Re: [Qemu-devel] [PATCH v6 2/3] linux-aio: handling -EAGAIN for !s->io_q.plugged case

[Kevin Wolf <kwolf@redhat.com>]

Am 25.11.2014 um 08:23 hat Ming Lei geschrieben:
> Previously -EAGAIN is simply ignored for !s->io_q.plugged case,
> and sometimes it is easy to cause -EIO to VM, such as NVME device.
> 
> This patch handles -EAGAIN by io queue for !s->io_q.plugged case,
> and it will be retried in following aio completion cb.
> 
> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
> Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Ming Lei <ming.lei@canonical.com>
> ---
>  block/linux-aio.c |   24 ++++++++++++++++--------
>  1 file changed, 16 insertions(+), 8 deletions(-)
> 
> diff --git a/block/linux-aio.c b/block/linux-aio.c
> index 11ac828..ac25722 100644
> --- a/block/linux-aio.c
> +++ b/block/linux-aio.c
> @@ -282,8 +282,13 @@ static int ioq_enqueue(struct qemu_laio_state *s, struct iocb *iocb)
>      s->io_q.iocbs[idx++] = iocb;
>      s->io_q.idx = idx;
>  
> -    /* submit immediately if queue depth is above 2/3 */
> -    if (idx > s->io_q.size * 2 / 3) {
> +    /*
> +     * This is reached in two cases: queue not plugged but io_submit
> +     * returned -EAGAIN, or queue plugged.  In the latter case, start
> +     * submitting some I/O if the queue is getting too full.  In the
> +     * former case, instead, wait until an I/O operation is completed.
> +     */

Are we guaranteed that an I/O operation is in flight when we get
-EAGAIN? The manpage of io_submit isn't very clear on this,
"insufficient resources" could be for any reason.

Because otherwise we might not ever submit this request.

> +    if (s->io_q.plugged && unlikely(idx > s->io_q.size * 2 / 3)) {
>          ioq_submit(s);
>      }

Kevin