[PATCH v2] btrfs: use BTRFS_FS_UPDATE_UUID_TREE_GEN flag for UUID tree rescan check

[Dave Chen <davechen@synology.com>]

The UUID tree rescan check in open_ctree() compares
fs_info->generation with the superblock's uuid_tree_generation.
This comparison is not reliable because fs_info->generation is
bumped at transaction start time in join_transaction(), while
uuid_tree_generation is only updated at commit time via
update_super_roots().

Between the early BTRFS_FS_UPDATE_UUID_TREE_GEN flag check and the
late rescan decision, mount operations such as file orphan cleanup
from an unclean shutdown start transactions without committing
them. This advances fs_info->generation past uuid_tree_generation
and produces a false-positive mismatch.

Use the BTRFS_FS_UPDATE_UUID_TREE_GEN flag directly instead. The
flag was already set earlier in open_ctree() when the generations
were known to match, and accurately represents "UUID tree is up to
date" without being affected by subsequent transaction starts.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Dave Chen <davechen@synology.com>
Signed-off-by: Robbie Ko <robbieko@synology.com>
---
v2:
- Drop Fixes tag, no benchmark numbers to justify it (Filipe)
- Reword subject and commit message as a correctness improvement

 fs/btrfs/disk-io.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index 1b0eb246b7147..70357b12508d0 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -3674,7 +3674,7 @@ int __cold open_ctree(struct super_block *sb, struct btrfs_fs_devices *fs_device
 
 	if (fs_info->uuid_root &&
 	    (btrfs_test_opt(fs_info, RESCAN_UUID_TREE) ||
-	     fs_info->generation != btrfs_super_uuid_tree_generation(disk_super))) {
+	     !test_bit(BTRFS_FS_UPDATE_UUID_TREE_GEN, &fs_info->flags))) {
 		btrfs_info(fs_info, "checking UUID tree");
 		ret = btrfs_check_uuid_tree(fs_info);
 		if (ret) {
-- 
2.43.0


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