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From: Barry Song <21cnbao@gmail.com>
Date: Fri, 26 Apr 2024 09:45:27 +0800
Message-ID: <CAGsJ_4wa0LskQkoZf9r5bG5+wEkyfCYveMBSTbuDe0=t1QetTg@mail.gmail.com>
Subject: Re: [PATCH v4] mm/rmap: do not add fully unmapped large folio to
 deferred split list
To: Zi Yan <ziy@nvidia.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org, 
	"Matthew Wilcox (Oracle)" <willy@infradead.org>, Yang Shi <shy828301@gmail.com>, 
	Ryan Roberts <ryan.roberts@arm.com>, David Hildenbrand <david@redhat.com>, 
	Lance Yang <ioworker0@gmail.com>, linux-kernel@vger.kernel.org
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On Fri, Apr 26, 2024 at 5:11=E2=80=AFAM 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(st=
ruct 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 =3D=3D RMAP_LEVEL_PTE || nr < nr_pmdmap=
ped)
> -                               deferred_split_folio(folio);
> +               if (folio_test_large(folio) && folio_test_anon(folio) &&
> +                   list_empty(&folio->_deferred_list) &&
> +                   ((level =3D=3D RMAP_LEVEL_PTE && atomic_read(mapped))=
 ||
> +                    (level =3D=3D RMAP_LEVEL_PMD && nr < nr_pmdmapped)))
> +                       deferred_split_folio(folio);

Hi Zi Yan,
in case a mTHP is mapped by two processed (forked but not CoW yet), if we
unmap the whole folio by pte level in one process only, are we still adding=
 this
folio into deferred list?

>         }
>
>         /*
>
> base-commit: 66313c66dd90e8711a8b63fc047ddfc69c53636a
> --
> 2.43.0
>

Thanks
Barry