Re: [PATCH 4/8] mm/swap: Use PG_dropbehind instead of PG_reclaim

[Yosry Ahmed <yosryahmed@google.com>]

On Tue, Jan 14, 2025 at 12:12 AM Kirill A. Shutemov
<kirill.shutemov@linux.intel.com> wrote:
>
> On Mon, Jan 13, 2025 at 08:17:20AM -0800, Yosry Ahmed wrote:
> > On Mon, Jan 13, 2025 at 1:35 AM Kirill A. Shutemov
> > <kirill.shutemov@linux.intel.com> wrote:
> > >
> > > The recently introduced PG_dropbehind allows for freeing folios
> > > immediately after writeback. Unlike PG_reclaim, it does not need vmscan
> > > to be involved to get the folio freed.
> > >
> > > Instead of using folio_set_reclaim(), use folio_set_dropbehind() in
> > > lru_deactivate_file().
> > >
> > > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > > ---
> > >  mm/swap.c | 8 +-------
> > >  1 file changed, 1 insertion(+), 7 deletions(-)
> > >
> > > diff --git a/mm/swap.c b/mm/swap.c
> > > index fc8281ef4241..4eb33b4804a8 100644
> > > --- a/mm/swap.c
> > > +++ b/mm/swap.c
> > > @@ -562,14 +562,8 @@ static void lru_deactivate_file(struct lruvec *lruvec, struct folio *folio)
> > >         folio_clear_referenced(folio);
> > >
> > >         if (folio_test_writeback(folio) || folio_test_dirty(folio)) {
> > > -               /*
> > > -                * Setting the reclaim flag could race with
> > > -                * folio_end_writeback() and confuse readahead.  But the
> > > -                * race window is _really_ small and  it's not a critical
> > > -                * problem.
> > > -                */
> > >                 lruvec_add_folio(lruvec, folio);
> > > -               folio_set_reclaim(folio);
> > > +               folio_set_dropbehind(folio);
> > >         } else {
> > >                 /*
> > >                  * The folio's writeback ended while it was in the batch.
> >
> > Now there's a difference in behavior here depending on whether or not
> > the folio is under writeback (or will be written back soon). If it is,
> > we set PG_dropbehind to get it freed right after, but if writeback has
> > already ended we put it on the tail of the LRU to be freed later.
> >
> > It's a bit counterintuitive to me that folios with pending writeback
> > get freed faster than folios that completed their writeback already.
> > Am I missing something?
>
> Yeah, it is strange.
>
> I think we can drop the writeback/dirty check. Set PG_dropbehind and put
> the page on the tail of LRU unconditionally. The check was required to
> avoid confusion with PG_readahead.
>
> Comment above the function is not valid anymore.

My read is that we don't put dirty/writeback folios at the tail of the
LRU because they cannot be freed immediately and we want to give them
time to be written back before reclaim reaches them. So I don't think
we want to change that and always put the pages at the tail.

>
> But the folio that is still dirty under writeback will be freed faster as
> we get rid of the folio just after writeback is done while clean page can
> dangle on LRU for a while.

Yeah if we reuse PG_dropbehind then we cannot avoid
folio_end_writeback() freeing the folio faster than clean ones.

>
> I don't think we have any convenient place to free clean dropbehind page
> other than shrink_folio_list(). Or do we?

Not sure tbh. FWIW I am not saying it's necessarily a bad thing to
free dirty/writeback folios before clean ones when deactivated, it's
just strange and a behavioral change from today that I wanted to point
out. Perhaps that's the best we can do for now.

>
> Looking at shrink_folio_list(), I think we need to bypass page demotion
> for PG_dropbehind pages.
>
> --
>   Kiryl Shutsemau / Kirill A. Shutemov