btrfs: ensure releasing squota reserve on head refs

A reservation goes through a 3 step lifetime:

- generated during delalloc
- released/counted by ordered_extent allocation
- freed by running delayed ref

That third step depends on must_insert_reserved on the head ref, so the
head ref with that field set owns the reservation. Once you prepare to
run the head ref, must_insert_reserved is unset, which means that
running the ref must free the reservation, whether or not it succeeds,
or else the reservation is leaked. That results in either a risk of
spurious ENOSPC if the fs stays writeable or a warning on unmount if it
is readonly.

The existing squota code was aware of these invariants, but missed a few
cases. Improve it by adding a helper function to use in the cleanup
paths and call it from the existing early returns in running delayed
refs. This also simplifies btrfs_record_squota_delta and struct
btrfs_quota_delta.

This fixes (or at least improves the reliability of) generic/475 with
"mkfs -O squota". On my machine, that test failed ~4/10 times without
this patch and passed 100/100 times with it.

Signed-off-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 11 insertions, 3 deletions

diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index daec90342dad..e46774e8f49f 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -4661,6 +4661,17 @@ void btrfs_qgroup_destroy_extent_records(struct btrfs_transaction *trans)
 	*root = RB_ROOT;
 }
 
+void btrfs_free_squota_rsv(struct btrfs_fs_info *fs_info, u64 root, u64 rsv_bytes)
+{
+	if (btrfs_qgroup_mode(fs_info) != BTRFS_QGROUP_MODE_SIMPLE)
+		return;
+
+	if (!is_fstree(root))
+		return;
+
+	btrfs_qgroup_free_refroot(fs_info, root, rsv_bytes, BTRFS_QGROUP_RSV_DATA);
+}
+
 int btrfs_record_squota_delta(struct btrfs_fs_info *fs_info,
 			      struct btrfs_squota_delta *delta)
 {
@@ -4705,8 +4716,5 @@ int btrfs_record_squota_delta(struct btrfs_fs_info *fs_info,
 
 out:
 	spin_unlock(&fs_info->qgroup_lock);
-	if (!ret && delta->rsv_bytes)
-		btrfs_qgroup_free_refroot(fs_info, root, delta->rsv_bytes,
-					  BTRFS_QGROUP_RSV_DATA);
 	return ret;
 }