linux-kernel - Re: [PATCH v4 34/36] rmap: add folio_add_file_rmap

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Date:   Wed, 15 Mar 2023 13:34:40 +0000
From:   Ryan Roberts <ryan.roberts@....com>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        linux-arch@...r.kernel.org
Cc:     Yin Fengwei <fengwei.yin@...el.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 34/36] rmap: add folio_add_file_rmap_range()

On 15/03/2023 05:14, Matthew Wilcox (Oracle) wrote:
> From: Yin Fengwei <fengwei.yin@...el.com>
> 
> folio_add_file_rmap_range() allows to add pte mapping to a specific
> range of file folio. Comparing to page_add_file_rmap(), it batched
> updates __lruvec_stat for large folio.
> 
> Signed-off-by: Yin Fengwei <fengwei.yin@...el.com>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> ---
>  include/linux/rmap.h |  2 ++
>  mm/rmap.c            | 60 +++++++++++++++++++++++++++++++++-----------
>  2 files changed, 48 insertions(+), 14 deletions(-)
> 
> diff --git a/include/linux/rmap.h b/include/linux/rmap.h
> index b87d01660412..a3825ce81102 100644
> --- a/include/linux/rmap.h
> +++ b/include/linux/rmap.h
> @@ -198,6 +198,8 @@ void folio_add_new_anon_rmap(struct folio *, struct vm_area_struct *,
>  		unsigned long address);
>  void page_add_file_rmap(struct page *, struct vm_area_struct *,
>  		bool compound);
> +void folio_add_file_rmap_range(struct folio *, struct page *, unsigned int nr,
> +		struct vm_area_struct *, bool compound);
>  void page_remove_rmap(struct page *, struct vm_area_struct *,
>  		bool compound);
>  
> diff --git a/mm/rmap.c b/mm/rmap.c
> index 4898e10c569a..a91906b28835 100644
> --- a/mm/rmap.c
> +++ b/mm/rmap.c
> @@ -1301,31 +1301,39 @@ void folio_add_new_anon_rmap(struct folio *folio, struct vm_area_struct *vma,
>  }
>  
>  /**
> - * page_add_file_rmap - add pte mapping to a file page
> - * @page:	the page to add the mapping to
> + * folio_add_file_rmap_range - add pte mapping to page range of a folio
> + * @folio:	The folio to add the mapping to
> + * @page:	The first page to add
> + * @nr_pages:	The number of pages which will be mapped
>   * @vma:	the vm area in which the mapping is added
>   * @compound:	charge the page as compound or small page
>   *
> + * The page range of folio is defined by [first_page, first_page + nr_pages)
> + *
>   * The caller needs to hold the pte lock.
>   */
> -void page_add_file_rmap(struct page *page, struct vm_area_struct *vma,
> -		bool compound)
> +void folio_add_file_rmap_range(struct folio *folio, struct page *page,
> +			unsigned int nr_pages, struct vm_area_struct *vma,
> +			bool compound)
>  {
> -	struct folio *folio = page_folio(page);
>  	atomic_t *mapped = &folio->_nr_pages_mapped;
> -	int nr = 0, nr_pmdmapped = 0;
> -	bool first;
> +	unsigned int nr_pmdmapped = 0, first;
> +	int nr = 0;
>  
> -	VM_BUG_ON_PAGE(compound && !PageTransHuge(page), page);
> +	VM_WARN_ON_FOLIO(compound && !folio_test_pmd_mappable(folio), folio);
>  
>  	/* Is page being mapped by PTE? Is this its first map to be added? */
>  	if (likely(!compound)) {
> -		first = atomic_inc_and_test(&page->_mapcount);
> -		nr = first;
> -		if (first && folio_test_large(folio)) {
> -			nr = atomic_inc_return_relaxed(mapped);
> -			nr = (nr < COMPOUND_MAPPED);
> -		}
> +		do {
> +			first = atomic_inc_and_test(&page->_mapcount);
> +			if (first && folio_test_large(folio)) {
> +				first = atomic_inc_return_relaxed(mapped);
> +				first = (nr < COMPOUND_MAPPED);

This still contains the typo that Yin Fengwei spotted in the previous version:
https://lore.kernel.org/linux-mm/20230228213738.272178-1-willy@infradead.org/T/#m84673899e25bc31356093a1177941f2cc35e5da8

FYI, I'm seeing a perf regression of about 1% when compiling the kernel on
Ampere Altra (arm64) with this whole series on top of v6.3-rc1 (In a VM using
ext4 filesystem). Looks like instruction aborts are taking much longer and a
selection of syscalls are a bit slower. Still hunting down the root cause. Will
report once I have conclusive diagnosis.

Thanks,
Ryan


> +			}
> +
> +			if (first)
> +				nr++;
> +		} while (page++, --nr_pages > 0);
>  	} else if (folio_test_pmd_mappable(folio)) {
>  		/* That test is redundant: it's for safety or to optimize out */
>  
> @@ -1354,6 +1362,30 @@ void page_add_file_rmap(struct page *page, struct vm_area_struct *vma,
>  	mlock_vma_folio(folio, vma, compound);
>  }
>  
> +/**
> + * page_add_file_rmap - add pte mapping to a file page
> + * @page:	the page to add the mapping to
> + * @vma:	the vm area in which the mapping is added
> + * @compound:	charge the page as compound or small page
> + *
> + * The caller needs to hold the pte lock.
> + */
> +void page_add_file_rmap(struct page *page, struct vm_area_struct *vma,
> +		bool compound)
> +{
> +	struct folio *folio = page_folio(page);
> +	unsigned int nr_pages;
> +
> +	VM_WARN_ON_ONCE_PAGE(compound && !PageTransHuge(page), page);
> +
> +	if (likely(!compound))
> +		nr_pages = 1;
> +	else
> +		nr_pages = folio_nr_pages(folio);
> +
> +	folio_add_file_rmap_range(folio, page, nr_pages, vma, compound);
> +}
> +
>  /**
>   * page_remove_rmap - take down pte mapping from a page
>   * @page:	page to remove mapping from