u32 reserved;
+};
+
+struct pcache_cache_kset_onmedia {
+	__u32 crc;
+	union {
+		__u32 key_num;
+		__u32 next_cache_seg_id;
+	};
+	__u64 magic;
+	__u64 flags;
+	struct pcache_cache_key_onmedia data[];
+};
+
+struct pcache_cache {
+	struct pcache_backing_dev	*backing_dev;
+	struct pcache_cache_dev		*cache_dev;
+	struct pcache_cache_ctrl	*cache_ctrl;
+	u64				dev_size;
+
+	struct pcache_cache_data_head __percpu *data_heads;
+
+	spinlock_t		key_head_lock;
+	struct pcache_cache_pos	key_head;
+	u32			n_ksets;
+	struct pcache_cache_kset	*ksets;
+
+	struct mutex		key_tail_lock;
+	struct pcache_cache_pos	key_tail;
+	u64			key_tail_seq;
+	u32			key_tail_index;
+
+	struct mutex		dirty_tail_lock;
+	struct pcache_cache_pos	dirty_tail;
+	u64			dirty_tail_seq;
+	u32			dirty_tail_index;
+
+	struct pcache_cache_tree	req_key_tree;
+	struct work_struct	clean_work;
+
+	struct mutex		writeback_lock;
+	char wb_kset_onmedia_buf[PCACHE_KSET_ONMEDIA_SIZE_MAX];
+	struct pcache_cache_tree	writeback_key_tree;
+	struct delayed_work	writeback_work;
+	struct {
+		atomic_t pending;
+		u32 advance;
+		int ret;
+	} writeback_ctx;
+
+	char gc_kset_onmedia_buf[PCACHE_KSET_ONMEDIA_SIZE_MAX];
+	struct delayed_work	gc_work;
+	atomic_t		gc_errors;
+
+	struct mutex			cache_info_lock;
+	struct pcache_cache_info	cache_info;
+	struct pcache_cache_info	*cache_info_addr;
+	u32				info_index;
+
+	u32			n_segs;
+	unsigned long		*seg_map;
+	u32			last_cache_seg;
+	bool			cache_full;
+	spinlock_t		seg_map_lock;
+	struct pcache_cache_segment *segments;
+};
+
+struct workqueue_struct *cache_get_wq(struct pcache_cache *cache);
+
+struct dm_pcache;
+struct pcache_cache_options {
+	u32	cache_mode:4;
+	u32	data_crc:1;
+};
+int pcache_cache_start(struct dm_pcache *pcache);
+void pcache_cache_stop(struct dm_pcache *pcache);
+
+struct pcache_cache_ctrl {
+	/* Updated by gc_thread */
+	struct pcache_cache_pos_onmedia key_tail_pos[PCACHE_META_INDEX_MAX];
+
+	/* Updated by writeback_thread */
+	struct pcache_cache_pos_onmedia dirty_tail_pos[PCACHE_META_INDEX_MAX];
+};
+
+struct pcache_cache_data_head {
+	struct pcache_cache_pos head_pos;
+};
+
+static inline u16 pcache_cache_get_gc_percent(struct pcache_cache *cache)
+{
+	return FIELD_GET(PCACHE_CACHE_FLAGS_GC_PERCENT_MASK, cache->cache_info.flags);
+}
+
+int pcache_cache_set_gc_percent(struct pcache_cache *cache, u8 percent);
+
+/* cache key */
+struct pcache_cache_key *cache_key_alloc(struct pcache_cache_tree *cache_tree);
+void cache_key_init(struct pcache_cache_tree *cache_tree, struct pcache_cache_key *key);
+void cache_key_get(struct pcache_cache_key *key);
+void cache_key_put(struct pcache_cache_key *key);
+int cache_key_append(struct pcache_cache *cache, struct pcache_cache_key *key, bool force_close);
+int cache_key_insert(struct pcache_cache_tree *cache_tree, struct pcache_cache_key *key, bool fixup);
+int cache_key_decode(struct pcache_cache *cache,
+			struct pcache_cache_key_onmedia *key_onmedia,
+			struct pcache_cache_key *key);
+void cache_pos_advance(struct pcache_cache_pos *pos, u32 len);
+
+#define PCACHE_KSET_FLAGS_LAST		BIT(0)
+#define PCACHE_KSET_MAGIC		0x676894a64e164f1aULL
+
+struct pcache_cache_kset {
+	struct pcache_cache *cache;
+	spinlock_t        kset_lock;
+	struct delayed_work flush_work;
+	struct pcache_cache_kset_onmedia kset_onmedia;
+};
+
+extern struct pcache_cache_kset_onmedia pcache_empty_kset;
+
+#define SUBTREE_WALK_RET_OK		0
+#define SUBTREE_WALK_RET_ERR		1
+#define SUBTREE_WALK_RET_NEED_KEY	2
+#define SUBTREE_WALK_RET_NEED_REQ	3
+#define SUBTREE_WALK_RET_RESEARCH	4
+
+struct pcache_cache_subtree_walk_ctx {
+	struct pcache_cache_tree *cache_tree;
+	struct rb_node *start_node;
+	struct pcache_request *pcache_req;
+	struct pcache_cache_key *key;
+	u32	req_done;
+	int	ret;
+
+	/* pre-allocated key and backing_dev_req */
+	struct pcache_cache_key		*pre_alloc_key;
+	struct pcache_backing_dev_req	*pre_alloc_req;
+
+	struct list_head *delete_key_list;
+	struct list_head *submit_req_list;
+
+	/*
+	 *	  |--------|		key_tmp
+	 * |====|			key
+	 */
+	int (*before)(struct pcache_cache_key *key, struct pcache_cache_key *key_tmp,
+			struct pcache_cache_subtree_walk_ctx *ctx);
+
+	/*
+	 * |----------|			key_tmp
+	 *		|=====|		key
+	 */
+	int (*after)(struct pcache_cache_key *key, struct pcache_cache_key *key_tmp,
+			struct pcache_cache_subtree_walk_ctx *ctx);
+
+	/*
+	 *     |----------------|	key_tmp
+	 * |===========|		key
+	 */
+	int (*overlap_tail)(struct pcache_cache_key *key, struct pcache_cache_key *key_tmp,
+			struct pcache_cache_subtree_walk_ctx *ctx);
+
+	/*
+	 * |--------|			key_tmp
+	 *   |==========|		key
+	 */
+	int (*overlap_head)(struct pcache_cache_key *key, struct pcache_cache_key *key_tmp,
+			struct pcache_cache_subtree_walk_ctx *ctx);
+
+	/*
+	 *    |----|			key_tmp
+	 * |==========|			key
+	 */
+	int (*overlap_contain)(struct pcache_cache_key *key, struct pcache_cache_key *key_tmp,
+			struct pcache_cache_subtree_walk_ctx *ctx);
+
+	/*
+	 * |-----------|		key_tmp
+	 *   |====|			key
+	 */
+	int (*overlap_contained)(struct pcache_cache_key *key, struct pcache_cache_key *key_tmp,
+			struct pcache_cache_subtree_walk_ctx *ctx);
+
+	int (*walk_finally)(struct pcache_cache_subtree_walk_ctx *ctx, int ret);
+	bool (*walk_done)(struct pcache_cache_subtree_walk_ctx *ctx);
+};
+
+int cache_subtree_walk(struct pcache_cache_subtree_walk_ctx *ctx);
+struct rb_node *cache_subtree_search(struct pcache_cache_subtree *cache_subtree, struct pcache_cache_key *key,
+				  struct rb_node **parentp, struct rb_node ***newp,
+				  struct list_head *delete_key_list);
+int cache_kset_close(struct pcache_cache *cache, struct pcache_cache_kset *kset);
+void clean_fn(struct work_struct *work);
+void kset_flush_fn(struct work_struct *work);
+int cache_replay(struct pcache_cache *cache);
+int cache_tree_init(struct pcache_cache *cache, struct pcache_cache_tree *cache_tree, u32 n_subtrees);
+void cache_tree_exit(struct pcache_cache_tree *cache_tree);
+
+/* cache segments */
+struct pcache_cache_segment *get_cache_segment(struct pcache_cache *cache);
+int cache_seg_init(struct pcache_cache *cache, u32 seg_id, u32 cache_seg_id,
+		   bool new_cache);
+void cache_seg_get(struct pcache_cache_segment *cache_seg);
+void cache_seg_put(struct pcache_cache_segment *cache_seg);
+void cache_seg_set_next_seg(struct pcache_cache_segment *cache_seg, u32 seg_id);
+
+/* cache request*/
+int cache_flush(struct pcache_cache *cache);
+void miss_read_end_work_fn(struct work_struct *work);
+int pcache_cache_handle_req(struct pcache_cache *cache, struct pcache_request *pcache_req);
+
+/* gc */
+void pcache_cache_gc_fn(struct work_struct *work);
+
+/* writeback */
+void cache_writeback_exit(struct pcache_cache *cache);
+int cache_writeback_init(struct pcache_cache *cache);
+void cache_writeback_fn(struct work_struct *work);
+
+/* inline functions */
+static inline struct pcache_cache_subtree *get_subtree(struct pcache_cache_tree *cache_tree, u64 off)
+{
+	if (cache_tree->n_subtrees == 1)
+		return &cache_tree->subtrees[0];
+
+	return &cache_tree->subtrees[off >> PCACHE_CACHE_SUBTREE_SIZE_SHIFT];
+}
+
+static inline void *cache_pos_addr(struct pcache_cache_pos *pos)
+{
+	return (pos->cache_seg->segment.data + pos->seg_off);
+}
+
+static inline void *get_key_head_addr(struct pcache_cache *cache)
+{
+	return cache_pos_addr(&cache->key_head);
+}
+
+static inline u32 get_kset_id(struct pcache_cache *cache, u64 off)
+{
+	return (off >> PCACHE_CACHE_SUBTREE_SIZE_SHIFT) % cache->n_ksets;
+}
+
+static inline struct pcache_cache_kset *get_kset(struct pcache_cache *cache, u32 kset_id)
+{
+	return (void *)cache->ksets + PCACHE_KSET_SIZE * kset_id;
+}
+
+static inline struct pcache_cache_data_head *get_data_head(struct pcache_cache *cache)
+{
+	return this_cpu_ptr(cache->data_heads);
+}
+
+static inline bool cache_key_empty(struct pcache_cache_key *key)
+{
+	return key->flags & PCACHE_CACHE_KEY_FLAGS_EMPTY;
+}
+
+static inline bool cache_key_clean(struct pcache_cache_key *key)
+{
+	return key->flags & PCACHE_CACHE_KEY_FLAGS_CLEAN;
+}
+
+static inline void cache_pos_copy(struct pcache_cache_pos *dst, struct pcache_cache_pos *src)
+{
+	memcpy(dst, src, sizeof(struct pcache_cache_pos));
+}
+
+/**
+ * cache_seg_is_ctrl_seg - Checks if a cache segment is a cache ctrl segment.
+ * @cache_seg_id: ID of the cache segment.
+ *
+ * Returns true if the cache segment ID corresponds to a cache ctrl segment.
+ *
+ * Note: We extend the segment control of the first cache segment
+ * (cache segment ID 0) to serve as the cache control (pcache_cache_ctrl)
+ * for the entire PCACHE cache. This function determines whether the given
+ * cache segment is the one storing the pcache_cache_ctrl information.
+ */
+static inline bool cache_seg_is_ctrl_seg(u32 cache_seg_id)
+{
+	return (cache_seg_id == 0);
+}
+
+/**
+ * cache_key_cutfront - Cuts a specified length from the front of a cache key.
+ * @key: Pointer to pcache_cache_key structure.
+ * @cut_len: Length to cut from the front.
+ *
+ * Advances the cache key position by cut_len and adjusts offset and length accordingly.
+ */
+static inline void cache_key_cutfront(struct pcache_cache_key *key, u32 cut_len)
+{
+	if (key->cache_pos.cache_seg)
+		cache_pos_advance(&key->cache_pos, cut_len);
+
+	key->off += cut_len;
+	key->len -= cut_len;
+}
+
+/**
+ * cache_key_cutback - Cuts a specified length from the back of a cache key.
+ * @key: Pointer to pcache_cache_key structure.
+ * @cut_len: Length to cut from the back.
+ *
+ * Reduces the length of the cache key by cut_len.
+ */
+static inline void cache_key_cutback(struct pcache_cache_key *key, u32 cut_len)
+{
+	key->len -= cut_len;
+}
+
+static inline void cache_key_delete(struct pcache_cache_key *key)
+{
+	struct pcache_cache_subtree *cache_subtree;
+
+	cache_subtree = key->cache_subtree;
+	BUG_ON(!cache_subtree);
+
+	rb_erase(&key->rb_node, &cache_subtree->root);
+	key->flags = 0;
+	cache_key_put(key);
+}
+
+static inline bool cache_data_crc_on(struct pcache_cache *cache)
+{
+	return (cache->cache_info.flags & PCACHE_CACHE_FLAGS_DATA_CRC);
+}
+
+/**
+ * cache_key_data_crc - Calculates CRC for data in a cache key.
+ * @key: Pointer to the pcache_cache_key structure.
+ *
+ * Returns the CRC-32 checksum of the data within the cache key's position.
+ */
+static inline u32 cache_key_data_crc(struct pcache_cache_key *key)
+{
+	void *data;
+
+	data = cache_pos_addr(&key->cache_pos);
+
+	return crc32c(PCACHE_CRC_SEED, data, key->len);
+}
+
+static inline u32 cache_kset_crc(struct pcache_cache_kset_onmedia *kset_onmedia)
+{
+	u32 crc_size;
+
+	if (kset_onmedia->flags & PCACHE_KSET_FLAGS_LAST)
+		crc_size = sizeof(struct pcache_cache_kset_onmedia) - 4;
+	else
+		crc_size = struct_size(kset_onmedia, data, kset_onmedia->key_num) - 4;
+
+	return crc32c(PCACHE_CRC_SEED, (void *)kset_onmedia + 4, crc_size);
+}
+
+static inline u32 get_kset_onmedia_size(struct pcache_cache_kset_onmedia *kset_onmedia)
+{
+	return struct_size_t(struct pcache_cache_kset_onmedia, data, kset_onmedia->key_num);
+}
+
+/**
+ * cache_seg_remain - Computes remaining space in a cache segment.
+ * @pos: Pointer to pcache_cache_pos structure.
+ *
+ * Returns the amount of remaining space in the segment data starting from
+ * the current position offset.
+ */
+static inline u32 cache_seg_remain(struct pcache_cache_pos *pos)
+{
+	struct pcache_cache_segment *cache_seg;
+	struct pcache_segment *segment;
+	u32 seg_remain;
+
+	cache_seg = pos->cache_seg;
+	segment = &cache_seg->segment;
+	seg_remain = segment->data_size - pos->seg_off;
+
+	return seg_remain;
+}
+
+/**
+ * cache_key_invalid - Checks if a cache key is invalid.
+ * @key: Pointer to pcache_cache_key structure.
+ *
+ * Returns true if the cache key is invalid due to its generation being
+ * less than the generation of its segment; otherwise returns false.
+ *
+ * When the GC (garbage collection) thread identifies a segment
+ * as reclaimable, it increments the segment's generation (gen). However,
+ * it does not immediately remove all related cache keys. When accessing
+ * such a cache key, this function can be used to determine if the cache
+ * key has already become invalid.
+ */
+static inline bool cache_key_invalid(struct pcache_cache_key *key)
+{
+	if (cache_key_empty(key))
+		return false;
+
+	return (key->seg_gen < key->cache_pos.cache_seg->gen);
+}
+
+/**
+ * cache_key_lstart - Retrieves the logical start offset of a cache key.
+ * @key: Pointer to pcache_cache_key structure.
+ *
+ * Returns the logical start offset for the cache key.
+ */
+static inline u64 cache_key_lstart(struct pcache_cache_key *key)
+{
+	return key->off;
+}
+
+/**
+ * cache_key_lend - Retrieves the logical end offset of a cache key.
+ * @key: Pointer to pcache_cache_key structure.
+ *
+ * Returns the logical end offset for the cache key.
+ */
+static inline u64 cache_key_lend(struct pcache_cache_key *key)
+{
+	return key->off + key->len;
+}
+
+static inline void cache_key_copy(struct pcache_cache_key *key_dst, struct pcache_cache_key *key_src)
+{
+	key_dst->off = key_src->off;
+	key_dst->len = key_src->len;
+	key_dst->seg_gen = key_src->seg_gen;
+	key_dst->cache_tree = key_src->cache_tree;
+	key_dst->cache_subtree = key_src->cache_subtree;
+	key_dst->flags = key_src->flags;
+
+	cache_pos_copy(&key_dst->cache_pos, &key_src->cache_pos);
+}
+
+/**
+ * cache_pos_onmedia_crc - Calculates the CRC for an on-media cache position.
+ * @pos_om: Pointer to pcache_cache_pos_onmedia structure.
+ *
+ * Calculates the CRC-32 checksum of the position, excluding the first 4 bytes.
+ * Returns the computed CRC value.
+ */
+static inline u32 cache_pos_onmedia_crc(struct pcache_cache_pos_onmedia *pos_om)
+{
+	return pcache_meta_crc(&pos_om->header, sizeof(struct pcache_cache_pos_onmedia));
+}
+
+void cache_pos_encode(struct pcache_cache *cache,
+			     struct pcache_cache_pos_onmedia *pos_onmedia,
+			     struct pcache_cache_pos *pos, u64 seq, u32 *index);
+int cache_pos_decode(struct pcache_cache *cache,
+			    struct pcache_cache_pos_onmedia *pos_onmedia,
+			    struct pcache_cache_pos *pos, u64 *seq, u32 *index);
+
+static inline void cache_encode_key_tail(struct pcache_cache *cache)
+{
+	cache_pos_encode(cache, cache->cache_ctrl->key_tail_pos,
+			&cache->key_tail, ++cache->key_tail_seq,
+			&cache->key_tail_index);
+}
+
+static inline int cache_decode_key_tail(struct pcache_cache *cache)
+{
+	return cache_pos_decode(cache, cache->cache_ctrl->key_tail_pos,
+				&cache->key_tail, &cache->key_tail_seq,
+				&cache->key_tail_index);
+}
+
+static inline void cache_encode_dirty_tail(struct pcache_cache *cache)
+{
+	cache_pos_encode(cache, cache->cache_ctrl->dirty_tail_pos,
+			&cache->dirty_tail, ++cache->dirty_tail_seq,
+			&cache->dirty_tail_index);
+}
+
+static inline int cache_decode_dirty_tail(struct pcache_cache *cache)
+{
+	return cache_pos_decode(cache, cache->cache_ctrl->dirty_tail_pos,
+				&cache->dirty_tail, &cache->dirty_tail_seq,
+				&cache->dirty_tail_index);
+}
+#endif /* _PCACHE_CACHE_H */
-- 
2.43.0

[PATCH v2 11/11] dm-pcache: initial dm-pcache target

[Dongsheng Yang]

Add the top-level integration pieces that make the new persistent-memory
cache target usable from device-mapper:

* Documentation
  - `Documentation/admin-guide/device-mapper/dm-pcache.rst` explains the
    design, table syntax, status fields and runtime messages.

* Core target implementation
  - `dm_pcache.c` and `dm_pcache.h` register the `"pcache"` DM target,
    parse constructor arguments, create workqueues, and forward BIOS to
    the cache core added in earlier patches.
  - Supports flush/FUA, status reporting, and a “gc_percent” message.
  - Dont support discard currently.
  - Dont support table reload for live target currently.

* Device-mapper tables now accept lines like
    pcache <pmem_dev> <backing_dev> writeback <true|false>

Signed-off-by: Dongsheng Yang <dongsheng.yang@linux.dev>
---
 .../admin-guide/device-mapper/dm-pcache.rst   | 201 ++++++++
 MAINTAINERS                                   |   8 +
 drivers/md/Kconfig                            |   2 +
 drivers/md/Makefile                           |   1 +
 drivers/md/dm-pcache/Kconfig                  |  17 +
 drivers/md/dm-pcache/Makefile                 |   3 +
 drivers/md/dm-pcache/dm_pcache.c              | 466 ++++++++++++++++++
 drivers/md/dm-pcache/dm_pcache.h              |  65 +++
 8 files changed, 763 insertions(+)
 create mode 100644 Documentation/admin-guide/device-mapper/dm-pcache.rst
 create mode 100644 drivers/md/dm-pcache/Kconfig
 create mode 100644 drivers/md/dm-pcache/Makefile
 create mode 100644 drivers/md/dm-pcache/dm_pcache.c
 create mode 100644 drivers/md/dm-pcache/dm_pcache.h

diff --git a/Documentation/admin-guide/device-mapper/dm-pcache.rst b/Documentation/admin-guide/device-mapper/dm-pcache.rst
new file mode 100644
index 000000000000..e6433fab7bd6
--- /dev/null
+++ b/Documentation/admin-guide/device-mapper/dm-pcache.rst
@@ -0,0 +1,201 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=================================
+dm-pcache — Persistent Cache
+=================================
+
+*Author: Dongsheng Yang <dongsheng.yang@linux.dev>*
+
+This document describes *dm-pcache*, a Device-Mapper target that lets a
+byte-addressable *DAX* (persistent-memory, “pmem”) region act as a
+high-performance, crash-persistent cache in front of a slower block
+device.  The code lives in `drivers/md/dm-pcache/`.
+
+Quick feature summary
+=====================
+
+* *Write-back* caching (only mode currently supported).
+* *16 MiB segments* allocated on the pmem device.
+* *Data CRC32* verification (optional, per cache).
+* Crash-safe: every metadata structure is duplicated (`PCACHE_META_INDEX_MAX
+  == 2`) and protected with CRC+sequence numbers.
+* *Multi-tree indexing* (one radix tree per CPU backend) for high PMem
+  parallelism
+* Pure *DAX path* I/O – no extra BIO round-trips
+* *Log-structured write-back* that preserves backend crash-consistency
+
+-------------------------------------------------------------------------------
+Constructor
+===========
+
+::
+
+    pcache <cache_dev> <backing_dev> [<number_of_optional_arguments> <cache_mode writeback> <data_crc true|false>]
+
+=========================  ====================================================
+``cache_dev``               Any DAX-capable block device (``/dev/pmem0``…).
+                            All metadata *and* cached blocks are stored here.
+
+``backing_dev``             The slow block device to be cached.
+
+``cache_mode``              Optional, Only ``writeback`` is accepted at the moment.
+
+``data_crc``                Optional, default to ``false``
+                            ``true``  – store CRC32 for every cached entry and
+                                      verify on reads
+                            ``false`` – skip CRC (faster)
+=========================  ====================================================
+
+Example
+-------
+
+.. code-block:: shell
+
+   dmsetup create pcache_sdb --table \
+     "0 $(blockdev --getsz /dev/sdb) pcache /dev/pmem0 /dev/sdb 4 cache_mode writeback data_crc true"
+
+The first time a pmem device is used, dm-pcache formats it automatically
+(super-block, cache_info, etc.).
+
+-------------------------------------------------------------------------------
+Status line
+===========
+
+``dmsetup status <device>`` (``STATUSTYPE_INFO``) prints:
+
+::
+
+   <sb_flags> <seg_total> <cache_segs> <segs_used> \
+   <gc_percent> <cache_flags> \
+   <key_head_seg>:<key_head_off> \
+   <dirty_tail_seg>:<dirty_tail_off> \
+   <key_tail_seg>:<key_tail_off>
+
+Field meanings
+--------------
+
+===============================  =============================================
+``sb_flags``                     Super-block flags (e.g. endian marker).
+
+``seg_total``                    Number of physical *pmem* segments.
+
+``cache_segs``                   Number of segments used for cache.
+
+``segs_used``                    Segments currently allocated (bitmap weight).
+
+``gc_percent``                   Current GC high-water mark (0-90).
+
+``cache_flags``                  Bit 0 – DATA_CRC enabled
+                                 Bit 1 – INIT_DONE (cache initialised)
+                                 Bits 2-5 – cache mode (0 == WB).
+
+``key_head``                     Where new key-sets are being written.
+
+``dirty_tail``                   First dirty key-set that still needs
+                                 write-back to the backing device.
+
+``key_tail``                     First key-set that may be reclaimed by GC.
+===============================  =============================================
+
+-------------------------------------------------------------------------------
+Messages
+========
+
+*Change GC trigger*
+
+::
+
+   dmsetup message <dev> 0 gc_percent <0-90>
+
+-------------------------------------------------------------------------------
+Theory of operation
+===================
+
+Sub-devices
+-----------
+
+====================  =========================================================
+backing_dev             Any block device (SSD/HDD/loop/LVM, etc.).
+cache_dev               DAX device; must expose direct-access memory.
+====================  =========================================================
+
+Segments and key-sets
+---------------------
+
+* The pmem space is divided into *16 MiB segments*.
+* Each write allocates space from a per-CPU *data_head* inside a segment.
+* A *cache-key* records a logical range on the origin and where it lives
+  inside pmem (segment + offset + generation).
+* 128 keys form a *key-set* (kset); ksets are written sequentially in pmem
+  and are themselves crash-safe (CRC).
+* The pair *(key_tail, dirty_tail)* delimit clean/dirty and live/dead ksets.
+
+Write-back
+----------
+
+Dirty keys are queued into a tree; a background worker copies data
+back to the backing_dev and advances *dirty_tail*.  A FLUSH/FUA bio from the
+upper layers forces an immediate metadata commit.
+
+Garbage collection
+------------------
+
+GC starts when ``segs_used >= seg_total * gc_percent / 100``.  It walks
+from *key_tail*, frees segments whose every key has been invalidated, and
+advances *key_tail*.
+
+CRC verification
+----------------
+
+If ``data_crc is enabled`` dm-pcache computes a CRC32 over every cached data
+range when it is inserted and stores it in the on-media key.  Reads
+validate the CRC before copying to the caller.
+
+-------------------------------------------------------------------------------
+Failure handling
+================
+
+* *pmem media errors* – all metadata copies are read with
+  ``copy_mc_to_kernel``; an uncorrectable error logs and aborts initialisation.
+* *Cache full* – if no free segment can be found, writes return ``-EBUSY``;
+  dm-pcache retries internally (request deferral).
+* *System crash* – on attach, the driver replays ksets from *key_tail* to
+  rebuild the in-core trees; every segment’s generation guards against
+  use-after-free keys.
+
+-------------------------------------------------------------------------------
+Limitations & TODO
+==================
+
+* Only *write-back* mode; other modes planned.
+* Only FIFO cache invalidate; other (LRU, ARC...) planned.
+* Table reload is not supported currently.
+* Discard planned.
+
+-------------------------------------------------------------------------------
+Example workflow
+================
+
+.. code-block:: shell
+
+   # 1.  Create devices
+   dmsetup create pcache_sdb --table \
+     "0 $(blockdev --getsz /dev/sdb) pcache /dev/pmem0 /dev/sdb 4 cache_mode writeback data_crc true"
+
+   # 2.  Put a filesystem on top
+   mkfs.ext4 /dev/mapper/pcache_sdb
+   mount /dev/mapper/pcache_sdb /mnt
+
+   # 3.  Tune GC threshold to 80 %
+   dmsetup message pcache_sdb 0 gc_percent 80
+
+   # 4.  Observe status
+   watch -n1 'dmsetup status pcache_sdb'
+
+   # 5.  Shutdown
+   umount /mnt
+   dmsetup remove pcache_sdb
+
+-------------------------------------------------------------------------------
+``dm-pcache`` is under active development; feedback, bug reports and patches
+are very welcome!
diff --git a/MAINTAINERS b/MAINTAINERS
index dd844ac8d910..0a7aa53eef96 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -6838,6 +6838,14 @@ S:	Maintained
 F:	Documentation/admin-guide/device-mapper/vdo*.rst
 F:	drivers/md/dm-vdo/
 
+DEVICE-MAPPER PCACHE TARGET
+M:	Dongsheng Yang <dongsheng.yang@linux.dev>
+M:	Zheng Gu <cengku@gmail.com>
+L:	dm-devel@lists.linux.dev
+S:	Maintained
+F:	Documentation/admin-guide/device-mapper/dm-pcache.rst
+F:	drivers/md/dm-pcache/
+
 DEVLINK
 M:	Jiri Pirko <jiri@resnulli.us>
 L:	netdev@vger.kernel.org
diff --git a/drivers/md/Kconfig b/drivers/md/Kconfig
index ddb37f6670de..cd4d8d1705bb 100644
--- a/drivers/md/Kconfig
+++ b/drivers/md/Kconfig
@@ -659,4 +659,6 @@ config DM_AUDIT
 
 source "drivers/md/dm-vdo/Kconfig"
 
+source "drivers/md/dm-pcache/Kconfig"
+
 endif # MD
diff --git a/drivers/md/Makefile b/drivers/md/Makefile
index 87bdfc9fe14c..f91a3133677f 100644
--- a/drivers/md/Makefile
+++ b/drivers/md/Makefile
@@ -71,6 +71,7 @@ obj-$(CONFIG_DM_RAID)		+= dm-raid.o
 obj-$(CONFIG_DM_THIN_PROVISIONING) += dm-thin-pool.o
 obj-$(CONFIG_DM_VERITY)		+= dm-verity.o
 obj-$(CONFIG_DM_VDO)            += dm-vdo/
+obj-$(CONFIG_DM_PCACHE)		+= dm-pcache/
 obj-$(CONFIG_DM_CACHE)		+= dm-cache.o
 obj-$(CONFIG_DM_CACHE_SMQ)	+= dm-cache-smq.o
 obj-$(CONFIG_DM_EBS)		+= dm-ebs.o
diff --git a/drivers/md/dm-pcache/Kconfig b/drivers/md/dm-pcache/Kconfig
new file mode 100644
index 000000000000..0e251eca892e
--- /dev/null
+++ b/drivers/md/dm-pcache/Kconfig
@@ -0,0 +1,17 @@
+config DM_PCACHE
+	tristate "Persistent cache for Block Device (Experimental)"
+	depends on BLK_DEV_DM
+	depends on DEV_DAX
+	help
+	  PCACHE provides a mechanism to use persistent memory (e.g., CXL persistent memory,
+	  DAX-enabled devices) as a high-performance cache layer in front of
+	  traditional block devices such as SSDs or HDDs.
+
+	  PCACHE is implemented as a kernel module that integrates with the block
+	  layer and supports direct access (DAX) to persistent memory for low-latency,
+	  byte-addressable caching.
+
+	  Note: This feature is experimental and should be tested thoroughly
+	  before use in production environments.
+
+	  If unsure, say 'N'.
diff --git a/drivers/md/dm-pcache/Makefile b/drivers/md/dm-pcache/Makefile
new file mode 100644
index 000000000000..86776e4acad2
--- /dev/null
+++ b/drivers/md/dm-pcache/Makefile
@@ -0,0 +1,3 @@
+dm-pcache-y := dm_pcache.o cache_dev.o segment.o backing_dev.o cache.o cache_gc.o cache_writeback.o cache_segment.o cache_key.o cache_req.o
+
+obj-m += dm-pcache.o
diff --git a/drivers/md/dm-pcache/dm_pcache.c b/drivers/md/dm-pcache/dm_pcache.c
new file mode 100644
index 000000000000..ee707d33635e
--- /dev/null
+++ b/drivers/md/dm-pcache/dm_pcache.c
@@ -0,0 +1,466 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+#include <linux/module.h>
+#include <linux/blkdev.h>
+#include <linux/bio.h>
+
+#include "../dm-core.h"
+#include "cache_dev.h"
+#include "backing_dev.h"
+#include "cache.h"
+#include "dm_pcache.h"
+
+void pcache_defer_reqs_kick(struct dm_pcache *pcache)
+{
+	struct pcache_cache *cache = &pcache->cache;
+
+	spin_lock(&cache->seg_map_lock);
+	if (!cache->cache_full)
+		queue_work(pcache->task_wq, &pcache->defered_req_work);
+	spin_unlock(&cache->seg_map_lock);
+}
+
+static void defer_req(struct pcache_request *pcache_req)
+{
+	struct dm_pcache *pcache = pcache_req->pcache;
+
+	BUG_ON(!list_empty(&pcache_req->list_node));
+
+	spin_lock(&pcache->defered_req_list_lock);
+	list_add(&pcache_req->list_node, &pcache->defered_req_list);
+	pcache_defer_reqs_kick(pcache);
+	spin_unlock(&pcache->defered_req_list_lock);
+}
+
+static void defered_req_fn(struct work_struct *work)
+{
+	struct dm_pcache *pcache = container_of(work, struct dm_pcache, defered_req_work);
+	struct pcache_request *pcache_req;
+	LIST_HEAD(tmp_list);
+	int ret;
+
+	if (pcache_is_stopping(pcache))
+		return;
+
+	spin_lock(&pcache->defered_req_list_lock);
+	list_splice_init(&pcache->defered_req_list, &tmp_list);
+	spin_unlock(&pcache->defered_req_list_lock);
+
+	while (!list_empty(&tmp_list)) {
+		pcache_req = list_first_entry(&tmp_list,
+					    struct pcache_request, list_node);
+		list_del_init(&pcache_req->list_node);
+		pcache_req->ret = 0;
+		ret = pcache_cache_handle_req(&pcache->cache, pcache_req);
+		if (ret == -EBUSY)
+			defer_req(pcache_req);
+		else
+			pcache_req_put(pcache_req, ret);
+	}
+}
+
+void pcache_req_get(struct pcache_request *pcache_req)
+{
+	kref_get(&pcache_req->ref);
+}
+
+static void end_req(struct kref *ref)
+{
+	struct pcache_request *pcache_req = container_of(ref, struct pcache_request, ref);
+	struct bio *bio = pcache_req->bio;
+	int ret = pcache_req->ret;
+
+	if (ret == -EBUSY) {
+		pcache_req_get(pcache_req);
+		defer_req(pcache_req);
+	} else {
+		bio->bi_status = errno_to_blk_status(ret);
+		bio_endio(bio);
+	}
+}
+
+void pcache_req_put(struct pcache_request *pcache_req, int ret)
+{
+	/* Set the return status if it is not already set */
+	if (ret && !pcache_req->ret)
+		pcache_req->ret = ret;
+
+	kref_put(&pcache_req->ref, end_req);
+}
+
+static bool at_least_one_arg(struct dm_arg_set *as, char **error)
+{
+	if (!as->argc) {
+		*error = "Insufficient args";
+		return false;
+	}
+
+	return true;
+}
+
+static int parse_cache_dev(struct dm_pcache *pcache, struct dm_arg_set *as,
+				char **error)
+{
+	int ret;
+
+	if (!at_least_one_arg(as, error))
+		return -EINVAL;
+	ret = dm_get_device(pcache->ti, dm_shift_arg(as),
+			  BLK_OPEN_READ | BLK_OPEN_WRITE,
+			  &pcache->cache_dev.dm_dev);
+	if (ret) {
+		*error = "Error opening cache device";
+		return ret;
+	}
+
+	return 0;
+}
+
+static int parse_backing_dev(struct dm_pcache *pcache, struct dm_arg_set *as,
+				char **error)
+{
+	int ret;
+
+	if (!at_least_one_arg(as, error))
+		return -EINVAL;
+
+	ret = dm_get_device(pcache->ti, dm_shift_arg(as),
+			  BLK_OPEN_READ | BLK_OPEN_WRITE,
+			  &pcache->backing_dev.dm_dev);
+	if (ret) {
+		*error = "Error opening backing device";
+		return ret;
+	}
+
+	return 0;
+}
+
+static void pcache_init_opts(struct pcache_cache_options *opts)
+{
+	opts->cache_mode = PCACHE_CACHE_MODE_WRITEBACK;
+	opts->data_crc = false;
+}
+
+static int parse_cache_opts(struct dm_pcache *pcache, struct dm_arg_set *as,
+			    char **error)
+{
+	struct pcache_cache_options *opts = &pcache->opts;
+	static const struct dm_arg _args[] = {
+		{0, 4, "Invalid number of cache option arguments"},
+	};
+	unsigned int argc;
+	const char *arg;
+	int ret;
+
+	pcache_init_opts(opts);
+	if (!as->argc)
+		return 0;
+
+	ret = dm_read_arg_group(_args, as, &argc, error);
+	if (ret)
+		return -EINVAL;
+
+	while (argc) {
+		arg = dm_shift_arg(as);
+		argc--;
+
+		if (!strcmp(arg, "cache_mode")) {
+			arg = dm_shift_arg(as);
+			if (!strcmp(arg, "writeback")) {
+				opts->cache_mode = PCACHE_CACHE_MODE_WRITEBACK;
+			} else {
+				*error = "Invalid cache mode parameter";
+				return -EINVAL;
+			}
+			argc--;
+		} else if (!strcmp(arg, "data_crc")) {
+			arg = dm_shift_arg(as);
+			if (!strcmp(arg, "true")) {
+				opts->data_crc = true;
+			} else if (!strcmp(arg, "false")) {
+				opts->data_crc = false;
+			} else {
+				*error = "Invalid data crc parameter";
+				return -EINVAL;
+			}
+			argc--;
+		} else {
+			*error = "Unrecognised cache option requested";
+			return -EINVAL;
+		}
+	}
+
+	return 0;
+}
+
+static int pcache_start(struct dm_pcache *pcache, char **error)
+{
+	int ret;
+
+	ret = cache_dev_start(pcache);
+	if (ret) {
+		*error = "Failed to start cache dev";
+		return ret;
+	}
+
+	ret = backing_dev_start(pcache);
+	if (ret) {
+		*error = "Failed to start backing dev";
+		goto stop_cache;
+	}
+
+	ret = pcache_cache_start(pcache);
+	if (ret) {
+		*error = "Failed to start pcache";
+		goto stop_backing;
+	}
+
+	return 0;
+stop_backing:
+	backing_dev_stop(pcache);
+stop_cache:
+	cache_dev_stop(pcache);
+
+	return ret;
+}
+
+static void pcache_destroy_args(struct dm_pcache *pcache)
+{
+	if (pcache->cache_dev.dm_dev)
+		dm_put_device(pcache->ti, pcache->cache_dev.dm_dev);
+	if (pcache->backing_dev.dm_dev)
+		dm_put_device(pcache->ti, pcache->backing_dev.dm_dev);
+}
+
+static int pcache_parse_args(struct dm_pcache *pcache, unsigned int argc, char **argv,
+				char **error)
+{
+	struct dm_arg_set as;
+	int ret;
+
+	as.argc = argc;
+	as.argv = argv;
+
+	/*
+	 * Parse cache device
+	 */
+	ret = parse_cache_dev(pcache, &as, error);
+	if (ret)
+		return ret;
+	/*
+	 * Parse backing device
+	 */
+	ret = parse_backing_dev(pcache, &as, error);
+	if (ret)
+		goto out;
+	/*
+	 * Parse optional arguments
+	 */
+	ret = parse_cache_opts(pcache, &as, error);
+	if (ret)
+		goto out;
+
+	return 0;
+out:
+	pcache_destroy_args(pcache);
+	return ret;
+}
+
+static int dm_pcache_ctr(struct dm_target *ti, unsigned int argc, char **argv)
+{
+	struct mapped_device *md = ti->table->md;
+	struct dm_pcache *pcache;
+	int ret;
+
+	if (md->map) {
+		ti->error = "Don't support table loading for live md";
+		return -EOPNOTSUPP;
+	}
+
+	/* Allocate memory for the cache structure */
+	pcache = kzalloc(sizeof(struct dm_pcache), GFP_KERNEL);
+	if (!pcache)
+		return -ENOMEM;
+
+	pcache->task_wq = alloc_workqueue("pcache-%s-wq",  WQ_UNBOUND | WQ_MEM_RECLAIM,
+					  0, md->name);
+	if (!pcache->task_wq) {
+		ret = -ENOMEM;
+		goto free_pcache;
+	}
+
+	spin_lock_init(&pcache->defered_req_list_lock);
+	INIT_LIST_HEAD(&pcache->defered_req_list);
+	INIT_WORK(&pcache->defered_req_work, defered_req_fn);
+	pcache->ti = ti;
+
+	ret = pcache_parse_args(pcache, argc, argv, &ti->error);
+	if (ret)
+		goto destroy_wq;
+
+	ret = pcache_start(pcache, &ti->error);
+	if (ret)
+		goto destroy_args;
+
+	ti->num_flush_bios = 1;
+	ti->flush_supported = true;
+	ti->per_io_data_size = sizeof(struct pcache_request);
+	ti->private = pcache;
+	atomic_set(&pcache->state, PCACHE_STATE_RUNNING);
+
+	return 0;
+destroy_args:
+	pcache_destroy_args(pcache);
+destroy_wq:
+	destroy_workqueue(pcache->task_wq);
+free_pcache:
+	kfree(pcache);
+
+	return ret;
+}
+
+static void defer_req_stop(struct dm_pcache *pcache)
+{
+	struct pcache_request *pcache_req;
+	LIST_HEAD(tmp_list);
+
+	flush_work(&pcache->defered_req_work);
+
+	spin_lock(&pcache->defered_req_list_lock);
+	list_splice_init(&pcache->defered_req_list, &tmp_list);
+	spin_unlock(&pcache->defered_req_list_lock);
+
+	while (!list_empty(&tmp_list)) {
+		pcache_req = list_first_entry(&tmp_list,
+					    struct pcache_request, list_node);
+		list_del_init(&pcache_req->list_node);
+		pcache_req_put(pcache_req, -EIO);
+	}
+}
+
+static void dm_pcache_dtr(struct dm_target *ti)
+{
+	struct dm_pcache *pcache;
+
+	pcache = ti->private;
+
+	atomic_set(&pcache->state, PCACHE_STATE_STOPPING);
+
+	defer_req_stop(pcache);
+	pcache_cache_stop(pcache);
+	backing_dev_stop(pcache);
+	cache_dev_stop(pcache);
+
+	pcache_destroy_args(pcache);
+	drain_workqueue(pcache->task_wq);
+	destroy_workqueue(pcache->task_wq);
+
+	kfree(pcache);
+}
+
+static int dm_pcache_map_bio(struct dm_target *ti, struct bio *bio)
+{
+	struct pcache_request *pcache_req = dm_per_bio_data(bio, sizeof(struct pcache_request));
+	struct dm_pcache *pcache = ti->private;
+	int ret;
+
+	pcache_req->pcache = pcache;
+	kref_init(&pcache_req->ref);
+	pcache_req->ret = 0;
+	pcache_req->bio = bio;
+	pcache_req->off = (u64)bio->bi_iter.bi_sector << SECTOR_SHIFT;
+	pcache_req->data_len = bio->bi_iter.bi_size;
+	INIT_LIST_HEAD(&pcache_req->list_node);
+
+	ret = pcache_cache_handle_req(&pcache->cache, pcache_req);
+	if (ret == -EBUSY)
+		defer_req(pcache_req);
+	else
+		pcache_req_put(pcache_req, ret);
+
+	return DM_MAPIO_SUBMITTED;
+}
+
+static void dm_pcache_status(struct dm_target *ti, status_type_t type,
+			     unsigned int status_flags, char *result,
+			     unsigned int maxlen)
+{
+	struct dm_pcache *pcache = ti->private;
+	struct pcache_cache_dev *cache_dev = &pcache->cache_dev;
+	struct pcache_backing_dev *backing_dev = &pcache->backing_dev;
+	struct pcache_cache *cache = &pcache->cache;
+	unsigned int sz = 0;
+
+	switch (type) {
+	case STATUSTYPE_INFO:
+		DMEMIT("%x %u %u %u %u %x %u:%u %u:%u %u:%u",
+		       cache_dev->sb_flags,
+		       cache_dev->seg_num,
+		       cache->n_segs,
+		       bitmap_weight(cache->seg_map, cache->n_segs),
+		       pcache_cache_get_gc_percent(cache),
+		       cache->cache_info.flags,
+		       cache->key_head.cache_seg->cache_seg_id,
+		       cache->key_head.seg_off,
+		       cache->dirty_tail.cache_seg->cache_seg_id,
+		       cache->dirty_tail.seg_off,
+		       cache->key_tail.cache_seg->cache_seg_id,
+		       cache->key_tail.seg_off);
+		break;
+	case STATUSTYPE_TABLE:
+		DMEMIT("%s %s writeback %s",
+		       cache_dev->dm_dev->name,
+		       backing_dev->dm_dev->name,
+		       cache_data_crc_on(cache) ? "true" : "false");
+		break;
+	case STATUSTYPE_IMA:
+		*result = '\0';
+		break;
+	}
+}
+
+static int dm_pcache_message(struct dm_target *ti, unsigned int argc,
+			     char **argv, char *result, unsigned int maxlen)
+{
+	struct dm_pcache *pcache = ti->private;
+	unsigned long val;
+
+	if (argc != 2)
+		goto err;
+
+	if (!strcasecmp(argv[0], "gc_percent")) {
+		if (kstrtoul(argv[1], 10, &val))
+			goto err;
+
+		return pcache_cache_set_gc_percent(&pcache->cache, val);
+	}
+err:
+	return -EINVAL;
+}
+
+static struct target_type dm_pcache_target = {
+	.name		= "pcache",
+	.version	= {0, 1, 0},
+	.module		= THIS_MODULE,
+	.features	= DM_TARGET_SINGLETON,
+	.ctr		= dm_pcache_ctr,
+	.dtr		= dm_pcache_dtr,
+	.map		= dm_pcache_map_bio,
+	.status		= dm_pcache_status,
+	.message	= dm_pcache_message,
+};
+
+static int __init dm_pcache_init(void)
+{
+	return dm_register_target(&dm_pcache_target);
+}
+module_init(dm_pcache_init);
+
+static void __exit dm_pcache_exit(void)
+{
+	dm_unregister_target(&dm_pcache_target);
+}
+module_exit(dm_pcache_exit);
+
+MODULE_DESCRIPTION("dm-pcache Persistent Cache for block device");
+MODULE_AUTHOR("Dongsheng Yang <dongsheng.yang@linux.dev>");
+MODULE_LICENSE("GPL");
diff --git a/drivers/md/dm-pcache/dm_pcache.h b/drivers/md/dm-pcache/dm_pcache.h
new file mode 100644
index 000000000000..c0d7b7078640
--- /dev/null
+++ b/drivers/md/dm-pcache/dm_pcache.h
@@ -0,0 +1,65 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+#ifndef _DM_PCACHE_H
+#define _DM_PCACHE_H
+#include <linux/device-mapper.h>
+
+#include "../dm-core.h"
+
+#define CACHE_DEV_TO_PCACHE(cache_dev)		(container_of(cache_dev, struct dm_pcache, cache_dev))
+#define BACKING_DEV_TO_PCACHE(backing_dev)	(container_of(backing_dev, struct dm_pcache, backing_dev))
+#define CACHE_TO_PCACHE(cache)			(container_of(cache, struct dm_pcache, cache))
+
+#define PCACHE_STATE_RUNNING			1
+#define PCACHE_STATE_STOPPING			2
+
+struct pcache_cache_dev;
+struct pcache_backing_dev;
+struct pcache_cache;
+struct pcache_cache_options;
+struct dm_pcache {
+	struct dm_target *ti;
+	struct pcache_cache_dev cache_dev;
+	struct pcache_backing_dev backing_dev;
+	struct pcache_cache cache;
+	struct pcache_cache_options opts;
+
+	spinlock_t			defered_req_list_lock;
+	struct list_head		defered_req_list;
+	struct workqueue_struct		*task_wq;
+
+	struct work_struct		defered_req_work;
+
+	atomic_t			state;
+};
+
+static inline bool pcache_is_stopping(struct dm_pcache *pcache)
+{
+	return (atomic_read(&pcache->state) == PCACHE_STATE_STOPPING);
+}
+
+#define pcache_dev_err(pcache, fmt, ...)							\
+	pcache_err("%s " fmt, pcache->ti->table->md->name, ##__VA_ARGS__)
+#define pcache_dev_info(pcache, fmt, ...)							\
+	pcache_info("%s " fmt, pcache->ti->table->md->name, ##__VA_ARGS__)
+#define pcache_dev_debug(pcache, fmt, ...)							\
+	pcache_debug("%s " fmt, pcache->ti->table->md->name, ##__VA_ARGS__)
+
+struct pcache_request {
+	struct dm_pcache	*pcache;
+	struct bio		*bio;
+
+	u64			off;
+	u32			data_len;
+
+	struct kref		ref;
+	int			ret;
+
+	struct list_head	list_node;
+};
+
+void pcache_req_get(struct pcache_request *pcache_req);
+void pcache_req_put(struct pcache_request *pcache_req, int ret);
+
+void pcache_defer_reqs_kick(struct dm_pcache *pcache);
+
+#endif /* _DM_PCACHE_H */
-- 
2.43.0

Re: [PATCH v2 00/11] dm-pcache – persistent-memory cache for block devices

[Mikulas Patocka]

On Mon, 7 Jul 2025, Dongsheng Yang wrote:

> Hi Mikulas,
> 	This is V2 for dm-pcache, please take a look.
> 
> Code:
>     https://github.com/DataTravelGuide/linux tags/pcache_v2
> 
> Changelogs
> 
> V2 from V1:
> 	- introduce req_alloc() and req_init() in backing_dev.c, then we
> 	  can do req_alloc() before holding spinlock and do req_init()
> 	  in subtree_walk().
> 	- introduce pre_alloc_key and pre_alloc_req in walk_ctx, that
> 	  means we can pre-allocate cache_key or backing_dev_request
> 	  before subtree walking.
> 	- use mempool_alloc() with NOIO for the allocation of cache_key
> 	  and backing_dev_req.
> 	- some coding style changes from comments of Jonathan.

Hi

mempool_alloc with GFP_NOIO never fails - so you don't have to check the 
returned value for NULL and propagate the error upwards.

"backing_req->kmem.bvecs = kmalloc_array(n_vecs, sizeof(struct bio_vec), 
GFP_NOIO)" - this call may fail and you should handle the error gracefully 
(i.e. don't end the bio with an error). Would it be possible to trim the 
request to BACKING_DEV_REQ_INLINE_BVECS vectors and retry it? 
Alternativelly, you can create a mempool for the largest possible n_vecs 
and allocate from this mempool if kmalloc_array fails.

I'm sending two patches for dm-pcache - the first patch adds the include 
file linux/bitfield.h - it is needed in my config. The second patch makes 
slab caches per-module rather than per-device, if you have them 
per-device, there are warnings about duplicate cache names.


BTW. What kind of persistent memory do you use? (afaik Intel killed the 
Optane products and I don't know of any replacement)

Some times ago I created a filesystem for persistent memory - see 
git://leontynka.twibright.com/nvfs.git - I'd be interested if you can test 
it on your persistent memory implementation.

Mikulas

Re: [PATCH v2 00/11] dm-pcache – persistent-memory cache for block devices

[Mikulas Patocka]

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
 drivers/md/dm-pcache/segment.h |    1 +
 1 file changed, 1 insertion(+)

Index: linux-2.6/drivers/md/dm-pcache/segment.h
===================================================================
--- linux-2.6.orig/drivers/md/dm-pcache/segment.h	2025-07-06 15:13:52.000000000 +0200
+++ linux-2.6/drivers/md/dm-pcache/segment.h	2025-07-06 15:14:17.000000000 +0200
@@ -3,6 +3,7 @@
 #define _PCACHE_SEGMENT_H
 
 #include <linux/bio.h>
+#include <linux/bitfield.h>
 
 #include "pcache_internal.h"
 

Re: [PATCH v2 00/11] dm-pcache – persistent-memory cache for block devices

[Mikulas Patocka]

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>

---
 drivers/md/dm-pcache/backing_dev.c |   14 +++-----------
 drivers/md/dm-pcache/backing_dev.h |    1 -
 drivers/md/dm-pcache/cache.h       |    1 -
 drivers/md/dm-pcache/cache_key.c   |   13 ++-----------
 drivers/md/dm-pcache/dm_pcache.c   |   28 +++++++++++++++++++++++++++-
 drivers/md/dm-pcache/dm_pcache.h   |    3 +++
 6 files changed, 35 insertions(+), 25 deletions(-)

Index: linux-2.6/drivers/md/dm-pcache/backing_dev.h
===================================================================
--- linux-2.6.orig/drivers/md/dm-pcache/backing_dev.h	2025-07-07 12:00:56.000000000 +0200
+++ linux-2.6/drivers/md/dm-pcache/backing_dev.h	2025-07-07 12:14:27.000000000 +0200
@@ -43,7 +43,6 @@ struct pcache_backing_dev {
 	struct pcache_cache		*cache;
 
 	struct dm_dev			*dm_dev;
-	struct kmem_cache		*backing_req_cache;
 	mempool_t			req_pool;
 
 	struct list_head		submit_list;
Index: linux-2.6/drivers/md/dm-pcache/cache.h
===================================================================
--- linux-2.6.orig/drivers/md/dm-pcache/cache.h	2025-07-07 12:00:56.000000000 +0200
+++ linux-2.6/drivers/md/dm-pcache/cache.h	2025-07-07 12:14:48.000000000 +0200
@@ -102,7 +102,6 @@ struct pcache_cache_subtree {
 struct pcache_cache_tree {
 	struct pcache_cache		*cache;
 	u32				n_subtrees;
-	struct kmem_cache		*key_cache;
 	mempool_t			key_pool;
 	struct pcache_cache_subtree	*subtrees;
 };
Index: linux-2.6/drivers/md/dm-pcache/dm_pcache.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-pcache/dm_pcache.c	2025-07-07 12:00:56.000000000 +0200
+++ linux-2.6/drivers/md/dm-pcache/dm_pcache.c	2025-07-07 12:23:13.000000000 +0200
@@ -9,6 +9,9 @@
 #include "cache.h"
 #include "dm_pcache.h"
 
+struct kmem_cache *backing_req_cache;
+struct kmem_cache *key_cache;
+
 void pcache_defer_reqs_kick(struct dm_pcache *pcache)
 {
 	struct pcache_cache *cache = &pcache->cache;
@@ -451,13 +454,36 @@ static struct target_type dm_pcache_targ
 
 static int __init dm_pcache_init(void)
 {
-	return dm_register_target(&dm_pcache_target);
+	int r;
+	backing_req_cache = KMEM_CACHE(pcache_backing_dev_req, 0);
+	if (!backing_req_cache) {
+		r = -ENOMEM;
+		goto err0;
+	}
+	key_cache = KMEM_CACHE(pcache_cache_key, 0);
+	if (!key_cache) {
+		r = -ENOMEM;
+		goto err1;
+	}
+	r = dm_register_target(&dm_pcache_target);
+	if (r)
+		goto err2;
+	return 0;
+
+err2:
+	kmem_cache_destroy(key_cache);
+err1:
+	kmem_cache_destroy(backing_req_cache);
+err0:
+	return r;
 }
 module_init(dm_pcache_init);
 
 static void __exit dm_pcache_exit(void)
 {
 	dm_unregister_target(&dm_pcache_target);
+	kmem_cache_destroy(key_cache);
+	kmem_cache_destroy(backing_req_cache);
 }
 module_exit(dm_pcache_exit);
 
Index: linux-2.6/drivers/md/dm-pcache/backing_dev.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-pcache/backing_dev.c	2025-07-07 12:00:56.000000000 +0200
+++ linux-2.6/drivers/md/dm-pcache/backing_dev.c	2025-07-07 12:21:27.000000000 +0200
@@ -11,7 +11,6 @@
 static void backing_dev_exit(struct pcache_backing_dev *backing_dev)
 {
 	mempool_exit(&backing_dev->req_pool);
-	kmem_cache_destroy(backing_dev->backing_req_cache);
 }
 
 static void req_submit_fn(struct work_struct *work);
@@ -21,15 +20,9 @@ static int backing_dev_init(struct dm_pc
 	struct pcache_backing_dev *backing_dev = &pcache->backing_dev;
 	int ret;
 
-	backing_dev->backing_req_cache = KMEM_CACHE(pcache_backing_dev_req, 0);
-	if (!backing_dev->backing_req_cache) {
-		ret = -ENOMEM;
-		goto err;
-	}
-
-	ret = mempool_init_slab_pool(&backing_dev->req_pool, 128, backing_dev->backing_req_cache);
+	ret = mempool_init_slab_pool(&backing_dev->req_pool, 128, backing_req_cache);
 	if (ret)
-		goto cache_destroy;
+		goto err;
 
 	INIT_LIST_HEAD(&backing_dev->submit_list);
 	INIT_LIST_HEAD(&backing_dev->complete_list);
@@ -39,8 +32,7 @@ static int backing_dev_init(struct dm_pc
 	INIT_WORK(&backing_dev->req_complete_work, req_complete_fn);
 
 	return 0;
-cache_destroy:
-	kmem_cache_destroy(backing_dev->backing_req_cache);
+
 err:
 	return ret;
 }
Index: linux-2.6/drivers/md/dm-pcache/cache_key.c
===================================================================
--- linux-2.6.orig/drivers/md/dm-pcache/cache_key.c	2025-07-07 12:00:56.000000000 +0200
+++ linux-2.6/drivers/md/dm-pcache/cache_key.c	2025-07-07 12:22:17.000000000 +0200
@@ -836,15 +836,9 @@ int cache_tree_init(struct pcache_cache
 	cache_tree->cache = cache;
 	cache_tree->n_subtrees = n_subtrees;
 
-	cache_tree->key_cache = KMEM_CACHE(pcache_cache_key, 0);
-	if (!cache_tree->key_cache) {
-		ret = -ENOMEM;
-		goto err;
-	}
-
-	ret = mempool_init_slab_pool(&cache_tree->key_pool, 1024, cache_tree->key_cache);
+	ret = mempool_init_slab_pool(&cache_tree->key_pool, 1024, key_cache);
 	if (ret)
-		goto destroy_key_cache;
+		goto err;
 
 	/*
 	 * Allocate and initialize the subtrees array.
@@ -868,8 +862,6 @@ int cache_tree_init(struct pcache_cache
 
 key_pool_exit:
 	mempool_exit(&cache_tree->key_pool);
-destroy_key_cache:
-	kmem_cache_destroy(cache_tree->key_cache);
 err:
 	return ret;
 }
@@ -896,5 +888,4 @@ void cache_tree_exit(struct pcache_cache
 	}
 	kvfree(cache_tree->subtrees);
 	mempool_exit(&cache_tree->key_pool);
-	kmem_cache_destroy(cache_tree->key_cache);
 }
Index: linux-2.6/drivers/md/dm-pcache/dm_pcache.h
===================================================================
--- linux-2.6.orig/drivers/md/dm-pcache/dm_pcache.h	2025-07-07 12:00:56.000000000 +0200
+++ linux-2.6/drivers/md/dm-pcache/dm_pcache.h	2025-07-07 12:19:42.000000000 +0200
@@ -32,6 +32,9 @@ struct dm_pcache {
 	atomic_t			state;
 };
 
+extern struct kmem_cache *backing_req_cache;
+extern struct kmem_cache *key_cache;
+
 static inline bool pcache_is_stopping(struct dm_pcache *pcache)
 {
 	return (atomic_read(&pcache->state) == PCACHE_STATE_STOPPING);

Re: [PATCH v2 00/11] dm-pcache – persistent-memory cache for block devices

[Dongsheng Yang]

在 7/8/2025 4:16 AM, Mikulas Patocka 写道:
>
> On Mon, 7 Jul 2025, Dongsheng Yang wrote:
>
>> Hi Mikulas,
>> 	This is V2 for dm-pcache, please take a look.
>>
>> Code:
>>      https://github.com/DataTravelGuide/linux tags/pcache_v2
>>
>> Changelogs
>>
>> V2 from V1:
>> 	- introduce req_alloc() and req_init() in backing_dev.c, then we
>> 	  can do req_alloc() before holding spinlock and do req_init()
>> 	  in subtree_walk().
>> 	- introduce pre_alloc_key and pre_alloc_req in walk_ctx, that
>> 	  means we can pre-allocate cache_key or backing_dev_request
>> 	  before subtree walking.
>> 	- use mempool_alloc() with NOIO for the allocation of cache_key
>> 	  and backing_dev_req.
>> 	- some coding style changes from comments of Jonathan.
> Hi
>
> mempool_alloc with GFP_NOIO never fails - so you don't have to check the
> returned value for NULL and propagate the error upwards.


Hi Mikulas:

    I noticed that the implementation of mempool_alloc—it waits for 5 
seconds and retries when allocation fails.

With this in mind, I propose that we handle -ENOMEM inside defer_req() 
using a similar mechanism. something like this commit:


https://github.com/DataTravelGuide/linux/commit/e6fc2e5012b1fe2312ed7dd02d6fbc2d038962c0


Here are two key reasons why:

(1) If we manage -ENOMEM in defer_req(), we don’t need to modify every 
lower-level allocation to use mempool to avoid failures—for example,

cache_key, backing_req, and the kmem.bvecs you mentioned. More 
importantly, there’s no easy way to prevent allocation failure in some 
places—for instance, bio_init_clone() could still return -ENOMEM.

(2) If we use a mempool, it will block and wait indefinitely when memory 
is unavailable, preventing the process from exiting.

But with defer_req(), the user can still manually stop the pcache device 
using dmsetup remove, releasing some memory if user want.


What do you think?

Thanx

Dongsheng

>
> "backing_req->kmem.bvecs = kmalloc_array(n_vecs, sizeof(struct bio_vec),
> GFP_NOIO)" - this call may fail and you should handle the error gracefully
> (i.e. don't end the bio with an error). Would it be possible to trim the
> request to BACKING_DEV_REQ_INLINE_BVECS vectors and retry it?
> Alternativelly, you can create a mempool for the largest possible n_vecs
> and allocate from this mempool if kmalloc_array fails.
>
> I'm sending two patches for dm-pcache - the first patch adds the include
> file linux/bitfield.h - it is needed in my config. The second patch makes
> slab caches per-module rather than per-device, if you have them
> per-device, there are warnings about duplicate cache names.
>
>
> BTW. What kind of persistent memory do you use? (afaik Intel killed the
> Optane products and I don't know of any replacement)
>
> Some times ago I created a filesystem for persistent memory - see
> git://leontynka.twibright.com/nvfs.git - I'd be interested if you can test
> it on your persistent memory implementation.
>
> Mikulas
>

Re: [PATCH v2 00/11] dm-pcache – persistent-memory cache for block devices

[Dongsheng Yang]

在 7/9/2025 5:45 PM, Dongsheng Yang 写道:
>
> 在 7/8/2025 4:16 AM, Mikulas Patocka 写道:
>>
>> On Mon, 7 Jul 2025, Dongsheng Yang wrote:
>>
>>> Hi Mikulas,
>>>     This is V2 for dm-pcache, please take a look.
>>>
>>> Code:
>>>      https://github.com/DataTravelGuide/linux tags/pcache_v2
>>>
>>> Changelogs
>>>
>>> V2 from V1:
>>>     - introduce req_alloc() and req_init() in backing_dev.c, then we
>>>       can do req_alloc() before holding spinlock and do req_init()
>>>       in subtree_walk().
>>>     - introduce pre_alloc_key and pre_alloc_req in walk_ctx, that
>>>       means we can pre-allocate cache_key or backing_dev_request
>>>       before subtree walking.
>>>     - use mempool_alloc() with NOIO for the allocation of cache_key
>>>       and backing_dev_req.
>>>     - some coding style changes from comments of Jonathan.
>> Hi
>>
>> mempool_alloc with GFP_NOIO never fails - so you don't have to check the
>> returned value for NULL and propagate the error upwards.
>
>
> Hi Mikulas:
>
>    I noticed that the implementation of mempool_alloc—it waits for 5 
> seconds and retries when allocation fails.
>
> With this in mind, I propose that we handle -ENOMEM inside defer_req() 
> using a similar mechanism. something like this commit:
>
>
> https://github.com/DataTravelGuide/linux/commit/e6fc2e5012b1fe2312ed7dd02d6fbc2d038962c0 
>
>
>
> Here are two key reasons why:
>
> (1) If we manage -ENOMEM in defer_req(), we don’t need to modify every 
> lower-level allocation to use mempool to avoid failures—for example,
>
> cache_key, backing_req, and the kmem.bvecs you mentioned. More 
> importantly, there’s no easy way to prevent allocation failure in some 
> places—for instance, bio_init_clone() could still return -ENOMEM.
>
> (2) If we use a mempool, it will block and wait indefinitely when 
> memory is unavailable, preventing the process from exiting.
>
> But with defer_req(), the user can still manually stop the pcache 
> device using dmsetup remove, releasing some memory if user want.
>
>
> What do you think?


BTW, I added a test case for NOMEM scenario by using failslab:


https://github.com/DataTravelGuide/dtg-tests/blob/main/pcache.py.data/pcache_failslab.sh

>
> Thanx
>
> Dongsheng
>
>>
>> "backing_req->kmem.bvecs = kmalloc_array(n_vecs, sizeof(struct bio_vec),
>> GFP_NOIO)" - this call may fail and you should handle the error 
>> gracefully
>> (i.e. don't end the bio with an error). Would it be possible to trim the
>> request to BACKING_DEV_REQ_INLINE_BVECS vectors and retry it?
>> Alternativelly, you can create a mempool for the largest possible n_vecs
>> and allocate from this mempool if kmalloc_array fails.
>>
>> I'm sending two patches for dm-pcache - the first patch adds the include
>> file linux/bitfield.h - it is needed in my config. The second patch 
>> makes
>> slab caches per-module rather than per-device, if you have them
>> per-device, there are warnings about duplicate cache names.
>>
>>
>> BTW. What kind of persistent memory do you use? (afaik Intel killed the
>> Optane products and I don't know of any replacement)
>>
>> Some times ago I created a filesystem for persistent memory - see
>> git://leontynka.twibright.com/nvfs.git - I'd be interested if you can 
>> test
>> it on your persistent memory implementation.
>>
>> Mikulas
>>
>

Re: [PATCH v2 00/11] dm-pcache – persistent-memory cache for block devices

[Mikulas Patocka]

[-- Attachment #1: Type: text/plain, Size: 3709 bytes --]



On Wed, 9 Jul 2025, Dongsheng Yang wrote:

> 
> 在 7/8/2025 4:16 AM, Mikulas Patocka 写道:
> > 
> > On Mon, 7 Jul 2025, Dongsheng Yang wrote:
> > 
> > > Hi Mikulas,
> > > 	This is V2 for dm-pcache, please take a look.
> > > 
> > > Code:
> > >      https://github.com/DataTravelGuide/linux tags/pcache_v2
> > > 
> > > Changelogs
> > > 
> > > V2 from V1:
> > > 	- introduce req_alloc() and req_init() in backing_dev.c, then we
> > > 	  can do req_alloc() before holding spinlock and do req_init()
> > > 	  in subtree_walk().
> > > 	- introduce pre_alloc_key and pre_alloc_req in walk_ctx, that
> > > 	  means we can pre-allocate cache_key or backing_dev_request
> > > 	  before subtree walking.
> > > 	- use mempool_alloc() with NOIO for the allocation of cache_key
> > > 	  and backing_dev_req.
> > > 	- some coding style changes from comments of Jonathan.
> > Hi
> > 
> > mempool_alloc with GFP_NOIO never fails - so you don't have to check the
> > returned value for NULL and propagate the error upwards.
> 
> 
> Hi Mikulas:
> 
>    I noticed that the implementation of mempool_alloc—it waits for 5 seconds
> and retries when allocation fails.

No, this is incorrect observation.

mempool_alloc will add the current process to a wait queue:
  prepare_to_wait(&pool->wait, &wait, TASK_UNINTERRUPTIBLE);
then, it will execute the wait:
  io_schedule_timeout(5*HZ);
and then remove the current process from a wait queue:
  finish_wait(&pool->wait, &wait);

but the io_schedule_timeout function will wait at most 5 seconds - it 
doesn't wait exactly 5 seconds. If some other piece of code frees some 
data into the mempool, it executes:
  wake_up(&pool->wait);
so that the process that is waiting in io_schedule_timeout(5*HZ) is woken 
up immediatelly.

See this comment in mempool_alloc:
        /*
         * FIXME: this should be io_schedule().  The timeout is there as a
         * workaround for some DM problems in 2.6.18.
         */
        io_schedule_timeout(5*HZ);

- the timeout is actually a workaround for some buggy code in device 
mapper where the code allocated more and more entries from the mempool 
without freeing them. I fixed this bug many years ago. But people 
shouldn't add more buggy code that depends on this timeout.

> With this in mind, I propose that we handle -ENOMEM inside defer_req() using a
> similar mechanism. something like this commit:
> 
> 
> https://github.com/DataTravelGuide/linux/commit/e6fc2e5012b1fe2312ed7dd02d6fbc2d038962c0
> 
> 
> Here are two key reasons why:
> 
> (1) If we manage -ENOMEM in defer_req(), we don’t need to modify every
> lower-level allocation to use mempool to avoid failures—for example,
> 
> cache_key, backing_req, and the kmem.bvecs you mentioned. More importantly,
> there’s no easy way to prevent allocation failure in some places—for instance,
> bio_init_clone() could still return -ENOMEM.
> 
> (2) If we use a mempool, it will block and wait indefinitely when memory is
> unavailable, preventing the process from exiting.

Mempool will wait until some other code frees some data into the mempool. 
So, as long as your code can make forward progress and finish some 
requests and free their data into the mempool, it should work properly.

On the other hand, non-mempool GFP_NOIO allocations can wait indefinitely.

> But with defer_req(), the user can still manually stop the pcache device using
> dmsetup remove, releasing some memory if user want.
> 
> What do you think?

I think that you should go back to version 2 (that had mempools) and 
change the non-mempool allocation "backing_req->kmem.bvecs = 
kmalloc_array(n_vecs, sizeof(struct bio_vec), GFP_NOIO);" to use a 
mempool.

> Thanx
> 
> Dongsheng

Mikulas