Re: [PATCH] mm: Do not start/end writeback for pages stored in zswap

[Yosry Ahmed <yosryahmed@google.com>]

On Mon, Jun 10, 2024 at 11:11 AM Usama Arif <usamaarif642@gmail.com> wrote:
>
>
> On 10/06/2024 18:31, Yosry Ahmed wrote:
> > On Mon, Jun 10, 2024 at 7:31 AM Usama Arif <usamaarif642@gmail.com> wrote:
> >> start/end writeback combination incorrectly increments NR_WRITTEN
> >> counter, eventhough the pages aren't written to disk. Pages successfully
> >> stored in zswap should just unlock folio and return from writepage.
> >>
> >> Signed-off-by: Usama Arif <usamaarif642@gmail.com>
> >> ---
> >>   mm/page_io.c | 2 --
> >>   1 file changed, 2 deletions(-)
> >>
> >> diff --git a/mm/page_io.c b/mm/page_io.c
> >> index a360857cf75d..501784d79977 100644
> >> --- a/mm/page_io.c
> >> +++ b/mm/page_io.c
> >> @@ -196,9 +196,7 @@ int swap_writepage(struct page *page, struct writeback_control *wbc)
> >>                  return ret;
> >>          }
> >>          if (zswap_store(folio)) {
> >> -               folio_start_writeback(folio);
> >>                  folio_unlock(folio);
> >> -               folio_end_writeback(folio);
> > Removing these calls will have several effects, I am not really sure it's safe.
> >
> > 1. As you note in the commit log, NR_WRITTEN stats (and apparently
> > others) will no longer be updated. While this may make sense, it's a
> > user-visible change. I am not sure if anyone relies on this.
>
> Thanks for the review.
>
> This patch would correct NR_WRITTEN stat, so I think its a good thing?
> But yeah as you said for people relying on that stat, suddenly this
> number would be lowered if they pick up a kernel with this patch, not
> sure how such changes would be dealt with in the kernel.
>
> > 2. folio_end_writeback() calls folio_rotate_reclaimable() after
> > writeback completes to put a folio that has been marked with
> > PG_reclaim at the tail of the LRU, to be reclaimed first next time. Do
> > we get this call through other paths now?
>
> We could add
>
> if (folio_test_reclaim(folio)) {
>          folio_clear_reclaim(folio);
>          folio_rotate_reclaimable(folio);
>      }
>
> to if zswap_store is successful to fix this?
>
> > 3. If I remember correctly, there was some sort of state machine where
> > folios go from dirty to writeback to clean. I am not sure what happens
> > if we take the writeback phase out of the equation.
> >
> > Overall, the change seems like it will special case the folios written
> > to zswap vs. to disk further, and we may end up missing important
> > things (like folio_rotate_reclaimable()). I would like to see a much
> > stronger argument for why it is safe and justified tbh :)
> >
> The patch came about from zero page swap optimization series
> (https://lore.kernel.org/all/ZmcITDhdBzUGEHuY@casper.infradead.org/),
> where Matthew pointed out that NR_WRITTEN would be wrong with the way I
> was doing it.
>
> Overall, I thought it would be good to have consistency with how
> zeropages and zswap pages would be dealt with, and have a more correct
> NR_WRITTEN stat.
>
> In the next revision of the zero page patch, I will just add
> folio_rotate_reclaimable after folio_unlock if folio is zero filled.

I would wait until others weigh in here.

I am not sure we can just change the stat handling from under the
users, even if it is the right thing to do :/

I also think we need more analysis before we decide it's safe to
remove the writeback calls otherwise. I noticed
folio_rotate_reclaimable() on a quick look, but there may be other
problems. I am not very familiar with the dirty -> writeback -> clean
state machine.

What's the benefit of this patch beyond making the code (and stats)
make more sense semantically? It feels like a significant risk with
little reward to me.