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

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Date: Mon, 10 Jun 2024 12:08:04 -0700
From: Shakeel Butt <shakeel.butt@...ux.dev>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Usama Arif <usamaarif642@...il.com>, akpm@...ux-foundation.org, 
	willy@...radead.org, hannes@...xchg.org, nphamcs@...il.com, chengming.zhou@...ux.dev, 
	linux-mm@...ck.org, linux-kernel@...r.kernel.org, kernel-team@...a.com
Subject: Re: [PATCH] mm: Do not start/end writeback for pages stored in zswap

On Mon, Jun 10, 2024 at 10:31:36AM GMT, Yosry Ahmed wrote:
> On Mon, Jun 10, 2024 at 7:31 AM Usama Arif <usamaarif642@...il.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@...il.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.
> 

I couldn't imagine how this stat can be useful for the zswap case and I
don't see much risk in changing this stat behavior for such cases.

> 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?
> 

The folio_rotate_reclaimable() only makes sense for async writeback
pages i.e. not for zswap where we synchronously reclaim the page.

> 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.
> 

Is there really such a state machine? We only trigger writeback if the
page is dirty and we have cleared it. The only thing I can think of is
the behavior of the waiters on PG_locked bit but the window of
PG_writeback is so small that it seems like it does not matter.