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qcow2: Flush metadata during read-only reopen

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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>
This commit is contained in:
Kevin Wolf 2014-04-03 13:47:50 +02:00
parent cbee81f6de
commit 4c2e5f8f46
3 changed files with 52 additions and 4 deletions
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20
tests/qemu-iotests/039
View file

@ -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

11
tests/qemu-iotests/039.out
View file

@ -54,4 +54,15 @@ wrote 512/512 bytes at offset 0
512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
incompatible_features 0x0
No errors were found on the image.
== Committing to a backing file with lazy_refcounts=on ==
Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=134217728
Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=134217728 backing_file='TEST_DIR/t.IMGFMT.base'
wrote 512/512 bytes at offset 0
512 bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
Image committed.
incompatible_features 0x0
incompatible_features 0x0
No errors were found on the image.
No errors were found on the image.
*** done