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qemu-img: Support bitmap --merge into backing image 

If you have the chain 'base.qcow2 <- top.qcow2' and want to merge a
bitmap from top into base, qemu-img was failing with:

qemu-img: Could not open 'top.qcow2': Could not open backing file: Failed to get shared "write" lock
Is another process using the image [base.qcow2]?

The easiest fix is to not open the entire backing chain of either
image (source or destination); after all, the point of 'qemu-img
bitmap' is solely to manipulate bitmaps directly within a single qcow2
image, and this is made more precise if we don't pay attention to
other images in the chain that may happen to have a bitmap by the same
name.

However, note that on a case-by-case analysis, there _are_ times where
we treat it as a feature that we can access a bitmap from a backing
layer in association with an overlay BDS.  A demonstration of this is
using NBD to expose both an overlay BDS (for constant contents) and a
bitmap (for learning which blocks are interesting) during an
incremental backup:

Base <- Active <- Temporary
          \--block job ->/

where Temporary is being fed by a backup 'sync=none' job.  When
exposing Temporary over NBD, referring to a bitmap that lives only in
Active is less effort than having to copy a bitmap into Temporary [1].
So the testsuite additions in this patch check both where bitmaps get
allocated (the qemu-img info output), and that qemu-nbd is indeed able
to access a bitmap inherited from the backing chain since it is a
different use case than 'qemu-img bitmap'.

[1] Full disclosure: prior to the recent commit 374eedd1c4 and
friends, we were NOT able to see bitmaps through filters, which meant
that we actually did not have nice clean semantics for uniformly being
able to pick up bitmaps from anywhere in the backing chain (seen as a
change in behavior between qemu 4.1 and 4.2 at commit 00e30f05de, when
block-copy swapped from a one-off to a filter).  Which means libvirt
was already coded to copy bitmaps around for the sake of older qemu,
even though modern qemu no longer needs it.  Oh well.

Fixes: http://bugzilla.redhat.com/1877209
Reported-by: Eyal Shenitzky <eshenitz@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20200914191009.644842-1-eblake@redhat.com>
[eblake: more commit message tweaks, per Max Reitz review]
Reviewed-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
stable-6.0
Eric Blake 2020-09-14 14:10:09 -05:00
parent 5df6c87e80
commit 14f16bf947
3 changed files with 76 additions and 3 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

11
qemu-img.c
View File

@ -4779,14 +4779,19 @@ static int img_bitmap(int argc, char **argv)
filename = argv[optind];
bitmap = argv[optind + 1];
blk = img_open(image_opts, filename, fmt, BDRV_O_RDWR, false, false,
false);
/*
* No need to open backing chains; we will be manipulating bitmaps
* directly in this image without reference to image contents.
*/
blk = img_open(image_opts, filename, fmt, BDRV_O_RDWR | BDRV_O_NO_BACKING,
false, false, false);
if (!blk) {
goto out;
}
bs = blk_bs(blk);
if (src_filename) {
src = img_open(false, src_filename, src_fmt, 0, false, false, false);
src = img_open(false, src_filename, src_fmt, BDRV_O_NO_BACKING,
false, false, false);
if (!src) {
goto out;
}

12
tests/qemu-iotests/291
View File

@ -91,6 +91,15 @@ $QEMU_IMG bitmap --remove --image-opts \
driver=$IMGFMT,file.driver=file,file.filename="$TEST_IMG" tmp
_img_info --format-specific
echo
echo "=== Merge from top layer into backing image ==="
echo
$QEMU_IMG rebase -u -F qcow2 -b "$TEST_IMG.base" "$TEST_IMG"
$QEMU_IMG bitmap --add --merge b2 -b "$TEST_IMG" -F $IMGFMT \
-f $IMGFMT "$TEST_IMG.base" b3
_img_info --format-specific --backing-chain
echo
echo "=== Check bitmap contents ==="
echo
@ -107,6 +116,9 @@ $QEMU_IMG map --output=json --image-opts \
nbd_server_start_unix_socket -r -f qcow2 -B b2 "$TEST_IMG"
$QEMU_IMG map --output=json --image-opts \
"$IMG,x-dirty-bitmap=qemu:dirty-bitmap:b2" | _filter_qemu_img_map
nbd_server_start_unix_socket -r -f qcow2 -B b3 "$TEST_IMG"
$QEMU_IMG map --output=json --image-opts \
"$IMG,x-dirty-bitmap=qemu:dirty-bitmap:b3" | _filter_qemu_img_map
# success, all done
echo '*** done'

56
tests/qemu-iotests/291.out
View File

@ -68,6 +68,59 @@ Format specific information:
corrupt: false
extended l2: false
=== Merge from top layer into backing image ===
image: TEST_DIR/t.IMGFMT
file format: IMGFMT
virtual size: 10 MiB (10485760 bytes)
cluster_size: 65536
backing file: TEST_DIR/t.IMGFMT.base
backing file format: IMGFMT
Format specific information:
compat: 1.1
compression type: zlib
lazy refcounts: false
bitmaps:
[0]:
flags:
name: b1
granularity: 524288
[1]:
flags:
[0]: auto
name: b2
granularity: 65536
[2]:
flags:
name: b0
granularity: 65536
refcount bits: 16
corrupt: false
extended l2: false
image: TEST_DIR/t.IMGFMT.base
file format: IMGFMT
virtual size: 10 MiB (10485760 bytes)
cluster_size: 65536
Format specific information:
compat: 1.1
compression type: zlib
lazy refcounts: false
bitmaps:
[0]:
flags:
[0]: auto
name: b0
granularity: 65536
[1]:
flags:
[0]: auto
name: b3
granularity: 65536
refcount bits: 16
corrupt: false
extended l2: false
=== Check bitmap contents ===
[{ "start": 0, "length": 3145728, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
@ -79,4 +132,7 @@ Format specific information:
[{ "start": 0, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
{ "start": 2097152, "length": 1048576, "depth": 0, "zero": false, "data": false},
{ "start": 3145728, "length": 7340032, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
[{ "start": 0, "length": 2097152, "depth": 0, "zero": false, "data": true, "offset": OFFSET},
{ "start": 2097152, "length": 1048576, "depth": 0, "zero": false, "data": false},
{ "start": 3145728, "length": 7340032, "depth": 0, "zero": false, "data": true, "offset": OFFSET}]
*** done