Re: [PATCH v2 1/4] mm/filemap: Add folio_lock_timeout()

[Doug Anderson <dianders@chromium.org>]

Hi,

On Wed, Apr 26, 2023 at 2:27 PM Matthew Wilcox <willy@infradead.org> wrote:
>
> On Wed, Apr 26, 2023 at 01:46:58PM -0700, Doug Anderson wrote:
> > On Wed, Apr 26, 2023 at 8:14 AM Matthew Wilcox <willy@infradead.org> wrote:
> > >
> > > I'm not generally a fan of lock-with-timeout approaches.  I think the
> > > rationale for this one makes sense, but we're going to see some people
> > > try to use this for situations where it doesn't make sense.
> >
> > Although it won't completely prevent the issue, I could add a comment
>
> People don't read comments.

Agreed, it's just better than nothing...


> > > Hm.  If the problem is that we want to wait for the lock unless the
> > > lock is being held for I/O, we can actually tell that in the caller.
> > >
> > >         if (folio_test_uptodate(folio))
> > >                 folio_lock(folio);
> > >         else
> > >                 folio_trylock(folio);
> > >
> > > (the folio lock isn't held for writeback, just taken and released;
> > > if the folio is uptodate, the folio lock should only be taken for a
> > > short time; if it's !uptodate then it's probably being read)
> >
> > The current place in patch #3 where I'm using folio_lock_timeout()
> > only calls it if a folio_trylock() already failed [2]. So I guess the
> > idea would be that if the trylock failed and folio_test_uptodate()
> > returns 0 then we immediately fail, otherwise we call the unbounded
> > folio_trylock()?
>
> Looking at the actual code, here's what I'd do:
>
> +++ b/mm/migrate.c
> @@ -1156,6 +1156,14 @@ static int migrate_folio_unmap(new_page_t get_new_page, free_page_t put_new_page
>                 if (current->flags & PF_MEMALLOC)
>                         goto out;
>
> +               /*
> +                * In "light" mode, we can wait for transient locks (eg
> +                * inserting a page into the page table), but it's not
> +                * worth waiting for I/O.
> +                */
> +               if (mode == MIGRATE_SYNC_LIGHT && !folio_test_uptodate(folio))
> +                       goto out;
> +
>                 folio_lock(src);
>         }
>         locked = true;
>
> > I put some traces in and ran my test and it turns out that in every
> > case (except one) where the tre initial folio_trylock() failed I saw
> > folio_test_uptodate() return 0. Assuming my test case is typical, I
> > think that means that coding it with folio_test_uptodate() is roughly
> > the same as just never waiting at all for the folio lock in the
> > SYNC_LIGHT case. In the original discussion of my v1 patch people
> > didn't like that idea. ...so I think that for now I'm going to keep it
> > with the timeout flow.
>
> I think that means that your specific test is generally going to
> exercise the case where the lock is held because we're waiting for I/O.
> That's exactly what you set it up to produce, after all!  But it won't
> affect the cases where the folio lock is being held for other reasons,
> which your testcase is incredibly unlikely to produce.

Sure, I'm happy to do it like you say. Do you have any suggestions for
the similar lock_buffer() case, or are you OK w/ the timeout there?

Mel: do you have any comments? In your previous response [1] you
seemed to indicate that you thought that short waits for read were a
good idea.

[1] https://lore.kernel.org/r/20230420102304.7wdquge2b7r3xerj@techsingularity.net

-Doug