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

[Yang Shi <shy828301@gmail.com>]

On Thu, Apr 25, 2024 at 7:53 AM Zi Yan <ziy@nvidia.com> wrote:
>
> On 25 Apr 2024, at 3:19, David Hildenbrand wrote:
>
> > On 25.04.24 00:46, Zi Yan 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
> >> 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
> >
> > Once again: your patch won't fix it either.
> >
> >> 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.
> >
> > It will still be present. Just less frequently.
>
> OK. Let me reread the email exchanges between you and Yang and clarify
> the details in the commit log.

There are still some places which may unmap PTE-mapped THP in page
granularity, for example, migration.

>
> >
> >>
> >> 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)))
> >> +                    deferred_split_folio(folio);
> >
> > Please refrain from posting a new patch before the discussion on the old one is done.
> >
> > See my comments on v2 why optimizing out the function call is a reasonable thing to do *where we cannot batch* and the misaccounting will still happen. But it can be done independently.
>
> Got it. Will keep the deferred list checking here and send a new one with commit
> log changes too.
>
> Thank you for the reviews.
>
>
> --
> Best Regards,
> Yan, Zi