Re: [PATCH v7 15/15] xfs: Add async buffered write support

Darrick J. Wong
2022-07-01T18:05:34Z

On Fri, Jul 01, 2022 at 08:38:07AM -0600, Jens Axboe wrote:
> On 7/1/22 8:30 AM, Jens Axboe wrote:
> > On 7/1/22 8:19 AM, Jens Axboe wrote:
> >> On 6/30/22 10:39 PM, Al Viro wrote:
> >>> On Wed, Jun 01, 2022 at 02:01:41PM -0700, Stefan Roesch wrote:
> >>>> This adds the async buffered write support to XFS. For async buffered
> >>>> write requests, the request will return -EAGAIN if the ilock cannot be
> >>>> obtained immediately.
> >>>
> >>> breaks generic/471...
> >>
> >> That test case is odd, because it makes some weird assumptions about
> >> what RWF_NOWAIT means. Most notably that it makes it mean if we should
> >> instantiate blocks or not. Where did those assumed semantics come from?
> >> On the read side, we have clearly documented that it should "not wait
> >> for data which is not immediately available".
> >>
> >> Now it is possible that we're returning a spurious -EAGAIN here when we
> >> should not be. And that would be a bug imho. I'll dig in and see what's
> >> going on.
> >
> > This is the timestamp update that needs doing which will now return
> > -EAGAIN if IOCB_NOWAIT is set as it may block.
> >
> > I do wonder if we should just allow inode time updates with IOCB_NOWAIT,
> > even on the io_uring side. Either that, or passed in RWF_NOWAIT
> > semantics don't map completely to internal IOCB_NOWAIT semantics. At
> > least in terms of what generic/471 is doing, but I'm not sure who came
> > up with that and if it's established semantics or just some made up ones
> > from whomever wrote that test. I don't think they make any sense, to be
> > honest.
>
> Further support that generic/471 is just randomly made up semantics,
> it needs to special case btrfs with nocow or you'd get -EAGAIN anyway
> for that test.
>
> And it's relying on some random timing to see if this works. I really
> think that test case is just hot garbage, and doesn't test anything
> meaningful.

<shrug> I had thought that NOWAIT means "don't wait for *any*thing",
which would include timestamp updates... but then I've never been all
that clear on what specifically NOWAIT will and won't wait for. :/

--D

> --
> Jens Axboe
>