Re: [PATCH v14 016/138] mm/util: Add folio_mapping() and folio_file_mapping()

Mike Rapoport
2021-07-20T10:55:04Z

On Thu, Jul 15, 2021 at 04:35:02AM +0100, Matthew Wilcox (Oracle) wrote:
> These are the folio equivalent of page_mapping() and page_file_mapping().
> Add an out-of-line page_mapping() wrapper around folio_mapping()
> in order to prevent the page_folio() call from bloating every caller
> of page_mapping(). Adjust page_file_mapping() and page_mapping_file()
> to use folios internally. Rename __page_file_mapping() to
> swapcache_mapping() and change it to take a folio.
>
> This ends up saving 122 bytes of text overall. folio_mapping() is
> 45 bytes shorter than page_mapping() was, but the new page_mapping()
> wrapper is 30 bytes. The major reduction is a few bytes less in dozens
> of nfs functions (which call page_file_mapping()). Most of these appear
> to be a slight change in gcc's register allocation decisions, which allow:
>
> 48 8b 56 08 mov 0x8(%rsi),%rdx
> 48 8d 42 ff lea -0x1(%rdx),%rax
> 83 e2 01 and $0x1,%edx
> 48 0f 44 c6 cmove %rsi,%rax
>
> to become:
>
> 48 8b 46 08 mov 0x8(%rsi),%rax
> 48 8d 78 ff lea -0x1(%rax),%rdi
> a8 01 test $0x1,%al
> 48 0f 44 fe cmove %rsi,%rdi
>
> for a reduction of a single byte. Once the NFS client is converted to
> use folios, this entire sequence will disappear.
>
> Also add folio_mapping() documentation.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> Acked-by: Jeff Layton <jlayton@kernel.org>
> Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> Acked-by: Vlastimil Babka <vbabka@suse.cz>
> Reviewed-by: William Kucharski <william.kucharski@oracle.com>
> Reviewed-by: David Howells <dhowells@redhat.com>
> ---
> Documentation/core-api/mm-api.rst | 2 ++
> include/linux/mm.h | 14 -------------
> include/linux/pagemap.h | 35 +++++++++++++++++++++++++++++--
> include/linux/swap.h | 6 ++++++
> mm/Makefile | 2 +-
> mm/folio-compat.c | 13 ++++++++++++
> mm/swapfile.c | 8 +++----
> mm/util.c | 30 +++++++++++++++-----------
> 8 files changed, 77 insertions(+), 33 deletions(-)
> create mode 100644 mm/folio-compat.c
>
> diff --git a/Documentation/core-api/mm-api.rst b/Documentation/core-api/mm-api.rst
> index 5c459ee2acce..dcce6605947a 100644
> --- a/Documentation/core-api/mm-api.rst
> +++ b/Documentation/core-api/mm-api.rst
> @@ -100,3 +100,5 @@ More Memory Management Functions
> :internal:
> .. kernel-doc:: include/linux/page_ref.h
> .. kernel-doc:: include/linux/mmzone.h
> +.. kernel-doc:: mm/util.c
> + :functions: folio_mapping
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 788fbc4cde0c..9d28f5b2e983 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1753,19 +1753,6 @@ void page_address_init(void);
>
> extern void *page_rmapping(struct page *page);
> extern struct anon_vma *page_anon_vma(struct page *page);
> -extern struct address_space *page_mapping(struct page *page);
> -
> -extern struct address_space *__page_file_mapping(struct page *);
> -
> -static inline
> -struct address_space *page_file_mapping(struct page *page)
> -{
> - if (unlikely(PageSwapCache(page)))
> - return __page_file_mapping(page);
> -
> - return page->mapping;
> -}
> -
> extern pgoff_t __page_file_index(struct page *page);
>
> /*
> @@ -1780,7 +1767,6 @@ static inline pgoff_t page_index(struct page *page)
> }
>
> bool page_mapped(struct page *page);
> -struct address_space *page_mapping(struct page *page);
>
> /*
> * Return true only if the page has been allocated with
> diff --git a/include/linux/pagemap.h b/include/linux/pagemap.h
> index aa71fa82d6be..a0925a89ba11 100644
> --- a/include/linux/pagemap.h
> +++ b/include/linux/pagemap.h
> @@ -162,14 +162,45 @@ static inline void filemap_nr_thps_dec(struct address_space *mapping)
>
> void release_pages(struct page **pages, int nr);
>
> +struct address_space *page_mapping(struct page *);
> +struct address_space *folio_mapping(struct folio *);
> +struct address_space *swapcache_mapping(struct folio *);
> +
> +/**
> + * folio_file_mapping - Find the mapping this folio belongs to.
> + * @folio: The folio.
> + *
> + * For folios which are in the page cache, return the mapping that this
> + * page belongs to. Folios in the swap cache return the mapping of the
> + * swap file or swap device where the data is stored. This is different
> + * from the mapping returned by folio_mapping(). The only reason to
> + * use it is if, like NFS, you return 0 from ->activate_swapfile.
> + *
> + * Do not call this for folios which aren't in the page cache or swap cache.

Missing return value description.

> + */
> +static inline struct address_space *folio_file_mapping(struct folio *folio)
> +{
> + if (unlikely(folio_test_swapcache(folio)))
> + return swapcache_mapping(folio);
> +
> + return folio->mapping;
> +}
> +
> +static inline struct address_space *page_file_mapping(struct page *page)
> +{
> + return folio_file_mapping(page_folio(page));
> +}
> +

[ snip ]

> diff --git a/mm/util.c b/mm/util.c
> index 9043d03750a7..1cde6218d6d1 100644
> --- a/mm/util.c
> +++ b/mm/util.c
> @@ -686,30 +686,36 @@ struct anon_vma *page_anon_vma(struct page *page)
> return __page_rmapping(page);
> }
>
> -struct address_space *page_mapping(struct page *page)
> +/**
> + * folio_mapping - Find the mapping where this folio is stored.
> + * @folio: The folio.
> + *
> + * For folios which are in the page cache, return the mapping that this
> + * page belongs to. Folios in the swap cache return the swap mapping
> + * this page is stored in (which is different from the mapping for the
> + * swap file or swap device where the data is stored).
> + *
> + * You can call this for folios which aren't in the swap cache or page
> + * cache and it will return NULL.
> + */

Missing return value description.

> +struct address_space *folio_mapping(struct folio *folio)
> {
> struct address_space *mapping;
>
> - page = compound_head(page);
> -
> /* This happens if someone calls flush_dcache_page on slab page */
> - if (unlikely(PageSlab(page)))
> + if (unlikely(folio_test_slab(folio)))
> return NULL;
>
> - if (unlikely(PageSwapCache(page))) {
> - swp_entry_t entry;
> -
> - entry.val = page_private(page);
> - return swap_address_space(entry);
> - }
> + if (unlikely(folio_test_swapcache(folio)))
> + return swap_address_space(folio_swap_entry(folio));
>
> - mapping = page->mapping;
> + mapping = folio->mapping;
> if ((unsigned long)mapping & PAGE_MAPPING_ANON)
> return NULL;
>
> return (void *)((unsigned long)mapping & ~PAGE_MAPPING_FLAGS);
> }
> -EXPORT_SYMBOL(page_mapping);
> +EXPORT_SYMBOL(folio_mapping);
>
> /* Slow path of page_mapcount() for compound pages */
> int __page_mapcount(struct page *page)
> --
> 2.30.2
>
>

--
Sincerely yours,
Mike.