Patch "btrfs: clean up our handling of refs == 0 in snapshot delete" has been added to the 6.6-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    btrfs: clean up our handling of refs == 0 in snapshot delete

to the 6.6-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     btrfs-clean-up-our-handling-of-refs-0-in-snapshot-de.patch
and it can be found in the queue-6.6 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 0bd6504e69a7e239b76deebdb1f55b82d271dabf
Author: Josef Bacik <josef@xxxxxxxxxxxxxx>
Date:   Tue May 7 14:12:13 2024 -0400

    btrfs: clean up our handling of refs == 0 in snapshot delete
    
    [ Upstream commit b8ccef048354074a548f108e51d0557d6adfd3a3 ]
    
    In reada we BUG_ON(refs == 0), which could be unkind since we aren't
    holding a lock on the extent leaf and thus could get a transient
    incorrect answer.  In walk_down_proc we also BUG_ON(refs == 0), which
    could happen if we have extent tree corruption.  Change that to return
    -EUCLEAN.  In do_walk_down() we catch this case and handle it correctly,
    however we return -EIO, which -EUCLEAN is a more appropriate error code.
    Finally in walk_up_proc we have the same BUG_ON(refs == 0), so convert
    that to proper error handling.  Also adjust the error message so we can
    actually do something with the information.
    
    Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
    Reviewed-by: David Sterba <dsterba@xxxxxxxx>
    Signed-off-by: David Sterba <dsterba@xxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 2d39481fb3ce..72851adc1fee 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -5085,7 +5085,15 @@ static noinline void reada_walk_down(struct btrfs_trans_handle *trans,
 		/* We don't care about errors in readahead. */
 		if (ret < 0)
 			continue;
-		BUG_ON(refs == 0);
+
+		/*
+		 * This could be racey, it's conceivable that we raced and end
+		 * up with a bogus refs count, if that's the case just skip, if
+		 * we are actually corrupt we will notice when we look up
+		 * everything again with our locks.
+		 */
+		if (refs == 0)
+			continue;
 
 		if (wc->stage == DROP_REFERENCE) {
 			if (refs == 1)
@@ -5152,7 +5160,11 @@ static noinline int walk_down_proc(struct btrfs_trans_handle *trans,
 		BUG_ON(ret == -ENOMEM);
 		if (ret)
 			return ret;
-		BUG_ON(wc->refs[level] == 0);
+		if (unlikely(wc->refs[level] == 0)) {
+			btrfs_err(fs_info, "bytenr %llu has 0 references, expect > 0",
+				  eb->start);
+			return -EUCLEAN;
+		}
 	}
 
 	if (wc->stage == DROP_REFERENCE) {
@@ -5286,8 +5298,9 @@ static noinline int do_walk_down(struct btrfs_trans_handle *trans,
 		goto out_unlock;
 
 	if (unlikely(wc->refs[level - 1] == 0)) {
-		btrfs_err(fs_info, "Missing references.");
-		ret = -EIO;
+		btrfs_err(fs_info, "bytenr %llu has 0 references, expect > 0",
+			  bytenr);
+		ret = -EUCLEAN;
 		goto out_unlock;
 	}
 	*lookup_info = 0;
@@ -5487,7 +5500,12 @@ static noinline int walk_up_proc(struct btrfs_trans_handle *trans,
 				path->locks[level] = 0;
 				return ret;
 			}
-			BUG_ON(wc->refs[level] == 0);
+			if (unlikely(wc->refs[level] == 0)) {
+				btrfs_tree_unlock_rw(eb, path->locks[level]);
+				btrfs_err(fs_info, "bytenr %llu has 0 references, expect > 0",
+					  eb->start);
+				return -EUCLEAN;
+			}
 			if (wc->refs[level] == 1) {
 				btrfs_tree_unlock_rw(eb, path->locks[level]);
 				path->locks[level] = 0;