[PATCH v2] btrfs: defrag: don't try to merge regular extents with preallocated extents

* [PATCH v2] btrfs: defrag: don't try to merge regular extents with preallocated extents
@ 2022-01-25  7:06 Qu Wenruo
  2022-01-25 10:26 ` Filipe Manana
  0 siblings, 1 reply; 2+ messages in thread
From: Qu Wenruo @ 2022-01-25  7:06 UTC (permalink / raw)
  To: linux-btrfs

[BUG]
With older kernels (before v5.16), btrfs will defrag preallocated extents.
While with newer kernels (v5.16 and newer) btrfs will not defrag
preallocated extents, but it will defrag the extent just before the
preallocated extent, even it's just a single sector.

This can be exposed by the following small script:

	mkfs.btrfs -f $dev > /dev/null

	mount $dev $mnt
	xfs_io -f -c "pwrite 0 4k" -c sync -c "falloc 4k 16K" $mnt/file
	xfs_io -c "fiemap -v" $mnt/file
	btrfs fi defrag $mnt/file
	sync
	xfs_io -c "fiemap -v" $mnt/file

The output looks like this on older kernels:

/mnt/btrfs/file:
 EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
   0: [0..7]:          26624..26631         8   0x0
   1: [8..39]:         26632..26663        32 0x801
/mnt/btrfs/file:
 EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
   0: [0..39]:         26664..26703        40   0x1

Which defrags the single sector along with the preallocated extent, and
replace them with an regular extent into a new location (caused by data
COW).
This wastes most of the data IO just for the preallocated range.

On the other hand, v5.16 is slightly better:

/mnt/btrfs/file:
 EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
   0: [0..7]:          26624..26631         8   0x0
   1: [8..39]:         26632..26663        32 0x801
/mnt/btrfs/file:
 EXT: FILE-OFFSET      BLOCK-RANGE      TOTAL FLAGS
   0: [0..7]:          26664..26671         8   0x0
   1: [8..39]:         26632..26663        32 0x801

The preallocated range is not defragged, but the sector before it still
gets defragged, which has no need for it.

[CAUSE]
One of the function reused by the old and new behavior is
defrag_check_next_extent(), it will determine if we should defrag
current extent by checking the next one.

It only checks if the next extent is a hole or inlined, but it doesn't
check if it's preallocated.

On the other hand, out of the function, both old and new kernel will
reject preallocated extents.

Such inconsistent behavior causes above behavior.

[FIX]
- Also check if next extent is preallocated
  If so, don't defrag current extent.

- Make extent size check consistent
  By passing @extent_thresh from the caller.

- Remove an ambiguous check based on physical address
  There used to be an check on physically adjacent and large enough file
  extents.
  That check is too specific, we already add proper size check.
  And for physically adjacent extents we may also want to merge them as
  that can reduce the number of extents.

This will reduce the IO caused by defrag ioctl and autodefrag.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
v2:
- Use @extent_thresh from caller to replace the harded coded threshold
  Now caller has full control over the extent threshold value.

- Remove the old ambiguous check based on physical address
  The original check is too specific, only reject extents which are
  physically adjacent, AND too large.
  Since we have correct size check now, and the physically adjacent check
  is not always a win.
  So remove the old check completely.

---
 fs/btrfs/ioctl.c | 36 ++++++++++++++++++++++++++++--------
 1 file changed, 28 insertions(+), 8 deletions(-)

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 91ba2efe9792..d19bb882c3c4 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -1049,23 +1049,43 @@ static struct extent_map *defrag_lookup_extent(struct inode *inode, u64 start,
 	return em;
 }
 
+/*
+ * Return if current extent @em is a good candidate for defrag.
+ *
+ * This is done by checking against the next extent after @em.
+ */
 static bool defrag_check_next_extent(struct inode *inode, struct extent_map *em,
-				     bool locked)
+				     u32 extent_thresh, bool locked)
 {
 	struct extent_map *next;
-	bool ret = true;
+	bool ret = false;
 
 	/* this is the last extent */
 	if (em->start + em->len >= i_size_read(inode))
-		return false;
+		return ret;
 
 	next = defrag_lookup_extent(inode, em->start + em->len, locked);
+	/* No next extent or a hole, no way to merge */
 	if (!next || next->block_start >= EXTENT_MAP_LAST_BYTE)
-		ret = false;
-	else if ((em->block_start + em->block_len == next->block_start) &&
-		 (em->block_len > SZ_128K && next->block_len > SZ_128K))
-		ret = false;
+		goto out;
 
+	/* Next extent is preallocated, no sense to defrag current extent */
+	if (test_bit(EXTENT_FLAG_PREALLOC, &next->flags))
+		goto out;
+
+	/* Next extent is already large enough */
+	if (next->len >= extent_thresh)
+		goto out;
+	/*
+	 * There used to be a check based on em->block_start and
+	 * next->block_start, but merging physically adjacent
+	 * extents still has its own benefit, like reduce the number
+	 * of extent items.
+	 * So here we don't reject physically adjacent extents, only
+	 * reject hole/preallocated or large enough extents.
+	 */
+	ret = true;
+out:
 	free_extent_map(next);
 	return ret;
 }
@@ -1225,7 +1245,7 @@ static int defrag_collect_targets(struct btrfs_inode *inode,
 			goto next;
 
 		next_mergeable = defrag_check_next_extent(&inode->vfs_inode, em,
-							  locked);
+							  extent_thresh, locked);
 		if (!next_mergeable) {
 			struct defrag_target_range *last;
 
-- 
2.34.1


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