Re: [Qemu-devel] [PATCH v5 2/3] linux-aio: handling -EAGAIN for !s->io_q.plugged case

[Stefan Hajnoczi]

On Mon, Nov 24, 2014 at 10:29:38PM +0800, Ming Lei wrote:
> 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.
> 
> Suggested-by: Paolo Bonzini <address@hidden>
> Signed-off-by: Ming Lei <address@hidden>
> ---
>  block/linux-aio.c |   28 ++++++++++++++++++----------
>  1 file changed, 18 insertions(+), 10 deletions(-)
> 
> diff --git a/block/linux-aio.c b/block/linux-aio.c
> index 70312a4..0cb098d 100644
> --- a/block/linux-aio.c
> +++ b/block/linux-aio.c
> @@ -282,12 +282,17 @@ 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) {
> -        return ioq_submit(s);
> +    /*
> +     * 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.
> +     */
> +    if (!s->io_q.plugged || likely(idx < s->io_q.size * 2 / 3)) {
> +        return 0;
>      }
>  
> -    return 0;
> +    return ioq_submit(s);

Same as in the previous patch, the ioq_submit() return value is not
meaningful to ioq_enqueue() callers.  It should not be returned.

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