Re: [PATCH v4] mm/rmap: do not add fully unmapped large folio to deferred split list

[Lance Yang <ioworker0@gmail.com>]

On Fri, Apr 26, 2024 at 11:45 AM Lance Yang <ioworker0@gmail.com> wrote:
>
> On Fri, Apr 26, 2024 at 5:11 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. But it is possible that
> > the folio is fully unmapped and adding it to deferred split list is
> > unnecessary.
> >
> > For PMD-mapped THPs, that was not really an issue, because removing the
> > last PMD mapping in the absence of PTE mappings would not have added the
> > folio to the deferred split queue.
> >
> > However, for PTE-mapped THPs, which are now more prominent due to mTHP,
> > they are always added to the deferred split queue. One side effect
> > is that the THP_DEFERRED_SPLIT_PAGE stat for a PTE-mapped folio can be
> > unintentionally increased, making it look like there are many partially
> > mapped folios -- although the whole folio is fully unmapped stepwise.
> >
> > Core-mm now tries batch-unmapping consecutive PTEs of PTE-mapped THPs
> > where possible starting from commit b06dc281aa99 ("mm/rmap: introduce
> > folio_remove_rmap_[pte|ptes|pmd]()"). When it happens, a whole PTE-mapped
> > folio is unmapped in one go and can avoid being added to deferred split
> > list, reducing the THP_DEFERRED_SPLIT_PAGE noise. But there will still be
> > noise when we cannot batch-unmap a complete PTE-mapped folio in one go
> > -- or where this type of batching is not implemented yet, e.g., migration.
> >
> > To avoid the unnecessary addition, folio->_nr_pages_mapped is checked
> > to tell if the whole folio is unmapped. If the folio is already on
> > deferred split list, it will be skipped, too.
> >
> > Note: 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().
> >
> > Signed-off-by: Zi Yan <ziy@nvidia.com>
> > Reviewed-by: Yang Shi <shy828301@gmail.com>
> > ---
> >  mm/rmap.c | 8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> >
> > diff --git a/mm/rmap.c b/mm/rmap.c
> > index a7913a454028..220ad8a83589 100644
> > --- a/mm/rmap.c
> > +++ b/mm/rmap.c
> > @@ -1553,9 +1553,11 @@ 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) &&
> > +                   list_empty(&folio->_deferred_list) &&
>
> FWIW
>
> Perhaps it would achieve the same check, ensuring that at least one
> page of the folio is unmapped while at least one page remains mapped.
>
>  +                   atomic_read(mapped) && nr < folio_nr_pages(folio))
>  -                   ((level == RMAP_LEVEL_PTE && atomic_read(mapped)) ||
>  -                    (level == RMAP_LEVEL_PMD && nr < nr_pmdmapped)))

Second thought: it’s probably best to leave it as is. The compiler should
optimize out based on the level enum, which is what I overlooked.

Thanks,
Lance

>
> Thanks,
> Lance
>
>
> > +                   ((level == RMAP_LEVEL_PTE && atomic_read(mapped)) ||
> > +                    (level == RMAP_LEVEL_PMD && nr < nr_pmdmapped)))
> > +                       deferred_split_folio(folio);
> >         }
> >
> >         /*
> >
> > base-commit: 66313c66dd90e8711a8b63fc047ddfc69c53636a
> > --
> > 2.43.0
> >