qcow2: Flush metadata during read-only reopen

If lazy refcounts are enabled for a backing file, committing to this
backing file may leave it in a dirty state even if the commit succeeds.
The reason is that the bdrv_flush() call in bdrv_commit() doesn't flush
refcount updates with lazy refcounts enabled, and qcow2_reopen_prepare()
doesn't take care to flush metadata.

In order to fix this, this patch also fixes qcow2_mark_clean(), which
contains another ineffective bdrv_flush() call beause lazy refcounts are
disabled only afterwards. All existing callers of qcow2_mark_clean()
either don't modify refcounts or already flush manually, so that this
fixes only a latent, but not yet actually triggerable bug.

Another instance of the same problem is live snapshots. Again, a real
corruption is prevented by an explicit flush for non-read-only images in
external_snapshot_prepare(), but images using lazy refcounts stay dirty.

Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 20 insertions, 0 deletions

diff --git a/tests/qemu-iotests/039 b/tests/qemu-iotests/039
index 9b355c0977..b9cbe99560 100755
--- a/tests/qemu-iotests/039
+++ b/tests/qemu-iotests/039
@@ -131,6 +131,26 @@ ulimit -c "$old_ulimit"
 ./qcow2.py "$TEST_IMG" dump-header | grep incompatible_features
 _check_test_img
 
+echo
+echo "== Committing to a backing file with lazy_refcounts=on =="
+
+IMGOPTS="compat=1.1,lazy_refcounts=on"
+TEST_IMG="$TEST_IMG".base _make_test_img $size
+
+IMGOPTS="compat=1.1,lazy_refcounts=on,backing_file=$TEST_IMG.base"
+_make_test_img $size
+
+$QEMU_IO -c "write 0 512" "$TEST_IMG" | _filter_qemu_io
+$QEMU_IMG commit "$TEST_IMG"
+
+# The dirty bit must not be set
+./qcow2.py "$TEST_IMG" dump-header | grep incompatible_features
+./qcow2.py "$TEST_IMG".base dump-header | grep incompatible_features
+
+_check_test_img
+TEST_IMG="$TEST_IMG".base _check_test_img
+
+
 # success, all done
 echo "*** done"
 rm -f $seq.full