[PATCH v2 0/2] block/io: Update BSC only if want_zero is true

[PATCH v2 0/2] block/io: Update BSC only if want_zero is true

[Hanna Reitz]

Hi,

Cover letter from v1:
https://lists.nongnu.org/archive/html/qemu-block/2022-01/msg00314.html

In v2, I’ve added a comment to the new test explaining why we need to
pass --allocation-depth to qemu-nbd, as requested by Nir; and as he has
implied, using the short options is kind of silly because they are
anything but self-explanatory, so in v2, I’ve switched to exclusively
using long options for the qemu-nbd invocation.

(Also, added “Cc: qemu-stable” tag in patch 1.)

git-backport-diff against v1:

Key:
[----] : patches are identical
[####] : number of functional differences between upstream/downstream patch
[down] : patch is downstream-only
The flags [FC] indicate (F)unctional and (C)ontextual differences, respectively

001/2:[----] [--] 'block/io: Update BSC only if want_zero is true'
002/2:[0013] [FC] 'iotests/block-status-cache: New test'


Hanna Reitz (2):
  block/io: Update BSC only if want_zero is true
  iotests/block-status-cache: New test

 block/io.c                                    |   6 +-
 tests/qemu-iotests/tests/block-status-cache   | 139 ++++++++++++++++++
 .../qemu-iotests/tests/block-status-cache.out |   5 +
 3 files changed, 149 insertions(+), 1 deletion(-)
 create mode 100755 tests/qemu-iotests/tests/block-status-cache
 create mode 100644 tests/qemu-iotests/tests/block-status-cache.out

-- 
2.33.1

[PATCH v2 1/2] block/io: Update BSC only if want_zero is true

[Hanna Reitz]

We update the block-status cache whenever we get new information from a
bdrv_co_block_status() call to the block driver.  However, if we have
passed want_zero=false to that call, it may flag areas containing zeroes
as data, and so we would update the block-status cache with wrong
information.

Therefore, we should not update the cache with want_zero=false.

Reported-by: Nir Soffer <nsoffer@redhat.com>
Fixes: 0bc329fbb00 ("block: block-status cache for data regions")
Reviewed-by: Nir Soffer <nsoffer@redhat.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Hanna Reitz <hreitz@redhat.com>
---
 block/io.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/block/io.c b/block/io.c
index bb0a254def..4e4cb556c5 100644
--- a/block/io.c
+++ b/block/io.c
@@ -2497,8 +2497,12 @@ static int coroutine_fn bdrv_co_block_status(BlockDriverState *bs,
              * non-protocol nodes, and then it is never used.  However, filling
              * the cache requires an RCU update, so double check here to avoid
              * such an update if possible.
+             *
+             * Check want_zero, because we only want to update the cache when we
+             * have accurate information about what is zero and what is data.
              */
-            if (ret == (BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID) &&
+            if (want_zero &&
+                ret == (BDRV_BLOCK_DATA | BDRV_BLOCK_OFFSET_VALID) &&
                 QLIST_EMPTY(&bs->children))
             {
                 /*
-- 
2.33.1

[PATCH v2 2/2] iotests/block-status-cache: New test

[Hanna Reitz]

Add a new test to verify that want_zero=false block-status calls do not
pollute the block-status cache for want_zero=true calls.

We check want_zero=true calls and their results using `qemu-img map`
(over NBD), and want_zero=false calls also using `qemu-img map` over
NBD, but using the qemu:allocation-depth context.

(This test case cannot be integrated into nbd-qemu-allocation, because
that is a qcow2 test, and this is a raw test.)

Signed-off-by: Hanna Reitz <hreitz@redhat.com>
---
 tests/qemu-iotests/tests/block-status-cache   | 139 ++++++++++++++++++
 .../qemu-iotests/tests/block-status-cache.out |   5 +
 2 files changed, 144 insertions(+)
 create mode 100755 tests/qemu-iotests/tests/block-status-cache
 create mode 100644 tests/qemu-iotests/tests/block-status-cache.out

diff --git a/tests/qemu-iotests/tests/block-status-cache b/tests/qemu-iotests/tests/block-status-cache
new file mode 100755
index 0000000000..6fa10bb8f8
--- /dev/null
+++ b/tests/qemu-iotests/tests/block-status-cache
@@ -0,0 +1,139 @@
+#!/usr/bin/env python3
+# group: rw quick
+#
+# Test cases for the block-status cache.
+#
+# Copyright (C) 2022 Red Hat, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+
+import os
+import signal
+import iotests
+from iotests import qemu_img_create, qemu_img_pipe, qemu_nbd
+
+
+image_size = 1 * 1024 * 1024
+test_img = os.path.join(iotests.test_dir, 'test.img')
+
+nbd_pidfile = os.path.join(iotests.test_dir, 'nbd.pid')
+nbd_sock = os.path.join(iotests.sock_dir, 'nbd.sock')
+
+
+class TestBscWithNbd(iotests.QMPTestCase):
+    def setUp(self) -> None:
+        """Just create an empty image with a read-only NBD server on it"""
+        assert qemu_img_create('-f', iotests.imgfmt, test_img,
+                               str(image_size)) == 0
+
+        # Pass --allocation-depth to enable the qemu:allocation-depth context,
+        # which we are going to query to provoke a block-status inquiry with
+        # want_zero=false.
+        assert qemu_nbd(f'--socket={nbd_sock}',
+                        f'--format={iotests.imgfmt}',
+                        '--persistent',
+                        '--allocation-depth',
+                        '--read-only',
+                        f'--pid-file={nbd_pidfile}',
+                        test_img) \
+            == 0
+
+    def tearDown(self) -> None:
+        with open(nbd_pidfile, encoding='utf-8') as f:
+            pid = int(f.read())
+        os.kill(pid, signal.SIGTERM)
+        os.remove(nbd_pidfile)
+        os.remove(test_img)
+
+    def test_with_zero_bug(self) -> None:
+        """
+        Verify that the block-status cache is not corrupted by a
+        want_zero=false call.
+        We can provoke a want_zero=false call with `qemu-img map` over NBD with
+        x-dirty-bitmap=qemu:allocation-depth, so we first run a normal `map`
+        (which results in want_zero=true), then using said
+        qemu:allocation-depth context, and finally another normal `map` to
+        verify that the cache has not been corrupted.
+        """
+
+        nbd_img_opts = f'driver=nbd,server.type=unix,server.path={nbd_sock}'
+        nbd_img_opts_alloc_depth = nbd_img_opts + \
+            ',x-dirty-bitmap=qemu:allocation-depth'
+
+        # Normal map, results in want_zero=true.
+        # This will probably detect an allocated data sector first (qemu likes
+        # to allocate the first sector to facilitate alignment probing), and
+        # then the rest to be zero.  The BSC will thus contain (if anything)
+        # one range covering the first sector.
+        map_pre = qemu_img_pipe('map', '--output=json', '--image-opts',
+                                nbd_img_opts)
+
+        # qemu:allocation-depth maps for want_zero=false.
+        # want_zero=false should (with the file driver, which the server is
+        # using) report everything as data.  While this is sufficient for
+        # want_zero=false, this is nothing that should end up in the
+        # block-status cache.
+        # Due to a bug, this information did end up in the cache, though, and
+        # this would lead to wrong information being returned on subsequent
+        # want_zero=true calls.
+        #
+        # We need to run this map twice: On the first call, we probably still
+        # have the first sector in the cache, and so this will be served from
+        # the cache; and only the subsequent range will be queried from the
+        # block driver.  This subsequent range will then be entered into the
+        # cache.
+        # If we did a want_zero=true call at this point, we would thus get
+        # correct information: The first sector is not covered by the cache, so
+        # we would get fresh block-status information from the driver, which
+        # would return a data range, and this would then go into the cache,
+        # evicting the wrong range from the want_zero=false call before.
+        #
+        # Therefore, we need a second want_zero=false map to reproduce:
+        # Since the first sector is not in the cache, the query for its status
+        # will go to the driver, which will return a result that reports the
+        # whole image to be a single data area.  This result will then go into
+        # the cache, and so the cache will then report the whole image to
+        # contain data.
+        #
+        # Note that once the cache reports the whole image to contain data, any
+        # subsequent map operation will be served from the cache, and so we can
+        # never loop too many times here.
+        for _ in range(2):
+            # (Ignore the result, this is just to contaminate the cache)
+            qemu_img_pipe('map', '--output=json', '--image-opts',
+                          nbd_img_opts_alloc_depth)
+
+        # Now let's see whether the cache reports everything as data, or
+        # whether we get correct information (i.e. the same as we got on our
+        # first attempt).
+        map_post = qemu_img_pipe('map', '--output=json', '--image-opts',
+                                 nbd_img_opts)
+
+        if map_pre != map_post:
+            print('ERROR: Map information differs before and after querying ' +
+                  'qemu:allocation-depth')
+            print('Before:')
+            print(map_pre)
+            print('After:')
+            print(map_post)
+
+            self.fail("Map information differs")
+
+
+if __name__ == '__main__':
+    # The block-status cache only works on the protocol layer, so to test it,
+    # we can only use the raw format
+    iotests.main(supported_fmts=['raw'],
+                 supported_protocols=['file'])
diff --git a/tests/qemu-iotests/tests/block-status-cache.out b/tests/qemu-iotests/tests/block-status-cache.out
new file mode 100644
index 0000000000..ae1213e6f8
--- /dev/null
+++ b/tests/qemu-iotests/tests/block-status-cache.out
@@ -0,0 +1,5 @@
+.
+----------------------------------------------------------------------
+Ran 1 tests
+
+OK
-- 
2.33.1

Re: [PATCH v2 2/2] iotests/block-status-cache: New test

[Nir Soffer]

On Tue, Jan 18, 2022 at 7:12 PM Hanna Reitz <hreitz@redhat.com> wrote:
>
> Add a new test to verify that want_zero=false block-status calls do not
> pollute the block-status cache for want_zero=true calls.
>
> We check want_zero=true calls and their results using `qemu-img map`
> (over NBD), and want_zero=false calls also using `qemu-img map` over
> NBD, but using the qemu:allocation-depth context.
>
> (This test case cannot be integrated into nbd-qemu-allocation, because
> that is a qcow2 test, and this is a raw test.)
>
> Signed-off-by: Hanna Reitz <hreitz@redhat.com>
> ---
>  tests/qemu-iotests/tests/block-status-cache   | 139 ++++++++++++++++++
>  .../qemu-iotests/tests/block-status-cache.out |   5 +
>  2 files changed, 144 insertions(+)
>  create mode 100755 tests/qemu-iotests/tests/block-status-cache
>  create mode 100644 tests/qemu-iotests/tests/block-status-cache.out
>
> diff --git a/tests/qemu-iotests/tests/block-status-cache b/tests/qemu-iotests/tests/block-status-cache
> new file mode 100755
> index 0000000000..6fa10bb8f8
> --- /dev/null
> +++ b/tests/qemu-iotests/tests/block-status-cache
> @@ -0,0 +1,139 @@
> +#!/usr/bin/env python3
> +# group: rw quick
> +#
> +# Test cases for the block-status cache.
> +#
> +# Copyright (C) 2022 Red Hat, Inc.
> +#
> +# This program is free software; you can redistribute it and/or modify
> +# it under the terms of the GNU General Public License as published by
> +# the Free Software Foundation; either version 2 of the License, or
> +# (at your option) any later version.
> +#
> +# This program is distributed in the hope that it will be useful,
> +# but WITHOUT ANY WARRANTY; without even the implied warranty of
> +# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +# GNU General Public License for more details.
> +#
> +# You should have received a copy of the GNU General Public License
> +# along with this program.  If not, see <http://www.gnu.org/licenses/>.
> +#
> +
> +import os
> +import signal
> +import iotests
> +from iotests import qemu_img_create, qemu_img_pipe, qemu_nbd
> +
> +
> +image_size = 1 * 1024 * 1024
> +test_img = os.path.join(iotests.test_dir, 'test.img')
> +
> +nbd_pidfile = os.path.join(iotests.test_dir, 'nbd.pid')
> +nbd_sock = os.path.join(iotests.sock_dir, 'nbd.sock')
> +
> +
> +class TestBscWithNbd(iotests.QMPTestCase):
> +    def setUp(self) -> None:
> +        """Just create an empty image with a read-only NBD server on it"""
> +        assert qemu_img_create('-f', iotests.imgfmt, test_img,
> +                               str(image_size)) == 0
> +
> +        # Pass --allocation-depth to enable the qemu:allocation-depth context,
> +        # which we are going to query to provoke a block-status inquiry with
> +        # want_zero=false.
> +        assert qemu_nbd(f'--socket={nbd_sock}',
> +                        f'--format={iotests.imgfmt}',
> +                        '--persistent',
> +                        '--allocation-depth',
> +                        '--read-only',
> +                        f'--pid-file={nbd_pidfile}',
> +                        test_img) \
> +            == 0
> +
> +    def tearDown(self) -> None:
> +        with open(nbd_pidfile, encoding='utf-8') as f:
> +            pid = int(f.read())
> +        os.kill(pid, signal.SIGTERM)
> +        os.remove(nbd_pidfile)
> +        os.remove(test_img)
> +
> +    def test_with_zero_bug(self) -> None:
> +        """
> +        Verify that the block-status cache is not corrupted by a
> +        want_zero=false call.
> +        We can provoke a want_zero=false call with `qemu-img map` over NBD with
> +        x-dirty-bitmap=qemu:allocation-depth, so we first run a normal `map`
> +        (which results in want_zero=true), then using said
> +        qemu:allocation-depth context, and finally another normal `map` to
> +        verify that the cache has not been corrupted.
> +        """
> +
> +        nbd_img_opts = f'driver=nbd,server.type=unix,server.path={nbd_sock}'
> +        nbd_img_opts_alloc_depth = nbd_img_opts + \
> +            ',x-dirty-bitmap=qemu:allocation-depth'
> +
> +        # Normal map, results in want_zero=true.
> +        # This will probably detect an allocated data sector first (qemu likes
> +        # to allocate the first sector to facilitate alignment probing), and
> +        # then the rest to be zero.  The BSC will thus contain (if anything)
> +        # one range covering the first sector.
> +        map_pre = qemu_img_pipe('map', '--output=json', '--image-opts',
> +                                nbd_img_opts)
> +
> +        # qemu:allocation-depth maps for want_zero=false.
> +        # want_zero=false should (with the file driver, which the server is
> +        # using) report everything as data.  While this is sufficient for
> +        # want_zero=false, this is nothing that should end up in the
> +        # block-status cache.
> +        # Due to a bug, this information did end up in the cache, though, and
> +        # this would lead to wrong information being returned on subsequent
> +        # want_zero=true calls.
> +        #
> +        # We need to run this map twice: On the first call, we probably still
> +        # have the first sector in the cache, and so this will be served from
> +        # the cache; and only the subsequent range will be queried from the
> +        # block driver.  This subsequent range will then be entered into the
> +        # cache.
> +        # If we did a want_zero=true call at this point, we would thus get
> +        # correct information: The first sector is not covered by the cache, so
> +        # we would get fresh block-status information from the driver, which
> +        # would return a data range, and this would then go into the cache,
> +        # evicting the wrong range from the want_zero=false call before.
> +        #
> +        # Therefore, we need a second want_zero=false map to reproduce:
> +        # Since the first sector is not in the cache, the query for its status
> +        # will go to the driver, which will return a result that reports the
> +        # whole image to be a single data area.  This result will then go into
> +        # the cache, and so the cache will then report the whole image to
> +        # contain data.
> +        #
> +        # Note that once the cache reports the whole image to contain data, any
> +        # subsequent map operation will be served from the cache, and so we can
> +        # never loop too many times here.
> +        for _ in range(2):
> +            # (Ignore the result, this is just to contaminate the cache)
> +            qemu_img_pipe('map', '--output=json', '--image-opts',
> +                          nbd_img_opts_alloc_depth)
> +
> +        # Now let's see whether the cache reports everything as data, or
> +        # whether we get correct information (i.e. the same as we got on our
> +        # first attempt).
> +        map_post = qemu_img_pipe('map', '--output=json', '--image-opts',
> +                                 nbd_img_opts)
> +
> +        if map_pre != map_post:
> +            print('ERROR: Map information differs before and after querying ' +
> +                  'qemu:allocation-depth')
> +            print('Before:')
> +            print(map_pre)
> +            print('After:')
> +            print(map_post)
> +
> +            self.fail("Map information differs")
> +
> +
> +if __name__ == '__main__':
> +    # The block-status cache only works on the protocol layer, so to test it,
> +    # we can only use the raw format
> +    iotests.main(supported_fmts=['raw'],
> +                 supported_protocols=['file'])
> diff --git a/tests/qemu-iotests/tests/block-status-cache.out b/tests/qemu-iotests/tests/block-status-cache.out
> new file mode 100644
> index 0000000000..ae1213e6f8
> --- /dev/null
> +++ b/tests/qemu-iotests/tests/block-status-cache.out
> @@ -0,0 +1,5 @@
> +.
> +----------------------------------------------------------------------
> +Ran 1 tests
> +
> +OK
> --
> 2.33.1
>

The out file is not very useful, and even fragile - if the test framework
will change the output format, the test will fail. Ideally we depend only
on the relevant output of our tools, and using a different version of the test
framework on replacing it (e.g pytest) will not require modifying the out
files.

Regardless I would like to see this fix merged and this issue already
exists in other tests. Some tests in tests/ do have useful output that can
make debugging failures easier.

Reviewed-by: Nir Soffer <nsoffer@redhat.com>

Re: [PATCH v2 1/2] block/io: Update BSC only if want_zero is true

[Eric Blake]

On Tue, Jan 18, 2022 at 05:59:59PM +0100, Hanna Reitz wrote:
> We update the block-status cache whenever we get new information from a
> bdrv_co_block_status() call to the block driver.  However, if we have
> passed want_zero=false to that call, it may flag areas containing zeroes
> as data, and so we would update the block-status cache with wrong
> information.
> 
> Therefore, we should not update the cache with want_zero=false.
> 
> Reported-by: Nir Soffer <nsoffer@redhat.com>
> Fixes: 0bc329fbb00 ("block: block-status cache for data regions")
> Reviewed-by: Nir Soffer <nsoffer@redhat.com>
> Cc: qemu-stable@nongnu.org
> Signed-off-by: Hanna Reitz <hreitz@redhat.com>
> ---
>  block/io.c | 6 +++++-
>  1 file changed, 5 insertions(+), 1 deletion(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

Re: [PATCH v2 2/2] iotests/block-status-cache: New test

[Eric Blake]

On Tue, Jan 18, 2022 at 06:00:00PM +0100, Hanna Reitz wrote:
> Add a new test to verify that want_zero=false block-status calls do not
> pollute the block-status cache for want_zero=true calls.
> 
> We check want_zero=true calls and their results using `qemu-img map`
> (over NBD), and want_zero=false calls also using `qemu-img map` over
> NBD, but using the qemu:allocation-depth context.
> 
> (This test case cannot be integrated into nbd-qemu-allocation, because
> that is a qcow2 test, and this is a raw test.)
> 
> Signed-off-by: Hanna Reitz <hreitz@redhat.com>
> ---
>  tests/qemu-iotests/tests/block-status-cache   | 139 ++++++++++++++++++
>  .../qemu-iotests/tests/block-status-cache.out |   5 +
>  2 files changed, 144 insertions(+)
>  create mode 100755 tests/qemu-iotests/tests/block-status-cache
>  create mode 100644 tests/qemu-iotests/tests/block-status-cache.out

Reviewed-by: Eric Blake <eblake@redhat.com>

As this was detected as a regression using NBD, I'm happy to add this
to my NBD queue, and will send a pull request soon.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

Re: [PATCH v2 2/2] iotests/block-status-cache: New test

[Eric Blake]

On Fri, Jan 28, 2022 at 02:50:51PM -0600, Eric Blake wrote:
> On Tue, Jan 18, 2022 at 06:00:00PM +0100, Hanna Reitz wrote:
> > Add a new test to verify that want_zero=false block-status calls do not
> > pollute the block-status cache for want_zero=true calls.
> > 
> > We check want_zero=true calls and their results using `qemu-img map`
> > (over NBD), and want_zero=false calls also using `qemu-img map` over
> > NBD, but using the qemu:allocation-depth context.
> > 
> > (This test case cannot be integrated into nbd-qemu-allocation, because
> > that is a qcow2 test, and this is a raw test.)
> > 
> > Signed-off-by: Hanna Reitz <hreitz@redhat.com>
> > ---
> >  tests/qemu-iotests/tests/block-status-cache   | 139 ++++++++++++++++++
> >  .../qemu-iotests/tests/block-status-cache.out |   5 +
> >  2 files changed, 144 insertions(+)
> >  create mode 100755 tests/qemu-iotests/tests/block-status-cache
> >  create mode 100644 tests/qemu-iotests/tests/block-status-cache.out
> 
> Reviewed-by: Eric Blake <eblake@redhat.com>

I also tested that applying this patch but not 1/2 reproduces the
failure for me, so I'm adding:

Tested-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org