[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;