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

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Date: Fri, 26 Apr 2024 11:45:25 +0800
From: Lance Yang <ioworker0@...il.com>
To: Zi Yan <ziy@...dia.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, 
	"Matthew Wilcox (Oracle)" <willy@...radead.org>, Yang Shi <shy828301@...il.com>, 
	Ryan Roberts <ryan.roberts@....com>, Barry Song <21cnbao@...il.com>, 
	David Hildenbrand <david@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] mm/rmap: do not add fully unmapped large folio to
 deferred split list

On Fri, Apr 26, 2024 at 5:11 AM Zi Yan <zi.yan@...t.com> wrote:
>
> From: Zi Yan <ziy@...dia.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@...dia.com>
> Reviewed-by: Yang Shi <shy828301@...il.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)))

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
>