Re: [RFC] xfs: reduce sub-block DIO serialisation

Dave Chinner
2021-01-13T20:38:55Z

On Wed, Jan 13, 2021 at 10:00:37AM +0200, Avi Kivity wrote:
> On 1/13/21 12:13 AM, Dave Chinner wrote:
> > On Tue, Jan 12, 2021 at 10:01:35AM +0200, Avi Kivity wrote:
> > > On 1/12/21 3:07 AM, Dave Chinner wrote:
> > > > Hi folks,
> > > >
> > > > This is the XFS implementation on the sub-block DIO optimisations
> > > > for written extents that I've mentioned on #xfs and a couple of
> > > > times now on the XFS mailing list.
> > > >
> > > > It takes the approach of using the IOMAP_NOWAIT non-blocking
> > > > IO submission infrastructure to optimistically dispatch sub-block
> > > > DIO without exclusive locking. If the extent mapping callback
> > > > decides that it can't do the unaligned IO without extent
> > > > manipulation, sub-block zeroing, blocking or splitting the IO into
> > > > multiple parts, it aborts the IO with -EAGAIN. This allows the high
> > > > level filesystem code to then take exclusive locks and resubmit the
> > > > IO once it has guaranteed no other IO is in progress on the inode
> > > > (the current implementation).
> > >
> > > Can you expand on the no-splitting requirement? Does it involve only
> > > splitting by XFS (IO spans >1 extents) or lower layers (RAID)?
> > XFS only.
>
>
> Ok, that is somewhat under control as I can provide an extent hint, and wish
> really hard that the filesystem isn't fragmented.
>
>
> > > The reason I'm concerned is that it's the constraint that the application
> > > has least control over. I guess I could use RWF_NOWAIT to avoid blocking my
> > > main thread (but last time I tried I'd get occasional EIOs that frightened
> > > me off that).
> > Spurious EIO from RWF_NOWAIT is a bug that needs to be fixed. DO you
> > have any details?
> >
>
> I reported it in [1]. It's long since gone since I disabled RWF_NOWAIT. It
> was relatively rare, sometimes happening in continuous integration runs that
> take hours, and sometimes not.
>
>
> I expect it's fixed by now since io_uring relies on it. Maybe I should turn
> it on for kernels > some_random_version.
>
>
> [1] https://lore.kernel.org/lkml/9bab0f40-5748-f147-efeb-5aac4fd44533@scylladb.com/t/#u

Yeah, as I thought. Usage of REQ_NOWAIT with filesystem based IO is
simply broken - it causes spurious IO failures to be reported to IO
completion callbacks and so are very difficult to track and/or
retry. iomap does not use REQ_NOWAIT at all, so you should not ever
see this from XFS or ext4 DIO anymore...

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com