linux-kernel - Re: [RFC PATCH] madvise: make madvise_cold_or_pageout_pte

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Date:   Fri, 14 Jul 2023 09:41:54 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     "Yin, Fengwei" <fengwei.yin@...el.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        akpm@...ux-foundation.org, willy@...radead.org, david@...hat.com,
        ryan.roberts@....com, shy828301@...il.com
Subject: Re: [RFC PATCH] madvise: make madvise_cold_or_pageout_pte_range()
 support large folio

On Thu, Jul 13, 2023 at 11:57 PM Yin, Fengwei <fengwei.yin@...el.com> wrote:
>
>
> >> -               if (pageout_anon_only_filter && !folio_test_anon(folio))
> >> +               /* Do not interfere with other mappings of this folio */
> >> +               if (folio_mapcount(folio) != 1)
> >>                         continue;
> >>
> >> -               VM_BUG_ON_FOLIO(folio_test_large(folio), folio);
> >> -
> >> -               if (pte_young(ptent)) {
> >> -                       ptent = ptep_get_and_clear_full(mm, addr, pte,
> >> -                                                       tlb->fullmm);
> >> -                       ptent = pte_mkold(ptent);
> >> -                       set_pte_at(mm, addr, pte, ptent);
> >> -                       tlb_remove_tlb_entry(tlb, pte, addr);
> >> -               }
> >> -
> >> -               /*
> >> -                * We are deactivating a folio for accelerating reclaiming.
> >> -                * VM couldn't reclaim the folio unless we clear PG_young.
> >> -                * As a side effect, it makes confuse idle-page tracking
> >> -                * because they will miss recent referenced history.
> >> -                */
> >> -               folio_clear_referenced(folio);
> >> -               folio_test_clear_young(folio);
> >> -               if (folio_test_active(folio))
> >> -                       folio_set_workingset(folio);
> >> +pageout_cold_folio:
> >>                 if (pageout) {
> >>                         if (folio_isolate_lru(folio)) {
> >>                                 if (folio_test_unevictable(folio))
> >> @@ -529,8 +542,30 @@ static int madvise_cold_or_pageout_pte_range(pmd_t *pmd,
> >>                 arch_leave_lazy_mmu_mode();
> >>                 pte_unmap_unlock(start_pte, ptl);
> >>         }
> >> -       if (pageout)
> >> -               reclaim_pages(&folio_list);
> >> +
> >> +       if (pageout) {
> >> +               LIST_HEAD(reclaim_list);
> >> +
> >> +               while (!list_empty(&folio_list)) {
> >> +                       int refs;
> >> +                       unsigned long flags;
> >> +                       struct mem_cgroup *memcg = folio_memcg(folio);
> >> +
> >> +                       folio = lru_to_folio(&folio_list);
> >> +                       list_del(&folio->lru);
> >> +
> >> +                       refs = folio_referenced(folio, 0, memcg, &flags);
> >> +
> >> +                       if ((flags & VM_LOCKED) || (refs == -1)) {
> >> +                               folio_putback_lru(folio);
> >> +                               continue;
> >> +                       }
> >> +
> >> +                       folio_test_clear_referenced(folio);
> >> +                       list_add(&folio->lru, &reclaim_list);
> >> +               }
> >> +               reclaim_pages(&reclaim_list);
> >> +       }
> >
> > i overlooked the chunk above -- it's unnecessary: after we split the
> > large folio (and splice the base folios onto the same LRU list), we
> > continue at the position of the first base folio because of:
> >
> >   pte--;
> >   addr -= PAGE_SIZE;
> >   continue;
> >
> > And then we do pte_mkold(), which takes care of the A-bit.
> This patch moves the A-bit clear out of the folio isolation loop. So
> even the folio is split and loop restarts from the first base folio,
> the A-bit is not cleared. A-bit is only cleared in reclaim loop.
>
> There is one option for A-bit clearing:
>   - clear A-bit of base 4K page in isolation loop and leave large folio
>     A-bit clearing to reclaim loop.
>
> This patch didn't use it because don't want to introduce A-bit clearing
> in two places. But I am open about clearing base 4K page A-bit cleared in
> isolation loop. Thanks.

Sorry but why are we trying to do multiple things in one patch that I
assumed is supposed to simply fix madvise() for large anon folios? And
none of those things seems to have a clear rationale behind it.

The only patch that makes sense at the moment (or the first patch of a
series) is what I said before:

-  if (folio_mapcount(folio) != 1)
+  if (folio_estimated_sharers(folio) != 1)

And probably clarify (preferrably in the comments above) this is an
estimate because we think it's a better tradeoff if we do so (less
code/overhead from checking the mapcounts of the rest of folios within
the range).