block/mirror: limit qiov to IOV_MAX elements

If mirror has more free buffers than IOV_MAX, preadv(2)/pwritev(2)
EINVAL failures may be encountered.

It is possible to trigger this by setting granularity to a low value
like 8192.

This patch stops appending chunks once IOV_MAX is reached.

The spurious EINVAL failure can be reproduced with a qcow2 image file
and the following QMP invocation:

  qmp.command('drive-mirror', device='virtio0', target='/tmp/r7.s1',
              granularity=8192, sync='full', mode='absolute-paths',
              format='raw')

While the guest is running dd if=/dev/zero of=/var/tmp/foo oflag=direct
bs=4k.

Cc: Jeff Cody <jcody@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: 1435761950-26714-1-git-send-email-stefanha@redhat.com
Signed-off-by: Jeff Cody <jcody@redhat.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/block/mirror.c b/block/mirror.c
index fc4d8f5..0841964 100644
--- a/block/mirror.c
+++ b/block/mirror.c
@@ -245,6 +245,10 @@ static uint64_t coroutine_fn mirror_iteration(MirrorBlockJob *s)
             trace_mirror_break_buf_busy(s, nb_chunks, s->in_flight);
             break;
         }
+        if (IOV_MAX < nb_chunks + added_chunks) {
+            trace_mirror_break_iov_max(s, nb_chunks, added_chunks);
+            break;
+        }
 
         /* We have enough free space to copy these sectors.  */
         bitmap_set(s->in_flight_bitmap, next_chunk, added_chunks);