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Subject: Re: [PATCH v10 12/33] mm/filemap: Add folio_index, folio_file_page
 and folio_contains
To:     "Matthew Wilcox (Oracle)" <willy@infradead.org>,
        akpm@linux-foundation.org
Cc:     linux-fsdevel@vger.kernel.org, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
        Jeff Layton <jlayton@kernel.org>
References: <20210511214735.1836149-1-willy@infradead.org>
 <20210511214735.1836149-13-willy@infradead.org>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <77357d4f-5f56-6c12-7602-697773c2f125@suse.cz>
Date:   Fri, 14 May 2021 17:55:46 +0200
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On 5/11/21 11:47 PM, Matthew Wilcox (Oracle) wrote:
> folio_index() is the equivalent of page_index() for folios.
> folio_file_page() is the equivalent of find_subpage().

find_subpage() special cases hugetlbfs, folio_file_page() doesn't.

> folio_contains() is the equivalent of thp_contains().

Yet here, both thp_contains() and folio_contains() does.

This patch doesn't add users so maybe it becomes obvious later, but perhaps
worth explaining in the changelog or comment?

> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Acked-by: Jeff Layton <jlayton@kernel.org>
> ---
>  include/linux/pagemap.h | 53 +++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 53 insertions(+)
> 
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index bc5fa3d7204e..8eaeffccfd38 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -386,6 +386,59 @@ static inline bool thp_contains(struct page *head, pgoff_t index)
>  	return page_index(head) == (index & ~(thp_nr_pages(head) - 1UL));
>  }
>  
> +#define swapcache_index(folio)	__page_file_index(&(folio)->page)
> +
> +/**
> + * folio_index - File index of a folio.
> + * @folio: The folio.
> + *
> + * For a folio which is either in the page cache or the swap cache,
> + * return its index within the address_space it belongs to.  If you know
> + * the page is definitely in the page cache, you can look at the folio's
> + * index directly.
> + *
> + * Return: The index (offset in units of pages) of a folio in its file.
> + */
> +static inline pgoff_t folio_index(struct folio *folio)
> +{
> +        if (unlikely(folio_swapcache(folio)))
> +                return swapcache_index(folio);
> +        return folio->index;
> +}
> +
> +/**
> + * folio_file_page - The page for a particular index.
> + * @folio: The folio which contains this index.
> + * @index: The index we want to look up.
> + *
> + * Sometimes after looking up a folio in the page cache, we need to
> + * obtain the specific page for an index (eg a page fault).
> + *
> + * Return: The page containing the file data for this index.
> + */
> +static inline struct page *folio_file_page(struct folio *folio, pgoff_t index)
> +{
> +	return folio_page(folio, index & (folio_nr_pages(folio) - 1));
> +}
> +
> +/**
> + * folio_contains - Does this folio contain this index?
> + * @folio: The folio.
> + * @index: The page index within the file.
> + *
> + * Context: The caller should have the page locked in order to prevent
> + * (eg) shmem from moving the page between the page cache and swap cache
> + * and changing its index in the middle of the operation.
> + * Return: true or false.
> + */
> +static inline bool folio_contains(struct folio *folio, pgoff_t index)
> +{
> +	/* HugeTLBfs indexes the page cache in units of hpage_size */
> +	if (folio_hugetlb(folio))
> +		return folio->index == index;
> +	return index - folio_index(folio) < folio_nr_pages(folio);
> +}
> +
>  /*
>   * Given the page we found in the page cache, return the page corresponding
>   * to this index in the file
>