[PATCH v2] btrfs: avoid defragging extents whose next extents are not targets

[PATCH v2] btrfs: avoid defragging extents whose next extents are not targets

[Qu Wenruo]

[BUG]
There is a report that autodefrag is defragging single sector, which
is completely waste of IO, and no help for defragging:

   btrfs-cleaner-808 defrag_one_locked_range: root=256 ino=651122 start=0 len=4096

[CAUSE]
In defrag_collect_targets(), we check if the current range (A) can be merged
with next one (B).

If mergeable, we will add range A into target for defrag.

However there is a catch for autodefrag, when checking mergebility against
range B, we intentionally pass 0 as @newer_than, hoping to get a
higher chance to merge with the next extent.

But in next iteartion, range B will looked up by defrag_lookup_extent(),
with non-zero @newer_than.

And if range B is not really newer, it will rejected directly, causing
only range A being defragged, while we expect to defrag both range A and
B.

[FIX]
Since the root cause is the difference in check condition of
defrag_check_next_extent() and defrag_collect_targets(), we fix it by:

1. Pass @newer_than to defrag_check_next_extent()
2. Pass @extent_thresh to defrag_check_next_extent()

This makes the check between defrag_collect_targets() and
defrag_check_next_extent() more consistent.

While there is still some minor difference, the remaining checks are
focus on runtime flags like writeback/delalloc, which are mostly
transient and safe to be checked only in defrag_collect_targets().

Link: https://github.com/btrfs/linux/issues/423#issuecomment-1066981856
Cc: stable@vger.kernel.org # 5.16+
Signed-off-by: Qu Wenruo <wqu@suse.com>
---
Changelog:
v2:
- Add proper CC to stable
- Use Link: tag to replace Issue: tag
  As every developer/maintainer has their own btrfs tree, Issue: tag is
  really confusing
---
 fs/btrfs/ioctl.c | 21 +++++++++++++++------
 1 file changed, 15 insertions(+), 6 deletions(-)

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 3d3d6e2f110a..7d7520a2e281 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -1189,7 +1189,7 @@ static u32 get_extent_max_capacity(const struct extent_map *em)
 }
 
 static bool defrag_check_next_extent(struct inode *inode, struct extent_map *em,
-				     bool locked)
+				     u32 extent_thresh, u64 newer_than, bool locked)
 {
 	struct extent_map *next;
 	bool ret = false;
@@ -1199,11 +1199,13 @@ static bool defrag_check_next_extent(struct inode *inode, struct extent_map *em,
 		return false;
 
 	/*
-	 * We want to check if the next extent can be merged with the current
-	 * one, which can be an extent created in a past generation, so we pass
-	 * a minimum generation of 0 to defrag_lookup_extent().
+	 * Here we need to pass @newer_then when checking the next extent, or
+	 * we will hit a case we mark current extent for defrag, but the next
+	 * one will not be a target.
+	 * This will just cause extra IO without really reducing the fragments.
 	 */
-	next = defrag_lookup_extent(inode, em->start + em->len, 0, locked);
+	next = defrag_lookup_extent(inode, em->start + em->len, newer_than,
+				    locked);
 	/* No more em or hole */
 	if (!next || next->block_start >= EXTENT_MAP_LAST_BYTE)
 		goto out;
@@ -1215,6 +1217,13 @@ static bool defrag_check_next_extent(struct inode *inode, struct extent_map *em,
 	 */
 	if (next->len >= get_extent_max_capacity(em))
 		goto out;
+	/* Skip older extent */
+	if (next->generation < newer_than)
+		goto out;
+	/* Also check extent size */
+	if (next->len >= extent_thresh)
+		goto out;
+
 	ret = true;
 out:
 	free_extent_map(next);
@@ -1420,7 +1429,7 @@ static int defrag_collect_targets(struct btrfs_inode *inode,
 			goto next;
 
 		next_mergeable = defrag_check_next_extent(&inode->vfs_inode, em,
-							  locked);
+						extent_thresh, newer_than, locked);
 		if (!next_mergeable) {
 			struct defrag_target_range *last;
 
-- 
2.35.1

Re: [PATCH v2] btrfs: avoid defragging extents whose next extents are not targets

[David Sterba]

On Tue, Mar 15, 2022 at 07:28:05PM +0800, Qu Wenruo wrote:
> [BUG]
> There is a report that autodefrag is defragging single sector, which
> is completely waste of IO, and no help for defragging:
> 
>    btrfs-cleaner-808 defrag_one_locked_range: root=256 ino=651122 start=0 len=4096
> 
> [CAUSE]
> In defrag_collect_targets(), we check if the current range (A) can be merged
> with next one (B).
> 
> If mergeable, we will add range A into target for defrag.
> 
> However there is a catch for autodefrag, when checking mergebility against
> range B, we intentionally pass 0 as @newer_than, hoping to get a
> higher chance to merge with the next extent.
> 
> But in next iteartion, range B will looked up by defrag_lookup_extent(),
> with non-zero @newer_than.
> 
> And if range B is not really newer, it will rejected directly, causing
> only range A being defragged, while we expect to defrag both range A and
> B.
> 
> [FIX]
> Since the root cause is the difference in check condition of
> defrag_check_next_extent() and defrag_collect_targets(), we fix it by:
> 
> 1. Pass @newer_than to defrag_check_next_extent()
> 2. Pass @extent_thresh to defrag_check_next_extent()
> 
> This makes the check between defrag_collect_targets() and
> defrag_check_next_extent() more consistent.
> 
> While there is still some minor difference, the remaining checks are
> focus on runtime flags like writeback/delalloc, which are mostly
> transient and safe to be checked only in defrag_collect_targets().
> 
> Link: https://github.com/btrfs/linux/issues/423#issuecomment-1066981856
> Cc: stable@vger.kernel.org # 5.16+
> Signed-off-by: Qu Wenruo <wqu@suse.com>
> ---
> Changelog:
> v2:
> - Add proper CC to stable
> - Use Link: tag to replace Issue: tag
>   As every developer/maintainer has their own btrfs tree, Issue: tag is
>   really confusing

Added to misc-next, thanks.