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

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Message-ID: <a5426fa6-442a-453f-a234-63d89675ea79@redhat.com>
Date: Thu, 25 Apr 2024 09:19:18 +0200
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.com>, linux-mm@...ck.org
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>,
 Yang Shi <shy828301@...il.com>, Ryan Roberts <ryan.roberts@....com>,
 Barry Song <21cnbao@...il.com>, Lance Yang <ioworker0@...il.com>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm/rmap: do not add fully unmapped large folio to
 deferred split list

On 25.04.24 00:46, Zi Yan 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. 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.

> 
> 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@...dia.com>
> Reviewed-by: Yang Shi <shy828301@...il.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.

-- 
Cheers,

David / dhildenb