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

[Ming Lei]

Hi Kevin,

On Wed, Nov 26, 2014 at 7:27 PM, Kevin Wolf <address@hidden> wrote:
> 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 <address@hidden>
>> Suggested-by: Paolo Bonzini <address@hidden>
>> Signed-off-by: Ming Lei <address@hidden>
>> ---
>>  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.
>

That is a good question.

>From fs/aio.c in linux kernel, io_submit_one() returns -EAGAIN when
either there isn't enough requests which are reserved in io_setup(), or
kmem_cache_alloc(GFP_KERNEL) returns NULL.

In the former case, it means I/O operation is in flight.

In the later case, it should be very difficult to trigger since GFP_KERNEL
allocation will wait for memory reclaiming.

So most of times, it is reasonable to resubmit in completion for
-EAGAIN.  When there is no pending I/O, we still can handle
the very unlikely case either by returning failure to caller or
try to submit in one BH. Does it make sense for you?

Thanks,
Ming Lei