Re: [RFC PATCH] madvise: make madvise_cold_or_pageout_pte_range() support large folio

[Yu Zhao]

On Sun, Jul 16, 2023 at 5:52 PM Yin, Fengwei <fengwei.yin@xxxxxxxxx> wrote:
>
> On 7/14/2023 11:41 PM, Yu Zhao wrote:
> > On Thu, Jul 13, 2023 at 11:57 PM Yin, Fengwei <fengwei.yin@xxxxxxxxx> 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)
> Definitely. As I replied to you, I will split the patch to two parts:
>   - just bug fixing. Include the filio_mapcount() -> folio_estimated_shares().

I'm onboard with this fix.

>     And using ptep_clear_flush_young_notify() to clear the young of PTEs.

This is another fix (if it's a real problem), hence a separate patch.

>   - refactor for large folio.

Minchan will look at the last two.

> Let me know if this is OK. Thanks.

SGTM. Thanks.