linux-kernel - Re: [PATCH v1 2/3] mm: Implement folio_remove_rmap

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Message-ID: <B4A5CF00-73E3-4AAA-9D39-1E7ABCCCFB06@nvidia.com>
Date:   Mon, 17 Jul 2023 11:09:19 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Ryan Roberts <ryan.roberts@....com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Matthew Wilcox <willy@...radead.org>,
        Yin Fengwei <fengwei.yin@...el.com>,
        David Hildenbrand <david@...hat.com>,
        Yu Zhao <yuzhao@...gle.com>, Yang Shi <shy828301@...il.com>,
        "Huang, Ying" <ying.huang@...el.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH v1 2/3] mm: Implement folio_remove_rmap_range()

On 17 Jul 2023, at 10:31, Ryan Roberts wrote:

> Like page_remove_rmap() but batch-removes the rmap for a range of pages
> belonging to a folio. This can provide a small speedup due to less
> manipuation of the various counters. But more crucially, if removing the
> rmap for all pages of a folio in a batch, there is no need to
> (spuriously) add it to the deferred split list, which saves significant
> cost when there is contention for the split queue lock.
>
> All contained pages are accounted using the order-0 folio (or base page)
> scheme.
>
> Signed-off-by: Ryan Roberts <ryan.roberts@....com>
> ---
>  include/linux/rmap.h |  2 ++
>  mm/rmap.c            | 65 ++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 67 insertions(+)
>
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index b87d01660412..f578975c12c0 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -200,6 +200,8 @@ void page_add_file_rmap(struct page *, struct vm_area_struct *,
>  		bool compound);
>  void page_remove_rmap(struct page *, struct vm_area_struct *,
>  		bool compound);
> +void folio_remove_rmap_range(struct folio *folio, struct page *page,
> +		int nr, struct vm_area_struct *vma);
>
>  void hugepage_add_anon_rmap(struct page *, struct vm_area_struct *,
>  		unsigned long address, rmap_t flags);
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 2baf57d65c23..1da05aca2bb1 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1359,6 +1359,71 @@ void page_add_file_rmap(struct page *page, struct vm_area_struct *vma,
>  	mlock_vma_folio(folio, vma, compound);
>  }
>
> +/*
> + * folio_remove_rmap_range - take down pte mappings from a range of pages
> + * belonging to a folio. All pages are accounted as small pages.
> + * @folio:	folio that all pages belong to
> + * @page:       first page in range to remove mapping from
> + * @nr:		number of pages in range to remove mapping from

We might need some checks to make sure [page, page+nr] is in the range of
the folio. Something like:

page >= &folio->page && page + nr < (&folio->page + folio_nr_pages(folio))

> + * @vma:        the vm area from which the mapping is removed
> + *
> + * The caller needs to hold the pte lock.
> + */
> +void folio_remove_rmap_range(struct folio *folio, struct page *page,
> +					int nr, struct vm_area_struct *vma)
> +{
> +	atomic_t *mapped = &folio->_nr_pages_mapped;
> +	int nr_unmapped = 0;
> +	int nr_mapped;
> +	bool last;
> +	enum node_stat_item idx;
> +
> +	if (unlikely(folio_test_hugetlb(folio))) {
> +		VM_WARN_ON_FOLIO(1, folio);
> +		return;
> +	}
> +
> +	if (!folio_test_large(folio)) {
> +		/* Is this the page's last map to be removed? */
> +		last = atomic_add_negative(-1, &page->_mapcount);
> +		nr_unmapped = last;
> +	} else {
> +		for (; nr != 0; nr--, page++) {
> +			/* Is this the page's last map to be removed? */
> +			last = atomic_add_negative(-1, &page->_mapcount);
> +			if (last) {
> +				/* Page still mapped if folio mapped entirely */
> +				nr_mapped = atomic_dec_return_relaxed(mapped);
> +				if (nr_mapped < COMPOUND_MAPPED)
> +					nr_unmapped++;
> +			}
> +		}
> +	}
> +
> +	if (nr_unmapped) {
> +		idx = folio_test_anon(folio) ? NR_ANON_MAPPED : NR_FILE_MAPPED;
> +		__lruvec_stat_mod_folio(folio, idx, -nr_unmapped);
> +
> +		/*
> +		 * Queue anon THP for deferred split if we have just unmapped at
> +		 * least 1 page, while at least 1 page remains mapped.
> +		 */
> +		if (folio_test_large(folio) && folio_test_anon(folio))
> +			if (nr_mapped)
> +				deferred_split_folio(folio);
> +	}
> +
> +	/*
> +	 * It would be tidy to reset folio_test_anon mapping when fully
> +	 * unmapped, but that might overwrite a racing page_add_anon_rmap
> +	 * which increments mapcount after us but sets mapping before us:
> +	 * so leave the reset to free_pages_prepare, and remember that
> +	 * it's only reliable while mapped.
> +	 */
> +
> +	munlock_vma_folio(folio, vma, false);
> +}
> +
>  /**
>   * page_remove_rmap - take down pte mapping from a page
>   * @page:	page to remove mapping from
> -- 
> 2.25.1

Everything else looks good to me. Reviewed-by: Zi Yan <ziy@...dia.com>

--
Best Regards,
Yan, Zi

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