Re: [PATCH v3] mm/rmap: do not add fully unmapped large folio to deferred split list

[Lance Yang <ioworker0@gmail.com>]

On Thu, Apr 25, 2024 at 3:21 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 25.04.24 05:45, Lance Yang wrote:
> > Hey Zi,
> >
> > On Thu, Apr 25, 2024 at 6:46 AM Zi Yan <zi.yan@sent.com> wrote:
> >>
> >> From: Zi Yan <ziy@nvidia.com>
> >>
> >> In __folio_remove_rmap(), a large folio is added to deferred split list
> >> if any page in a folio loses its final mapping. It is possible that
> >> the folio is unmapped fully, but it is unnecessary to add the folio
> >
> > Agreed. If a folio is fully unmapped, then that's unnecessary to add
> > to the deferred split list.
> >
> >> to deferred split list at all. Fix it by checking folio->_nr_pages_mapped
> >> before adding a folio to deferred split list. If the folio is already
> >> on the deferred split list, it will be skipped. This issue applies to
> >> both PTE-mapped THP and mTHP.
> >>
> >> Commit 98046944a159 ("mm: huge_memory: add the missing
> >> folio_test_pmd_mappable() for THP split statistics") tried to exclude
> >> mTHP deferred split stats from THP_DEFERRED_SPLIT_PAGE, but it does not
> >> fix the above issue. A fully unmapped PTE-mapped order-9 THP was still
> >> added to deferred split list and counted as THP_DEFERRED_SPLIT_PAGE,
> >> since nr is 512 (non zero), level is RMAP_LEVEL_PTE, and inside
> >> deferred_split_folio() the order-9 folio is folio_test_pmd_mappable().
> >> However, this miscount was present even earlier due to implementation,
> >> since PTEs are unmapped individually and first PTE unmapping adds the THP
> >> into the deferred split list.
> >>
> >> With commit b06dc281aa99 ("mm/rmap: introduce
> >> folio_remove_rmap_[pte|ptes|pmd]()"), kernel is able to unmap PTE-mapped
> >> folios in one shot without causing the miscount, hence this patch.
> >>
> >> Signed-off-by: Zi Yan <ziy@nvidia.com>
> >> Reviewed-by: Yang Shi <shy828301@gmail.com>
> >> ---
> >>   mm/rmap.c | 7 ++++---
> >>   1 file changed, 4 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/mm/rmap.c b/mm/rmap.c
> >> index a7913a454028..2809348add7b 100644
> >> --- a/mm/rmap.c
> >> +++ b/mm/rmap.c
> >> @@ -1553,9 +1553,10 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
> >>                   * page of the folio is unmapped and at least one page
> >>                   * is still mapped.
> >>                   */
> >> -               if (folio_test_large(folio) && folio_test_anon(folio))
> >> -                       if (level == RMAP_LEVEL_PTE || nr < nr_pmdmapped)
> >> -                               deferred_split_folio(folio);
> >> +               if (folio_test_large(folio) && folio_test_anon(folio) &&
> >> +                   ((level == RMAP_LEVEL_PTE && atomic_read(mapped)) ||
> >> +                    (level == RMAP_LEVEL_PMD && nr < nr_pmdmapped)))
> >
> > Perhaps we only need to check the mapcount?
> >
> > IIUC, if a large folio that was PMD/PTE mapped is fully unmapped here,
> > then folio_mapcount() will return 0.
>
> See discussion on v1. folio_large_mapcount() would achieve the same
> without another folio_test_large() check, but in the context of this
> patch it doesn't really matter.

Got it. Thanks for pointing that out!
I'll take a closer look at the discussion in v1.

Thanks,
Lance


>
> --
> Cheers,
>
> David / dhildenb
>