[RFC PATCH v1 09/10] mm/prmem: Implement DAX support for Persistent Ramdisks.

[madvenka@linux.microsoft.com]

From: "Madhavan T. Venkataraman" <madvenka@linux.microsoft.com>

One problem with using a ramdisk is that the page cache will contain
redundant copies of ramdisk data. To avoid this, implement DAX support
for persistent ramdisks.

To avail this, the filesystem that is installed on the ramdisk must
support DAX. Like ext4. Mount the filesystem with the dax option. E.g.,

	sudo mount -t ext4 -o dax /dev/pram0 /path/to/mountpoint

Signed-off-by: Madhavan T. Venkataraman <madvenka@linux.microsoft.com>
---
 drivers/block/brd.c | 106 ++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 106 insertions(+)

diff --git a/drivers/block/brd.c b/drivers/block/brd.c
index 3a05e56ca16f..d4a42d3bd212 100644
--- a/drivers/block/brd.c
+++ b/drivers/block/brd.c
@@ -25,6 +25,9 @@
 #include <linux/backing-dev.h>
 #include <linux/debugfs.h>
 #include <linux/prmem.h>
+#include <linux/pfn_t.h>
+#include <linux/dax.h>
+#include <linux/uio.h>
 
 #include <linux/uaccess.h>
 
@@ -42,6 +45,7 @@ struct brd_device {
 	enum brd_type		brd_type;
 	struct gendisk		*brd_disk;
 	struct list_head	brd_list;
+	struct dax_device	*brd_dax;
 
 	/*
 	 * Backing store of pages. This is the contents of the block device.
@@ -58,6 +62,8 @@ static void brd_free_page(struct brd_device *brd, struct page *page);
 static void brd_xa_init(struct brd_device *brd);
 static void brd_init_name(struct brd_device *brd, char *name);
 static void brd_set_capacity(struct brd_device *brd);
+static int brd_dax_init(struct brd_device *brd);
+static void brd_dax_cleanup(struct brd_device *brd);
 
 /*
  * Look up and return a brd's page for a given sector.
@@ -408,6 +414,9 @@ static int brd_alloc(int i)
 	strscpy(disk->disk_name, buf, DISK_NAME_LEN);
 	brd_set_capacity(brd);
 	
+	if (brd_dax_init(brd))
+		goto out_clean_dax;
+
 	/*
 	 * This is so fdisk will align partitions on 4k, because of
 	 * direct_access API needing 4k alignment, returning a PFN
@@ -421,6 +430,8 @@ static int brd_alloc(int i)
 	blk_queue_flag_set(QUEUE_FLAG_NONROT, disk->queue);
 	blk_queue_flag_set(QUEUE_FLAG_SYNCHRONOUS, disk->queue);
 	blk_queue_flag_set(QUEUE_FLAG_NOWAIT, disk->queue);
+	if (brd->brd_dax)
+		blk_queue_flag_set(QUEUE_FLAG_DAX, disk->queue);
 	err = add_disk(disk);
 	if (err)
 		goto out_cleanup_disk;
@@ -429,6 +440,8 @@ static int brd_alloc(int i)
 
 out_cleanup_disk:
 	put_disk(disk);
+out_clean_dax:
+	brd_dax_cleanup(brd);
 out_free_dev:
 	list_del(&brd->brd_list);
 	brd_free_device(brd);
@@ -447,6 +460,7 @@ static void brd_cleanup(void)
 	debugfs_remove_recursive(brd_debugfs_dir);
 
 	list_for_each_entry_safe(brd, next, &brd_devices, brd_list) {
+		brd_dax_cleanup(brd);
 		del_gendisk(brd->brd_disk);
 		put_disk(brd->brd_disk);
 		brd_free_pages(brd);
@@ -659,3 +673,95 @@ static void brd_set_capacity(struct brd_device *brd)
 		disksize = prd_data[brd->brd_number].size;
 	set_capacity(brd->brd_disk, disksize * 2);
 }
+
+static bool		prd_dax_enabled = IS_ENABLED(CONFIG_FS_DAX);
+
+static long brd_dax_direct_access(struct dax_device *dax_dev,
+				  pgoff_t pgoff, long nr_pages,
+				  enum dax_access_mode mode,
+				  void **kaddr, pfn_t *pfn);
+static int brd_dax_zero_page_range(struct dax_device *dax_dev,
+				   pgoff_t pgoff, size_t nr_pages);
+
+static const struct dax_operations brd_dax_ops = {
+	.direct_access = brd_dax_direct_access,
+	.zero_page_range = brd_dax_zero_page_range,
+};
+
+static int brd_dax_init(struct brd_device *brd)
+{
+	if (!prd_dax_enabled || brd->brd_type == BRD_NORMAL)
+		return 0;
+
+	brd->brd_dax = alloc_dax(brd, &brd_dax_ops);
+	if (IS_ERR(brd->brd_dax)) {
+		pr_warn("%s: DAX failed\n", __func__);
+		brd->brd_dax = NULL;
+		return -ENOMEM;
+	}
+
+	if (dax_add_host(brd->brd_dax, brd->brd_disk)) {
+		pr_warn("%s: DAX add failed\n", __func__);
+		return -ENOMEM;
+	}
+	return 0;
+}
+
+static void brd_dax_cleanup(struct brd_device *brd)
+{
+	if (!prd_dax_enabled || brd->brd_type == BRD_NORMAL)
+		return;
+
+	if (brd->brd_dax) {
+		dax_remove_host(brd->brd_disk);
+		kill_dax(brd->brd_dax);
+		put_dax(brd->brd_dax);
+	}
+}
+static int brd_dax_zero_page_range(struct dax_device *dax_dev,
+				   pgoff_t pgoff, size_t nr_pages)
+{
+	long rc;
+	void *kaddr;
+
+	rc = dax_direct_access(dax_dev, pgoff, nr_pages, DAX_ACCESS,
+			&kaddr, NULL);
+	if (rc < 0)
+		return rc;
+	memset(kaddr, 0, nr_pages << PAGE_SHIFT);
+	return 0;
+}
+
+static long __brd_direct_access(struct brd_device *brd, pgoff_t pgoff,
+		long nr_pages, void **kaddr, pfn_t *pfn)
+{
+	struct page *page;
+	sector_t sector = (sector_t) pgoff << PAGE_SECTORS_SHIFT;
+	int ret;
+
+	if (!brd)
+		return -ENODEV;
+
+	ret = brd_insert_page(brd, sector, GFP_NOWAIT);
+	if (ret)
+		return ret;
+
+	page = brd_lookup_page(brd, sector);
+	if (!page)
+		return -ENOSPC;
+
+	*kaddr = page_address(page);
+	if (pfn)
+		*pfn = page_to_pfn_t(page);
+
+	return 1;
+}
+
+static long brd_dax_direct_access(struct dax_device *dax_dev,
+		pgoff_t pgoff, long nr_pages, enum dax_access_mode mode,
+		void **kaddr, pfn_t *pfn)
+{
+	struct brd_device *brd = dax_get_private(dax_dev);
+
+	return __brd_direct_access(brd, pgoff, nr_pages, kaddr, pfn);
+}
-- 
2.25.1