linux-kernel - Re: [PATCH v5 04/38] mm: Add folio_flush

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Message-Id: <20230710161727.7a676996b64e9885cc15eaeb@linux-foundation.org>
Date:   Mon, 10 Jul 2023 16:17:27 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Mike Rapoport <rppt@...nel.org>,
        Anshuman Khandual <anshuman.khandual@....com>
Subject: Re: [PATCH v5 04/38] mm: Add folio_flush_mapping()

On Mon, 10 Jul 2023 21:43:05 +0100 "Matthew Wilcox (Oracle)" <willy@...radead.org> wrote:

> This is the folio equivalent of page_mapping_file(), but rename it
> to make it clear that it's very different from page_file_mapping().
> Theoretically, there's nothing flush-only about it, but there are no
> other users today, and I doubt there will be; it's almost always more
> useful to know the swapfile's mapping or the swapcache's mapping.
> 
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>
> +++ b/include/linux/pagemap.h
> @@ -389,6 +389,26 @@ static inline struct address_space *folio_file_mapping(struct folio *folio)
>  	return folio->mapping;
>  }
>  
> +/**
> + * folio_flush_mapping - Find the file mapping this folio belongs to.
> + * @folio: The folio.
> + *
> + * For folios which are in the page cache, return the mapping that this
> + * page belongs to.  Anonymous folios return NULL, even if they're in
> + * the swap cache.  Other kinds of folio also return NULL.
> + *
> + * This is ONLY used by architecture cache flushing code.  If you aren't
> + * writing cache flushing code, you want either folio_mapping() or
> + * folio_file_mapping().
> + */
> +static inline struct address_space *folio_flush_mapping(struct folio *folio)
> +{
> +	if (unlikely(folio_test_swapcache(folio)))
> +		return NULL;
> +
> +	return folio_mapping(folio);
> +}

The name makes it sound like it flushes something.  Wouldn't
folio_flushable_mapping() be clearer?

>  static inline struct address_space *page_file_mapping(struct page *page)
>  {
>  	return folio_file_mapping(page_folio(page));
> @@ -399,11 +419,7 @@ static inline struct address_space *page_file_mapping(struct page *page)
>   */
>  static inline struct address_space *page_mapping_file(struct page *page)
>  {
> -	struct folio *folio = page_folio(page);
> -
> -	if (unlikely(folio_test_swapcache(folio)))
> -		return NULL;
> -	return folio_mapping(folio);
> +	return folio_flush_mapping(page_folio(page));
>  }