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:
- Pass @newer_than to defrag_check_next_extent()
- 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.