Re: [PATCH 07/11] vfs: add nowait parameter for file_accessed()

Dave Chinner
2023-09-03T22:31:05Z

On Wed, Aug 30, 2023 at 02:11:31PM +0800, Hao Xu wrote:
> On 8/29/23 19:53, Matthew Wilcox wrote:
> > On Tue, Aug 29, 2023 at 03:46:13PM +0800, Hao Xu wrote:
> > > On 8/28/23 05:32, Matthew Wilcox wrote:
> > > > On Sun, Aug 27, 2023 at 09:28:31PM +0800, Hao Xu wrote:
> > > > > From: Hao Xu <howeyxu@tencent.com>
> > > > >
> > > > > Add a boolean parameter for file_accessed() to support nowait semantics.
> > > > > Currently it is true only with io_uring as its initial caller.
> > > >
> > > > So why do we need to do this as part of this series? Apparently it
> > > > hasn't caused any problems for filemap_read().
> > > >
> > >
> > > We need this parameter to indicate if nowait semantics should be enforced in
> > > touch_atime(), There are locks and maybe IOs in it.
> >
> > That's not my point. We currently call file_accessed() and
> > touch_atime() for nowait reads and nowait writes. You haven't done
> > anything to fix those.
> >
> > I suspect you can trim this patchset down significantly by avoiding
> > fixing the file_accessed() problem. And then come back with a later
> > patchset that fixes it for all nowait i/o. Or do a separate prep series
>
> I'm ok to do that.
>
> > first that fixes it for the existing nowait users, and then a second
> > series to do all the directory stuff.
> >
> > I'd do the first thing. Just ignore the problem. Directory atime
> > updates cause I/O so rarely that you can afford to ignore it. Almost
> > everyone uses relatime or nodiratime.
>
> Hi Matthew,
> The previous discussion shows this does cause issues in real
> producations: https://lore.kernel.org/io-uring/2785f009-2ebb-028d-8250-d5f3a30510f0@gmail.com/#:~:text=fwiw%2C%20we%27ve%20just%20recently%20had%20similar%20problems%20with%20io_uring%20read/write
>

Then separate it out into it's own patch set so we can have a
discussion on the merits of requiring using noatime, relatime or
lazytime for really latency sensitive IO applications. Changing code
is not always the right solution...

-Dave.
--
Dave Chinner
david@fromorbit.com