[PATCH v2] common/btrfs: handle dmdust as mounted device in _btrfs_buffered_read_on_mirror()

[Qu Wenruo <wqu@suse.com>]

[BUG]
After commit ab41f0bddb73 ("common/btrfs: use _scratch_cycle_mount to
ensure all page caches are dropped"), the test case btrfs/143 can fail
like below:

 btrfs/143 6s ... [failed, exit status 1]- output mismatch (see ~/xfstests/results//btrfs/143.out.bad)
    --- tests/btrfs/143.out 2020-06-10 19:29:03.818519162 +0100
    +++ ~/xfstests/results//btrfs/143.out.bad 2023-06-19 17:04:00.575033899 +0100
    @@ -1,37 +1,6 @@
     QA output created by 143
     wrote 131072/131072 bytes
     XXX Bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
    -XXXXXXXX:  aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa
................
    -XXXXXXXX:  aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa
................
    -XXXXXXXX:  aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa
................
    -XXXXXXXX:  aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa aa
................

[CAUSE]
Test case btrfs/143 uses dm-dust device to emulate read errors, this
means we can not use _scratch_cycle_mount to cycle mount $SCRATCH_MNT.

As it would go mount $SCRATCH_DEV, not the dm-dust device to
$SCRATCH_MNT.
This prevents us to trigger read-repair (since no error would be hit)
thus fail the test.

[FIX]
Since we can mount whatever device at $SCRATCH_MNT, we can not use
_scratch_cycle_mount in this case.

Instead implement a small helper to grab the mounted device and its
mount options, and use the same device and mount options to cycle
$SCRATCH_MNT mount.

This would fix btrfs/143 and hopefully future test cases which use dm
devices.

Reported-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
---
Changelog:
v2:
- Use findmnt command to grab mount options and source device
---
 common/btrfs | 13 +++++++++++--
 1 file changed, 11 insertions(+), 2 deletions(-)

diff --git a/common/btrfs b/common/btrfs
index 175b33ae..0fec093d 100644
--- a/common/btrfs
+++ b/common/btrfs
@@ -601,8 +601,17 @@ _btrfs_buffered_read_on_mirror()
 	# The drop_caches doesn't seem to drop every pages on aarch64 with
 	# 64K page size.
 	# So here as another workaround, cycle mount the SCRATCH_MNT to ensure
-	# the cache are dropped.
-	_scratch_cycle_mount
+	# the cache are dropped, but we can not use _scratch_cycle_mount, as
+	# we may mount whatever dm device at SCRATCH_MNT.
+	# So here we grab the mounted block device and its mount options, then
+	# unmount and re-mount with the same device and options.
+	local dev=$(findmnt -n -T $SCRATCH_MNT -o SOURCE)
+	local opts=$(findmnt -n -T $SCRATCH_MNT -o OPTIONS)
+	if [ -z "$dev" -o -z "$opts" ]; then
+		_fail "failed to grab mount info of $SCRATCH_MNT"
+	fi
+	_scratch_unmount
+	_mount $dev -o $opts $SCRATCH_MNT
 	while [[ -z $( (( BASHPID % nr_mirrors == mirror )) &&
 		exec $XFS_IO_PROG \
 			-c "pread -b $size $offset $size" $file) ]]; do
-- 
2.39.0