[RFC PATCH 03/11] pcache: introduce meta_segment abstraction

[Dongsheng Yang <dongsheng.yang@linux.dev>]

This patch introduces the `meta_segment` abstraction to persistently store
metadata for pcache, specifically the `pcache_backing_dev_info` structure.

Each `meta_segment` wraps a data segment and organizes metadata space into
multiple entries, each replicated multiple times for reliability. Metadata
integrity is ensured using a sequence counter and CRC per metadata header.

Key highlights:
- `struct pcache_meta_segment`: Manages a segment dedicated to metadata.
- `struct pcache_meta_segment_info`: Describes the layout and size of
  metadata entries.
- `meta_seg_info_write()`: Writes updated metadata info with CRC protection.
- `meta_seg_meta()`: Computes the address of a given metadata entry.
- `pcache_meta_seg_for_each_meta`: A convenient macro to iterate over all
  metadata entries in the segment, simplifying metadata scanning and
  management logic.

Currently, only `pcache_backing_dev_info` is stored via this mechanism.

This design provides a structured, verifiable, and extensible foundation
for storing persistent metadata in the pcache framework.

Signed-off-by: Dongsheng Yang <dongsheng.yang@linux.dev>
---
 drivers/block/pcache/meta_segment.c | 61 +++++++++++++++++++++++++++++
 drivers/block/pcache/meta_segment.h | 46 ++++++++++++++++++++++
 2 files changed, 107 insertions(+)
 create mode 100644 drivers/block/pcache/meta_segment.c
 create mode 100644 drivers/block/pcache/meta_segment.h

diff --git a/drivers/block/pcache/meta_segment.c b/drivers/block/pcache/meta_segment.c
new file mode 100644
index 000000000000..6a6dd9ad9041
--- /dev/null
+++ b/drivers/block/pcache/meta_segment.c
@@ -0,0 +1,61 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+#include "cache_dev.h"
+#include "cache.h"
+#include "backing_dev.h"
+#include "meta_segment.h"
+
+static void meta_seg_info_write(struct pcache_meta_segment *meta_seg)
+{
+	struct pcache_meta_segment_info *info_addr;
+
+	mutex_lock(&meta_seg->info_lock);
+	meta_seg->meta_seg_info.seg_info.header.seq++;
+
+	info_addr = CACHE_DEV_SEGMENT(meta_seg->cache_dev, meta_seg->meta_seg_info.seg_info.seg_id);
+	info_addr = pcache_meta_find_oldest(&info_addr->seg_info.header, PCACHE_SEG_INFO_SIZE);
+
+	memcpy(info_addr, &meta_seg->meta_seg_info, sizeof(struct pcache_meta_segment_info));
+	info_addr->seg_info.header.crc = pcache_meta_crc(&info_addr->seg_info.header, PCACHE_SEG_INFO_SIZE);
+
+	cache_dev_flush(meta_seg->cache_dev, info_addr, PCACHE_SEG_INFO_SIZE);
+	mutex_unlock(&meta_seg->info_lock);
+}
+
+static void meta_seg_init(struct pcache_cache_dev *cache_dev, struct pcache_meta_segment *meta_seg, u32 seg_id, u32 meta_size)
+{
+	struct pcache_segment_init_options seg_opts = { 0 };
+
+	meta_seg->cache_dev = cache_dev;
+	mutex_init(&meta_seg->info_lock);
+
+	seg_opts.type = PCACHES_TYPE_META;
+	seg_opts.state = PCACHE_SEGMENT_STATE_RUNNING;
+	seg_opts.seg_id = seg_id;
+	seg_opts.data_off = PCACHE_SEG_INFO_SIZE * PCACHE_META_INDEX_MAX;
+	seg_opts.seg_info = &meta_seg->meta_seg_info.seg_info;
+
+	pcache_segment_init(cache_dev, &meta_seg->segment, &seg_opts);
+
+	meta_seg->meta_seg_info.meta_size = meta_size;
+	meta_seg->meta_seg_info.meta_num = meta_seg->segment.data_size / (meta_size * PCACHE_META_INDEX_MAX);
+
+	meta_seg_info_write(meta_seg);
+}
+
+struct pcache_meta_segment *pcache_meta_seg_alloc(struct pcache_cache_dev *cache_dev, u32 seg_id, u32 meta_size)
+{
+	struct pcache_meta_segment *meta_seg;
+
+	meta_seg = kzalloc(sizeof(struct pcache_meta_segment), GFP_KERNEL);
+	if (!meta_seg)
+		return NULL;
+
+	meta_seg_init(cache_dev, meta_seg, seg_id, meta_size);
+
+	return meta_seg;
+}
+
+void pcache_meta_seg_free(struct pcache_meta_segment *meta_seg)
+{
+	kfree(meta_seg);
+}
diff --git a/drivers/block/pcache/meta_segment.h b/drivers/block/pcache/meta_segment.h
new file mode 100644
index 000000000000..3e0886d0bd2b
--- /dev/null
+++ b/drivers/block/pcache/meta_segment.h
@@ -0,0 +1,46 @@
+/* SPDX-License-Identifier: GPL-2.0-or-later */
+#ifndef _PCACHE_META_SEGMENT_H
+#define _PCACHE_META_SEGMENT_H
+
+#include <linux/bio.h>
+
+#include "pcache_internal.h"
+#include "cache_dev.h"
+#include "segment.h"
+
+struct pcache_cache_dev;
+struct pcache_backing_dev_info;
+
+struct pcache_meta_segment_info {
+	struct pcache_segment_info	seg_info;
+	u32 meta_size;
+	u32 meta_num;
+};
+
+struct pcache_meta_segment {
+	struct pcache_segment	segment;
+
+	struct pcache_cache_dev *cache_dev;
+
+	struct pcache_meta_segment_info meta_seg_info;
+	struct mutex info_lock;
+
+	struct pcache_meta_segment *next_meta_seg;
+};
+
+static inline void *meta_seg_meta(struct pcache_meta_segment *meta_seg, u32 meta_id)
+{
+	void *data = meta_seg->segment.data;
+
+	return (data + meta_id * meta_seg->meta_seg_info.meta_size * PCACHE_META_INDEX_MAX);
+}
+
+#define pcache_meta_seg_for_each_meta(meta_seg, i, meta)	\
+	for (i = 0;						\
+	     i < meta_seg->meta_seg_info.meta_num &&		\
+	     ((meta = meta_seg_meta(meta_seg, i)) || true);	\
+	     i++)
+
+struct pcache_meta_segment *pcache_meta_seg_alloc(struct pcache_cache_dev *cache_dev, u32 seg_id, u32 meta_size);
+void pcache_meta_seg_free(struct pcache_meta_segment *meta_seg);
+#endif /* _PCACHE_META_SEGMENT_H */
-- 
2.34.1