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 09:45:27 +0800
From: Barry Song <21cnbao@...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>, David Hildenbrand <david@...hat.com>, 
	Lance Yang <ioworker0@...il.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) &&
> +                   ((level == RMAP_LEVEL_PTE && atomic_read(mapped)) ||
> +                    (level == 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