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Subject: Re: [PATCH v10 15/33] mm/util: Add folio_mapping and
 folio_file_mapping
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>
 <20210511214735.1836149-16-willy@infradead.org>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <8e8f88b9-796c-8372-f16f-ad4716e7b454@suse.cz>
Date:   Fri, 14 May 2021 19:29:50 +0200
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On 5/11/21 11:47 PM, 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 186 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: Vlastimil Babka <vbabka@suse.cz>

> ---
>  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 feb4645ef4f2..dca39daf3495 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -1749,19 +1749,6 @@ 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);
>  
>  /*
> @@ -1776,7 +1763,6 @@ 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 448a2dfb5ff1..1f37d7656955 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.
> + */
> +static inline struct address_space *folio_file_mapping(struct folio *folio)
> +{
> +	if (unlikely(folio_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));
> +}
> +
>  /*
>   * For file cache pages, return the address_space, otherwise return NULL
>   */
>  static inline struct address_space *page_mapping_file(struct page *page)
>  {
> -	if (unlikely(PageSwapCache(page)))
> +	struct folio *folio = page_folio(page);
> +
> +	if (unlikely(folio_swapcache(folio)))
>  		return NULL;
> -	return page_mapping(page);
> +	return folio_mapping(folio);
>  }
>  
>  static inline bool page_cache_add_speculative(struct page *page, int count)
> diff --git a/include/linux/swap.h b/include/linux/swap.h
> index 144727041e78..20766342845b 100644
> --- a/include/linux/swap.h
> +++ b/include/linux/swap.h
> @@ -314,6 +314,12 @@ struct vma_swap_readahead {
>  #endif
>  };
>  
> +static inline swp_entry_t folio_swap_entry(struct folio *folio)
> +{
> +	swp_entry_t entry = { .val = page_private(&folio->page) };
> +	return entry;
> +}
> +
>  /* linux/mm/workingset.c */
>  void workingset_age_nonresident(struct lruvec *lruvec, unsigned long nr_pages);
>  void *workingset_eviction(struct page *page, struct mem_cgroup *target_memcg);
> diff --git a/mm/Makefile b/mm/Makefile
> index a9ad6122d468..434c2a46b6c5 100644
> --- a/mm/Makefile
> +++ b/mm/Makefile
> @@ -46,7 +46,7 @@ mmu-$(CONFIG_MMU)	+= process_vm_access.o
>  endif
>  
>  obj-y			:= filemap.o mempool.o oom_kill.o fadvise.o \
> -			   maccess.o page-writeback.o \
> +			   maccess.o page-writeback.o folio-compat.o \
>  			   readahead.o swap.o truncate.o vmscan.o shmem.o \
>  			   util.o mmzone.o vmstat.o backing-dev.o \
>  			   mm_init.o percpu.o slab_common.o \
> diff --git a/mm/folio-compat.c b/mm/folio-compat.c
> new file mode 100644
> index 000000000000..5e107aa30a62
> --- /dev/null
> +++ b/mm/folio-compat.c
> @@ -0,0 +1,13 @@
> +/*
> + * Compatibility functions which bloat the callers too much to make inline.
> + * All of the callers of these functions should be converted to use folios
> + * eventually.
> + */
> +
> +#include <linux/pagemap.h>
> +
> +struct address_space *page_mapping(struct page *page)
> +{
> +	return folio_mapping(page_folio(page));
> +}
> +EXPORT_SYMBOL(page_mapping);
> diff --git a/mm/swapfile.c b/mm/swapfile.c
> index 149e77454e3c..d0ee24239a83 100644
> --- a/mm/swapfile.c
> +++ b/mm/swapfile.c
> @@ -3533,13 +3533,13 @@ struct swap_info_struct *page_swap_info(struct page *page)
>  }
>  
>  /*
> - * out-of-line __page_file_ methods to avoid include hell.
> + * out-of-line methods to avoid include hell.
>   */
> -struct address_space *__page_file_mapping(struct page *page)
> +struct address_space *swapcache_mapping(struct folio *folio)
>  {
> -	return page_swap_info(page)->swap_file->f_mapping;
> +	return page_swap_info(&folio->page)->swap_file->f_mapping;
>  }
> -EXPORT_SYMBOL_GPL(__page_file_mapping);
> +EXPORT_SYMBOL_GPL(swapcache_mapping);
>  
>  pgoff_t __page_file_index(struct page *page)
>  {
> diff --git a/mm/util.c b/mm/util.c
> index 0b6dd9d81da7..245f5c7bedae 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.
> + */
> +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_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_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)
>