Re: Spurious EIO on AIO+DIO+RWF_NOWAIT

[Goldwyn Rodrigues]

On 14:05 12/12, Avi Kivity wrote:
> On 12/10/18 2:48 PM, Goldwyn Rodrigues wrote:
> > On 13:19 09/12, Avi Kivity wrote:
> > > I have an application that receives spurious EIO when running with
> > > RWF_NOWAIT enabled. Removing RWF_NOWAIT causes those EIOs to disappear. The
> > > application uses AIO+DIO, and errors were seen on both xfs and ext4.
> > > 
> > > 
> > > I suspect the following code:
> > > 
> > > 
> > > /*
> > >   * Process one completed BIO.  No locks are held.
> > >   */
> > > static blk_status_t dio_bio_complete(struct dio *dio, struct bio *bio)
> > > {
> > >          struct bio_vec *bvec;
> > >          unsigned i;
> > >          blk_status_t err = bio->bi_status;
> > > 
> > >          if (err) {
> > >                  if (err == BLK_STS_AGAIN && (bio->bi_opf & REQ_NOWAIT))
> > >                          dio->io_error = -EAGAIN;
> > >                  else
> > >                          dio->io_error = -EIO;
> > >          }
> > > 
> > > Could it be that REQ_NOWAIT was dropped from bio->bi_opf? or that
> > > bio->bi_status got changed along the way?
> > > 
> > I don't think REQ_NOWAIT is dropped. I am assuming bio->bi_status error
> > is set differently. Is the blk queue being stopped? Is it possible to
> > instrument the kernel in your testcase?
> > 
> 
> I traced the function, and I see bio->bi_status == BLK_STS_NOTSUPP and
> bio->bi_opf == REQ_OP_WRITE|REQ_SYNC|REQ_NOMERGE|REQ_FUA|REQ_NOWAIT.
> Presumably the NOTSUPP is the result of NOWAIT not being supported down the
> stack, but shouldn't it be detected earlier? And not converted to EIO?
> 

I don't think there is a way to detect it earlier. However, I think we should
return -EOPNOTSUPP if the lower layers do not support REQ_NOWAIT. I will write
a patch to modify this.

-- 
Goldwyn