Re: [PATCH RFC] mm: madvise: pageout: ignore references rather than clearing young

[Barry Song]

On Sat, Feb 24, 2024 at 7:24 AM Minchan Kim <minchan@xxxxxxxxxx> wrote:
>
> On Sat, Feb 24, 2024 at 11:20:36AM +1300, Barry Song wrote:
> > On Sat, Feb 24, 2024 at 11:09 AM Minchan Kim <minchan@xxxxxxxxxx> wrote:
> > >
> > > Hi Barry,
> > >
> > > On Fri, Feb 23, 2024 at 05:15:50PM +1300, Barry Song wrote:
> > > > From: Barry Song <v-songbaohua@xxxxxxxx>
> > > >
> > > > While doing MADV_PAGEOUT, the current code will clear PTE young
> > > > so that vmscan won't read young flags to allow the reclamation
> > > > of madvised folios to go ahead.
> > >
> > > Isn't it good to accelerate reclaiming? vmscan checks whether the
> > > page was accessed recenlty by the young bit from pte and if it is,
> > > it doesn't reclaim the page. Since we have cleared the young bit
> > > in pte in madvise_pageout, vmscan is likely to reclaim the page
> > > since it wouldn't see the ferencecd_ptes from folio_check_references.
> >
> > right, but the proposal is asking vmscan to skip the folio_check_references
> > if this is a PAGEOUT. so we remove both pte_clear_young and rmap
> > of folio_check_references.
> >
> > >
> > > Could you clarify if I miss something here?
> >
> > guest you missed we are skipping folio_check_references now.
> > we remove both, thus, make MADV_PAGEOUT 6% faster.
>
> This makes sense to me.
>
> Only concern was race with mlock during the reclaim but the race was already
> there for normal page reclaming. Thus, mlock would already handle it.

yes. in try_to_unmap_one(), mlock()'s vma is not reclaimed,
while (page_vma_mapped_walk(&pvmw)) {
      /* Unexpected PMD-mapped THP? */
      VM_BUG_ON_FOLIO(!pvmw.pte, folio);

      /*
       * If the folio is in an mlock()d vma, we must not swap it out.
       */
      if (!(flags & TTU_IGNORE_MLOCK) &&
               (vma->vm_flags & VM_LOCKED)) {
            /* Restore the mlock which got missed */
                 if (!folio_test_large(folio))
                           mlock_vma_folio(folio, vma);
                  page_vma_mapped_walk_done(&pvmw);
                  ret = false;
                  break;
  }

BTW，
Hi SeongJae,
I am not quite sure if damon also needs this, so I have kept damon as is by
setting ignore_references = false.  MADV_PAGEOUT is an explicit hint users
don't want the memory to be reclaimed, I don't know if it is true for damon as
well. If you have some comments, please chime in.

>
> Thanks.

Thanks
Barry