[PATCH 5.15 017/110] btrfs: dont start transaction for scrub if the fs is mounted read-only

Greg Kroah-Hartman
2022-02-07T11:55:48Z

From: Qu Wenruo <wqu@suse.com>

commit 2d192fc4c1abeb0d04d1c8cd54405ff4a0b0255b upstream.

[BUG]
The following super simple script would crash btrfs at unmount time, if
CONFIG_BTRFS_ASSERT() is set.

mkfs.btrfs -f $dev
mount $dev $mnt
xfs_io -f -c "pwrite 0 4k" $mnt/file
umount $mnt
mount -r ro $dev $mnt
btrfs scrub start -Br $mnt
umount $mnt

This will trigger the following ASSERT() introduced by commit
0a31daa4b602 ("btrfs: add assertion for empty list of transactions at
late stage of umount").

That patch is definitely not the cause, it just makes enough noise for
developers.

[CAUSE]
We will start transaction for the following call chain during scrub:

scrub_enumerate_chunks()
|- btrfs_inc_block_group_ro()
|- btrfs_join_transaction()

However for RO mount, there is no running transaction at all, thus
btrfs_join_transaction() will start a new transaction.

Furthermore, since it's read-only mount, btrfs_sync_fs() will not call
btrfs_commit_super() to commit the new but empty transaction.

And leads to the ASSERT().

The bug has been there for a long time. Only the new ASSERT() makes it
noisy enough to be noticed.

[FIX]
For read-only scrub on read-only mount, there is no need to start a
transaction nor to allocate new chunks in btrfs_inc_block_group_ro().

Just do extra read-only mount check in btrfs_inc_block_group_ro(), and
if it's read-only, skip all chunk allocation and go inc_block_group_ro()
directly.

CC: stable@vger.kernel.org # 5.4+
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/btrfs/block-group.c | 13 +++++++++++++
1 file changed, 13 insertions(+)

--- a/fs/btrfs/block-group.c
+++ b/fs/btrfs/block-group.c
@@ -2511,6 +2511,19 @@ int btrfs_inc_block_group_ro(struct btrf
int ret;
bool dirty_bg_running;

+ /*
+ * This can only happen when we are doing read-only scrub on read-only
+ * mount.
+ * In that case we should not start a new transaction on read-only fs.
+ * Thus here we skip all chunk allocations.
+ */
+ if (sb_rdonly(fs_info->sb)) {
+ mutex_lock(&fs_info->ro_block_group_mutex);
+ ret = inc_block_group_ro(cache, 0);
+ mutex_unlock(&fs_info->ro_block_group_mutex);
+ return ret;
+ }
+
do {
trans = btrfs_join_transaction(fs_info->extent_root);
if (IS_ERR(trans))