This is a note to let you know that I've just added the patch titled
btrfs: fix race between quota disable and relocation
to the 5.4-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-fix-race-between-quota-disable-and-relocation.patch
and it can be found in the queue-5.4 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 175a9a29703c734ac6cdc652deb0a3fda3444090
Author: Filipe Manana <fdmanana@xxxxxxxx>
Date: Mon Jun 19 17:21:50 2023 +0100
btrfs: fix race between quota disable and relocation
[ Upstream commit 8a4a0b2a3eaf75ca8854f856ef29690c12b2f531 ]
If we disable quotas while we have a relocation of a metadata block group
that has extents belonging to the quota root, we can cause the relocation
to fail with -ENOENT. This is because relocation builds backref nodes for
extents of the quota root and later needs to walk the backrefs and access
the quota root - however if in between a task disables quotas, it results
in deleting the quota root from the root tree (with btrfs_del_root(),
called from btrfs_quota_disable().
This can be sporadically triggered by test case btrfs/255 from fstests:
$ ./check btrfs/255
FSTYP -- btrfs
PLATFORM -- Linux/x86_64 debian0 6.4.0-rc6-btrfs-next-134+ #1 SMP PREEMPT_DYNAMIC Thu Jun 15 11:59:28 WEST 2023
MKFS_OPTIONS -- /dev/sdc
MOUNT_OPTIONS -- /dev/sdc /home/fdmanana/btrfs-tests/scratch_1
btrfs/255 6s ... _check_dmesg: something found in dmesg (see /home/fdmanana/git/hub/xfstests/results//btrfs/255.dmesg)
- output mismatch (see /home/fdmanana/git/hub/xfstests/results//btrfs/255.out.bad)
--- tests/btrfs/255.out 2023-03-02 21:47:53.876609426 +0000
+++ /home/fdmanana/git/hub/xfstests/results//btrfs/255.out.bad 2023-06-16 10:20:39.267563212 +0100
@@ -1,2 +1,4 @@
QA output created by 255
+ERROR: error during balancing '/home/fdmanana/btrfs-tests/scratch_1': No such file or directory
+There may be more info in syslog - try dmesg | tail
Silence is golden
...
(Run 'diff -u /home/fdmanana/git/hub/xfstests/tests/btrfs/255.out /home/fdmanana/git/hub/xfstests/results//btrfs/255.out.bad' to see the entire diff)
Ran: btrfs/255
Failures: btrfs/255
Failed 1 of 1 tests
To fix this make the quota disable operation take the cleaner mutex, as
relocation of a block group also takes this mutex. This is also what we
do when deleting a subvolume/snapshot, we take the cleaner mutex in the
cleaner kthread (at cleaner_kthread()) and then we call btrfs_del_root()
at btrfs_drop_snapshot() while under the protection of the cleaner mutex.
Fixes: bed92eae26cc ("Btrfs: qgroup implementation and prototypes")
CC: stable@xxxxxxxxxxxxxxx # 5.4+
Signed-off-by: Filipe Manana <fdmanana@xxxxxxxx>
Signed-off-by: David Sterba <dsterba@xxxxxxxx>
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index 7821bef061fe6..b6cce67520f04 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -1164,12 +1164,23 @@ int btrfs_quota_disable(struct btrfs_fs_info *fs_info)
int ret = 0;
/*
- * We need to have subvol_sem write locked, to prevent races between
- * concurrent tasks trying to disable quotas, because we will unlock
- * and relock qgroup_ioctl_lock across BTRFS_FS_QUOTA_ENABLED changes.
+ * We need to have subvol_sem write locked to prevent races with
+ * snapshot creation.
*/
lockdep_assert_held_write(&fs_info->subvol_sem);
+ /*
+ * Lock the cleaner mutex to prevent races with concurrent relocation,
+ * because relocation may be building backrefs for blocks of the quota
+ * root while we are deleting the root. This is like dropping fs roots
+ * of deleted snapshots/subvolumes, we need the same protection.
+ *
+ * This also prevents races between concurrent tasks trying to disable
+ * quotas, because we will unlock and relock qgroup_ioctl_lock across
+ * BTRFS_FS_QUOTA_ENABLED changes.
+ */
+ mutex_lock(&fs_info->cleaner_mutex);
+
mutex_lock(&fs_info->qgroup_ioctl_lock);
if (!fs_info->quota_root)
goto out;
@@ -1251,6 +1262,7 @@ int btrfs_quota_disable(struct btrfs_fs_info *fs_info)
btrfs_end_transaction(trans);
else if (trans)
ret = btrfs_end_transaction(trans);
+ mutex_unlock(&fs_info->cleaner_mutex);
return ret;
}
