The patch below does not apply to the 5.10-stable tree. If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to stable@vger.kernel.org.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 2d192fc4c1abeb0d04d1c8cd54405ff4a0b0255b Mon Sep 17 00:00:00 2001 From: Qu Wenruo wqu@suse.com Date: Thu, 16 Dec 2021 19:47:35 +0800 Subject: [PATCH] btrfs: don't start transaction for scrub if the fs is mounted read-only

[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

diff --git a/fs/btrfs/block-group.c b/fs/btrfs/block-group.c index 1db24e6d6d90..68feabc83a27 100644 --- a/fs/btrfs/block-group.c +++ b/fs/btrfs/block-group.c @@ -2544,6 +2544,19 @@ int btrfs_inc_block_group_ro(struct btrfs_block_group *cache, 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(root); if (IS_ERR(trans))