From 75a36a7d3ea904cef2e5b56af0c58cc60dcf947a Mon Sep 17 00:00:00 2001 From: Qu Wenruo Date: Tue, 15 Mar 2022 19:28:05 +0800 Subject: [PATCH] btrfs: avoid defragging extents whose next extents are not targets [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 mergeability against range B, we intentionally pass 0 as @newer_than, hoping to get a higher chance to merge with the next extent. But in the next iteration, 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+ Reviewed-by: Filipe Manana Signed-off-by: Qu Wenruo Signed-off-by: David Sterba --- fs/btrfs/ioctl.c | 20 ++++++++++++++------ 1 file changed, 14 insertions(+), 6 deletions(-) diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c index 238cee5b5254..f46e71061942 100644 --- a/fs/btrfs/ioctl.c +++ b/fs/btrfs/ioctl.c @@ -1239,7 +1239,7 @@ 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; @@ -1249,11 +1249,12 @@ 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; @@ -1265,6 +1266,13 @@ 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); @@ -1470,7 +1478,7 @@ 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; -- 2.11.0