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 3:29 PM David Hildenbrand <david@redhat.com> wrote:
>
> On 25.04.24 09:27, Lance Yang wrote:
> > 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.
>
> Forgot to add: as long as the large mapcount patches are not upstream,
> folio_large_mapcount() would be expensive. So this patch can be added
> independent of the other stuff.

Thanks for clarifying!
Lance

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