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Commit 4751832d authored by Qu Wenruo's avatar Qu Wenruo Committed by David Sterba
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btrfs: fiemap: Cache and merge fiemap extent before submit it to user 



[BUG]
Cycle mount btrfs can cause fiemap to return different result.
Like:
 # mount /dev/vdb5 /mnt/btrfs
 # dd if=/dev/zero bs=16K count=4 oflag=dsync of=/mnt/btrfs/file
 # xfs_io -c "fiemap -v" /mnt/btrfs/file
 /mnt/test/file:
 EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
   0: [0..127]:        25088..25215       128   0x1
 # umount /mnt/btrfs
 # mount /dev/vdb5 /mnt/btrfs
 # xfs_io -c "fiemap -v" /mnt/btrfs/file
 /mnt/test/file:
 EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
   0: [0..31]:         25088..25119        32   0x0
   1: [32..63]:        25120..25151        32   0x0
   2: [64..95]:        25152..25183        32   0x0
   3: [96..127]:       25184..25215        32   0x1
But after above fiemap, we get correct merged result if we call fiemap
again.
 # xfs_io -c "fiemap -v" /mnt/btrfs/file
 /mnt/test/file:
 EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
   0: [0..127]:        25088..25215       128   0x1

[REASON]
Btrfs will try to merge extent map when inserting new extent map.

btrfs_fiemap(start=0 len=(u64)-1)
|- extent_fiemap(start=0 len=(u64)-1)
   |- get_extent_skip_holes(start=0 len=64k)
   |  |- btrfs_get_extent_fiemap(start=0 len=64k)
   |     |- btrfs_get_extent(start=0 len=64k)
   |        |  Found on-disk (ino, EXTENT_DATA, 0)
   |        |- add_extent_mapping()
   |        |- Return (em->start=0, len=16k)
   |
   |- fiemap_fill_next_extent(logic=0 phys=X len=16k)
   |
   |- get_extent_skip_holes(start=0 len=64k)
   |  |- btrfs_get_extent_fiemap(start=0 len=64k)
   |     |- btrfs_get_extent(start=16k len=48k)
   |        |  Found on-disk (ino, EXTENT_DATA, 16k)
   |        |- add_extent_mapping()
   |        |  |- try_merge_map()
   |        |     Merge with previous em start=0 len=16k
   |        |     resulting em start=0 len=32k
   |        |- Return (em->start=0, len=32K)    << Merged result
   |- Stripe off the unrelated range (0~16K) of return em
   |- fiemap_fill_next_extent(logic=16K phys=X+16K len=16K)
      ^^^ Causing split fiemap extent.

And since in add_extent_mapping(), em is already merged, in next
fiemap() call, we will get merged result.

[FIX]
Here we introduce a new structure, fiemap_cache, which records previous
fiemap extent.

And will always try to merge current fiemap_cache result before calling
fiemap_fill_next_extent().
Only when we failed to merge current fiemap extent with cached one, we
will call fiemap_fill_next_extent() to submit cached one.

So by this method, we can merge all fiemap extents.

It can also be done in fs/ioctl.c, however the problem is if
fieinfo->fi_extents_max == 0, we have no space to cache previous fiemap
extent.
So I choose to merge it in btrfs.

Signed-off-by: default avatarQu Wenruo <quwenruo@cn.fujitsu.com>
Reviewed-by: default avatarLiu Bo <bo.li.liu@oracle.com>
Reviewed-by: default avatarDavid Sterba <dsterba@suse.com>
Signed-off-by: default avatarDavid Sterba <dsterba@suse.com>
parent 9bcaaea7

fs/btrfs/extent_io.c

+122 −2
Original line number Diff line number Diff line
@@ -4377,6 +4377,123 @@ static struct extent_map *get_extent_skip_holes(struct inode *inode, 
	return NULL;

}



/*

 * To cache previous fiemap extent

 *

 * Will be used for merging fiemap extent

 */

struct fiemap_cache {

	u64 offset;

	u64 phys;

	u64 len;

	u32 flags;

	bool cached;

};



/*

 * Helper to submit fiemap extent.

 *

 * Will try to merge current fiemap extent specified by @offset, @phys,

 * @len and @flags with cached one.

 * And only when we fails to merge, cached one will be submitted as

 * fiemap extent.

 *

 * Return value is the same as fiemap_fill_next_extent().

 */

static int emit_fiemap_extent(struct fiemap_extent_info *fieinfo,

				struct fiemap_cache *cache,

				u64 offset, u64 phys, u64 len, u32 flags)

{

	int ret = 0;



	if (!cache->cached)

		goto assign;



	/*

	 * Sanity check, extent_fiemap() should have ensured that new

	 * fiemap extent won't overlap with cahced one.

	 * Not recoverable.

	 *

	 * NOTE: Physical address can overlap, due to compression

	 */

	if (cache->offset + cache->len > offset) {

		WARN_ON(1);

		return -EINVAL;

	}



	/*

	 * Only merges fiemap extents if

	 * 1) Their logical addresses are continuous

	 *

	 * 2) Their physical addresses are continuous

	 *    So truly compressed (physical size smaller than logical size)

	 *    extents won't get merged with each other

	 *

	 * 3) Share same flags except FIEMAP_EXTENT_LAST

	 *    So regular extent won't get merged with prealloc extent

	 */

	if (cache->offset + cache->len  == offset &&

	    cache->phys + cache->len == phys  &&

	    (cache->flags & ~FIEMAP_EXTENT_LAST) ==

			(flags & ~FIEMAP_EXTENT_LAST)) {

		cache->len += len;

		cache->flags |= flags;

		goto try_submit_last;

	}



	/* Not mergeable, need to submit cached one */

	ret = fiemap_fill_next_extent(fieinfo, cache->offset, cache->phys,

				      cache->len, cache->flags);

	cache->cached = false;

	if (ret)

		return ret;

assign:

	cache->cached = true;

	cache->offset = offset;

	cache->phys = phys;

	cache->len = len;

	cache->flags = flags;

try_submit_last:

	if (cache->flags & FIEMAP_EXTENT_LAST) {

		ret = fiemap_fill_next_extent(fieinfo, cache->offset,

				cache->phys, cache->len, cache->flags);

		cache->cached = false;

	}

	return ret;

}



/*

 * Sanity check for fiemap cache

 *

 * All fiemap cache should be submitted by emit_fiemap_extent()

 * Iteration should be terminated either by last fiemap extent or

 * fieinfo->fi_extents_max.

 * So no cached fiemap should exist.

 */

static int check_fiemap_cache(struct btrfs_fs_info *fs_info,

			       struct fiemap_extent_info *fieinfo,

			       struct fiemap_cache *cache)

{

	int ret;



	if (!cache->cached)

		return 0;



	/* Small and recoverbale problem, only to info developer */

#ifdef CONFIG_BTRFS_DEBUG

	WARN_ON(1);

#endif

	btrfs_warn(fs_info,

		   "unhandled fiemap cache detected: offset=%llu phys=%llu len=%llu flags=0x%x",

		   cache->offset, cache->phys, cache->len, cache->flags);

	ret = fiemap_fill_next_extent(fieinfo, cache->offset, cache->phys,

				      cache->len, cache->flags);

	cache->cached = false;

	if (ret > 0)

		ret = 0;

	return ret;

}



int extent_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo,

		__u64 start, __u64 len, get_extent_t *get_extent)

{
@@ -4394,6 +4511,7 @@ int extent_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo, 
	struct extent_state *cached_state = NULL;

	struct btrfs_path *path;

	struct btrfs_root *root = BTRFS_I(inode)->root;

	struct fiemap_cache cache = { 0 };

	int end = 0;

	u64 em_start = 0;

	u64 em_len = 0;
@@ -4573,7 +4691,7 @@ int extent_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo, 
			flags |= FIEMAP_EXTENT_LAST;

			end = 1;

		}

		ret = fiemap_fill_next_extent(fieinfo, em_start, disko,

		ret = emit_fiemap_extent(fieinfo, &cache, em_start, disko,

					   em_len, flags);

		if (ret) {

			if (ret == 1)
@@ -4582,6 +4700,8 @@ int extent_fiemap(struct inode *inode, struct fiemap_extent_info *fieinfo, 
		}

	}

out_free:

	if (!ret)

		ret = check_fiemap_cache(root->fs_info, fieinfo, &cache);

	free_extent_map(em);

out:

	btrfs_free_path(path);