Hi Kevin,
On Wed, Nov 26, 2014 at 7:27 PM, Kevin Wolf <kw...@redhat.com> 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 <pbonz...@redhat.com>
>> Suggested-by: Paolo Bonzini <pbonz...@redhat.com>
>> Signed-off-by: Ming Lei <ming....@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.
>
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
