Re: [PATCH v2 5/5] iotests/migrate-bitmaps-postcopy-test: cover postcopy with a return path

[Eric Blake <eblake@redhat.com>]

On Tue, Nov 10, 2026 at 07:32:57PM +0100, Denis V. Lunev wrote:
> From: Denis V. Lunev <den@openvz.org>
> 
> The test migrates through an exec: fifo, which has no return path, so
> the source never waits for the destination's package-loaded pong.
> libvirt enables return-path for every migration that is not tunnelled.
> 
> Add a case that migrates one bitmap over a unix socket with return-path,
> unthrottled, and waits for the source to enter postcopy-active, so that
> the bitmap really goes through postcopy, and then for both sides to
> complete. It times out when the destination answers the pong from the
> run BH while dirty_bitmap_load() holds its lock across stream reads.
> 
> The existing cases are not run with a return path instead. They
> throttle the source, which lets the run BH take the lock between
> sections, and with a return path the source notices the destination
> shutdown and resumes the VM, as each VM has its own image.
> 
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: Fabiano Rosas <farosas@suse.de>
> Cc: Juraj Marcin <jmarcin@redhat.com>
> Cc: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> Cc: Eric Blake <eblake@redhat.com>
> Cc: Kevin Wolf <kwolf@redhat.com>
> Cc: Hanna Reitz <hreitz@redhat.com>
> Cc: qemu-block@nongnu.org
> ---
>  .../tests/migrate-bitmaps-postcopy-test       | 39 +++++++++++++++++++
>  .../tests/migrate-bitmaps-postcopy-test.out   |  4 +-
>  2 files changed, 41 insertions(+), 2 deletions(-)

I'm having trouble getting this test to fail when applied in isolation
without the rest of the series.

I even tried amending your patch as follows (to force more migration
traffic, and ensure that postcopy is in the loop), but still could not
get it to fail:

diff --git i/tests/qemu-iotests/tests/migrate-bitmaps-postcopy-test w/tests/qemu-iotests/tests/migrate-bitmaps-postcopy-test
index a6e21dec0a0..7eb729716ff 100755
--- i/tests/qemu-iotests/tests/migrate-bitmaps-postcopy-test
+++ w/tests/qemu-iotests/tests/migrate-bitmaps-postcopy-test
@@ -303,8 +303,8 @@ class TestDirtyBitmapPostcopyMigration(iotests.QMPTestCase):
 # libvirt's configuration: the source waits for the destination's pong
 class TestDirtyBitmapPostcopyReturnPath(iotests.QMPTestCase):
     def setUp(self):
-        qemu_img('create', '-f', iotests.imgfmt, disk_a, '1G')
-        qemu_img('create', '-f', iotests.imgfmt, disk_b, '1G')
+        qemu_img('create', '-f', iotests.imgfmt, disk_a, '256G')
+        qemu_img('create', '-f', iotests.imgfmt, disk_b, '256G')
         self.vm_a = iotests.VM(path_suffix='a').add_drive(disk_a)
         self.vm_b = iotests.VM(path_suffix='b').add_drive(disk_b)
         self.vm_b.add_incoming('unix:' + mig_sock)
@@ -318,13 +318,17 @@ class TestDirtyBitmapPostcopyReturnPath(iotests.QMPTestCase):
         os.remove(disk_b)

     def test_postcopy_success(self):
-        self.vm_a.cmd('block-dirty-bitmap-add', node='drive0',
-                      name='bitmap0', persistent=True)
+        for i in range(15):
+            self.vm_a.cmd('block-dirty-bitmap-add', node='drive0',
+                          name=f'bitmap{i}', granularity=512, persistent=True)
+        self.vm_a.hmp_qemu_io('drive0', 'write 0 256M')

         caps = [{'capability': 'dirty-bitmaps', 'state': True},
+                {'capability': 'postcopy-ram', 'state': True},
                 {'capability': 'events', 'state': True},
                 {'capability': 'return-path', 'state': True}]
         self.vm_a.cmd('migrate-set-capabilities', capabilities=caps)
+        self.vm_a.cmd('migrate-set-parameters', max_bandwidth=16536)
         self.vm_b.cmd('migrate-set-capabilities', capabilities=caps)

         self.vm_a.cmd('migrate', uri='unix:' + mig_sock)
@@ -332,10 +336,11 @@ class TestDirtyBitmapPostcopyReturnPath(iotests.QMPTestCase):

         match = {'data': {'status': 'postcopy-active'}}
         self.vm_a.event_wait('MIGRATION', match=match)
+        self.vm_a.cmd('migrate-set-parameters', max_bandwidth=524288)
         match = {'data': {'status': 'completed'}}
         self.vm_b.event_wait('MIGRATION', match=match)
         self.vm_a.event_wait('MIGRATION', match=match)
-        check_bitmaps(self.vm_b, 1)
+        check_bitmaps(self.vm_b, 15)


 if __name__ == '__main__':


While it is indeed valuable to add a test covering libvirt's setup, I
was hoping that the test would also make it more obvious why the rest
of the series is necessary; if the test passes without the rest of the
series, it is not testing what you fixed.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org