Re: [Qemu-devel] [PATCH] block/mirror: limit qiov to IOV_MAX elements

[Paolo Bonzini <pbonzini@redhat.com>]

On 01/07/2015 16:45, Stefan Hajnoczi wrote:
> 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>
> ---
>  block/mirror.c | 4 ++++
>  trace-events   | 1 +
>  2 files changed, 5 insertions(+)
> 
> diff --git a/block/mirror.c b/block/mirror.c
> index 048e452..985ad00 100644
> --- a/block/mirror.c
> +++ b/block/mirror.c
> @@ -241,6 +241,10 @@ 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) {

No Yoda conditions... apart from that,

Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>

> +            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);
> diff --git a/trace-events b/trace-events
> index 52b7efa..943cd0c 100644
> --- a/trace-events
> +++ b/trace-events
> @@ -94,6 +94,7 @@ mirror_yield(void *s, int64_t cnt, int buf_free_count, int in_flight) "s %p dirt
>  mirror_yield_in_flight(void *s, int64_t sector_num, int in_flight) "s %p sector_num %"PRId64" in_flight %d"
>  mirror_yield_buf_busy(void *s, int nb_chunks, int in_flight) "s %p requested chunks %d in_flight %d"
>  mirror_break_buf_busy(void *s, int nb_chunks, int in_flight) "s %p requested chunks %d in_flight %d"
> +mirror_break_iov_max(void *s, int nb_chunks, int added_chunks) "s %p requested chunks %d added_chunks %d"
>  
>  # block/backup.c
>  backup_do_cow_enter(void *job, int64_t start, int64_t sector_num, int nb_sectors) "job %p start %"PRId64" sector_num %"PRId64" nb_sectors %d"
>