linux-kernel - Re: [PATCH v5] mm/rmap: do not add fully unmapped large folio to deferred split list

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Message-ID: <e274ad3b-bbeb-49b0-81bf-24fd33106f05@redhat.com>
Date: Fri, 26 Apr 2024 21:08:46 +0200
From: David Hildenbrand <david@...hat.com>
To: Zi Yan <ziy@...dia.com>, Andrew Morton <akpm@...ux-foundation.org>,
 linux-mm@...ck.org
Cc: "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 v5] mm/rmap: do not add fully unmapped large folio to
 deferred split list

On 26.04.24 21:02, 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. 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().
> 
> Suggested-by: David Hildenbrand <david@...hat.com>
> Signed-off-by: Zi Yan <ziy@...dia.com>
> ---
>   mm/rmap.c | 12 +++++++++---
>   1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 2608c40dffad..a9bd64ebdd9a 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1495,6 +1495,7 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>   {
>   	atomic_t *mapped = &folio->_nr_pages_mapped;
>   	int last, nr = 0, nr_pmdmapped = 0;
> +	bool partially_mapped = false;
>   	enum node_stat_item idx;
>   
>   	__folio_rmap_sanity_checks(folio, page, nr_pages, level);
> @@ -1515,6 +1516,8 @@ static __always_inline void __folio_remove_rmap(struct folio *folio,
>   					nr++;
>   			}
>   		} while (page++, --nr_pages > 0);
> +
> +		partially_mapped = !!nr && !!atomic_read(mapped);

Nit: The && should remove the need for both !!.

Reviewed-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb