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    Subject[PATCH v11 02/33] mm: Introduce struct folio
    Date
    A struct folio is a new abstraction to replace the venerable struct page.
    A function which takes a struct folio argument declares that it will
    operate on the entire (possibly compound) page, not just PAGE_SIZE bytes.
    In return, the caller guarantees that the pointer it is passing does
    not point to a tail page. No change to generated code.

    Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
    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>
    ---
    Documentation/core-api/mm-api.rst | 1 +
    include/linux/mm.h | 74 +++++++++++++++++++++++++++++++
    include/linux/mm_types.h | 60 +++++++++++++++++++++++++
    include/linux/page-flags.h | 28 ++++++++++++
    4 files changed, 163 insertions(+)

    diff --git a/Documentation/core-api/mm-api.rst b/Documentation/core-api/mm-api.rst
    index a42f9baddfbf..2a94e6164f80 100644
    --- a/Documentation/core-api/mm-api.rst
    +++ b/Documentation/core-api/mm-api.rst
    @@ -95,6 +95,7 @@ More Memory Management Functions
    .. kernel-doc:: mm/mempolicy.c
    .. kernel-doc:: include/linux/mm_types.h
    :internal:
    +.. kernel-doc:: include/linux/page-flags.h
    .. kernel-doc:: include/linux/mm.h
    :internal:
    .. kernel-doc:: include/linux/mmzone.h
    diff --git a/include/linux/mm.h b/include/linux/mm.h
    index 877eb8730eea..19f2ec1a2221 100644
    --- a/include/linux/mm.h
    +++ b/include/linux/mm.h
    @@ -950,6 +950,20 @@ static inline unsigned int compound_order(struct page *page)
    return page[1].compound_order;
    }

    +/**
    + * folio_order - The allocation order of a folio.
    + * @folio: The folio.
    + *
    + * A folio is composed of 2^order pages. See get_order() for the definition
    + * of order.
    + *
    + * Return: The order of the folio.
    + */
    +static inline unsigned int folio_order(struct folio *folio)
    +{
    + return compound_order(&folio->page);
    +}
    +
    static inline bool hpage_pincount_available(struct page *page)
    {
    /*
    @@ -1595,6 +1609,65 @@ static inline void set_page_links(struct page *page, enum zone_type zone,
    #endif
    }

    +/**
    + * folio_nr_pages - The number of pages in the folio.
    + * @folio: The folio.
    + *
    + * Return: A number which is a power of two.
    + */
    +static inline unsigned long folio_nr_pages(struct folio *folio)
    +{
    + return compound_nr(&folio->page);
    +}
    +
    +/**
    + * folio_next - Move to the next physical folio.
    + * @folio: The folio we're currently operating on.
    + *
    + * If you have physically contiguous memory which may span more than
    + * one folio (eg a &struct bio_vec), use this function to move from one
    + * folio to the next. Do not use it if the memory is only virtually
    + * contiguous as the folios are almost certainly not adjacent to each
    + * other. This is the folio equivalent to writing ``page++``.
    + *
    + * Context: We assume that the folios are refcounted and/or locked at a
    + * higher level and do not adjust the reference counts.
    + * Return: The next struct folio.
    + */
    +static inline struct folio *folio_next(struct folio *folio)
    +{
    + return (struct folio *)folio_page(folio, folio_nr_pages(folio));
    +}
    +
    +/**
    + * folio_shift - The number of bits covered by this folio.
    + * @folio: The folio.
    + *
    + * A folio contains a number of bytes which is a power-of-two in size.
    + * This function tells you which power-of-two the folio is.
    + *
    + * Context: The caller should have a reference on the folio to prevent
    + * it from being split. It is not necessary for the folio to be locked.
    + * Return: The base-2 logarithm of the size of this folio.
    + */
    +static inline unsigned int folio_shift(struct folio *folio)
    +{
    + return PAGE_SHIFT + folio_order(folio);
    +}
    +
    +/**
    + * folio_size - The number of bytes in a folio.
    + * @folio: The folio.
    + *
    + * Context: The caller should have a reference on the folio to prevent
    + * it from being split. It is not necessary for the folio to be locked.
    + * Return: The number of bytes in this folio.
    + */
    +static inline size_t folio_size(struct folio *folio)
    +{
    + return PAGE_SIZE << folio_order(folio);
    +}
    +
    /*
    * Some inline functions in vmstat.h depend on page_zone()
    */
    @@ -1699,6 +1772,7 @@ extern void pagefault_out_of_memory(void);

    #define offset_in_page(p) ((unsigned long)(p) & ~PAGE_MASK)
    #define offset_in_thp(page, p) ((unsigned long)(p) & (thp_size(page) - 1))
    +#define offset_in_folio(folio, p) ((unsigned long)(p) & (folio_size(folio) - 1))

    /*
    * Flags passed to show_mem() and show_free_areas() to suppress output in
    diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
    index 5aacc1c10a45..3118ba8b5a4e 100644
    --- a/include/linux/mm_types.h
    +++ b/include/linux/mm_types.h
    @@ -224,6 +224,66 @@ struct page {
    #endif
    } _struct_page_alignment;

    +/**
    + * struct folio - Represents a contiguous set of bytes.
    + * @flags: Identical to the page flags.
    + * @lru: Least Recently Used list; tracks how recently this folio was used.
    + * @mapping: The file this page belongs to, or refers to the anon_vma for
    + * anonymous pages.
    + * @index: Offset within the file, in units of pages. For anonymous pages,
    + * this is the index from the beginning of the mmap.
    + * @private: Filesystem per-folio data (see folio_attach_private()).
    + * Used for swp_entry_t if folio_swapcache().
    + * @_mapcount: Do not access this member directly. Use folio_mapcount() to
    + * find out how many times this folio is mapped by userspace.
    + * @_refcount: Do not access this member directly. Use folio_ref_count()
    + * to find how many references there are to this folio.
    + * @memcg_data: Memory Control Group data.
    + *
    + * A folio is a physically, virtually and logically contiguous set
    + * of bytes. It is a power-of-two in size, and it is aligned to that
    + * same power-of-two. It is at least as large as %PAGE_SIZE. If it is
    + * in the page cache, it is at a file offset which is a multiple of that
    + * power-of-two. It may be mapped into userspace at an address which is
    + * at an arbitrary page offset, but its kernel virtual address is aligned
    + * to its size.
    + */
    +struct folio {
    + /* private: don't document the anon union */
    + union {
    + struct {
    + /* public: */
    + unsigned long flags;
    + struct list_head lru;
    + struct address_space *mapping;
    + pgoff_t index;
    + void *private;
    + atomic_t _mapcount;
    + atomic_t _refcount;
    +#ifdef CONFIG_MEMCG
    + unsigned long memcg_data;
    +#endif
    + /* private: the union with struct page is transitional */
    + };
    + struct page page;
    + };
    +};
    +
    +static_assert(sizeof(struct page) == sizeof(struct folio));
    +#define FOLIO_MATCH(pg, fl) \
    + static_assert(offsetof(struct page, pg) == offsetof(struct folio, fl))
    +FOLIO_MATCH(flags, flags);
    +FOLIO_MATCH(lru, lru);
    +FOLIO_MATCH(compound_head, lru);
    +FOLIO_MATCH(index, index);
    +FOLIO_MATCH(private, private);
    +FOLIO_MATCH(_mapcount, _mapcount);
    +FOLIO_MATCH(_refcount, _refcount);
    +#ifdef CONFIG_MEMCG
    +FOLIO_MATCH(memcg_data, memcg_data);
    +#endif
    +#undef FOLIO_MATCH
    +
    static inline atomic_t *compound_mapcount_ptr(struct page *page)
    {
    return &page[1].compound_mapcount;
    diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
    index d8e26243db25..df48da56cfba 100644
    --- a/include/linux/page-flags.h
    +++ b/include/linux/page-flags.h
    @@ -188,6 +188,34 @@ static inline unsigned long _compound_head(const struct page *page)

    #define compound_head(page) ((typeof(page))_compound_head(page))

    +/**
    + * page_folio - Converts from page to folio.
    + * @p: The page.
    + *
    + * Every page is part of a folio. This function cannot be called on a
    + * NULL pointer.
    + *
    + * Context: No reference, nor lock is required on @page. If the caller
    + * does not hold a reference, this call may race with a folio split, so
    + * it should re-check the folio still contains this page after gaining
    + * a reference on the folio.
    + * Return: The folio which contains this page.
    + */
    +#define page_folio(p) (_Generic((p), \
    + const struct page *: (const struct folio *)_compound_head(p), \
    + struct page *: (struct folio *)_compound_head(p)))
    +
    +/**
    + * folio_page - Return a page from a folio.
    + * @folio: The folio.
    + * @n: The page number to return.
    + *
    + * @n is relative to the start of the folio. This function does not
    + * check that the page number lies within @folio; the caller is presumed
    + * to have a reference to the page.
    + */
    +#define folio_page(folio, n) nth_page(&(folio)->page, n)
    +
    static __always_inline int PageTail(struct page *page)
    {
    return READ_ONCE(page->compound_head) & 1;
    --
    2.30.2
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