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    Subject[PATCH 23/29] mm: methods for teaching filesystems about PG_swapcache pages
    In order to teach filesystems to handle swap cache pages, two new page
    functions are introduced:

    pgoff_t page_file_index(struct page *);
    struct address_space *page_file_mapping(struct page *);

    page_file_index - gives the offset of this page in the file in PAGE_CACHE_SIZE
    blocks. Like page->index is for mapped pages, this function also gives the
    correct index for PG_swapcache pages.

    page_file_mapping - gives the mapping backing the actual page; that is for
    swap cache pages it will give swap_file->f_mapping.

    page_offset() is modified to use page_file_index(), so that it will give the
    expected result, even for PG_swapcache pages.

    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    CC: Trond Myklebust <trond.myklebust@fys.uio.no>
    ---
    include/linux/mm.h | 25 +++++++++++++++++++++++++
    include/linux/pagemap.h | 2 +-
    2 files changed, 26 insertions(+), 1 deletion(-)

    Index: linux-2.6-git/include/linux/mm.h
    ===================================================================
    --- linux-2.6-git.orig/include/linux/mm.h 2007-02-21 12:15:01.000000000 +0100
    +++ linux-2.6-git/include/linux/mm.h 2007-02-21 12:15:07.000000000 +0100
    @@ -594,6 +594,16 @@ static inline struct swap_info_struct *p
    return get_swap_info_struct(swp_type(swap));
    }

    +static inline
    +struct address_space *page_file_mapping(struct page *page)
    +{
    +#ifdef CONFIG_SWAP_FILE
    + if (unlikely(PageSwapCache(page)))
    + return page_swap_info(page)->swap_file->f_mapping;
    +#endif
    + return page->mapping;
    +}
    +
    static inline int PageAnon(struct page *page)
    {
    return ((unsigned long)page->mapping & PAGE_MAPPING_ANON) != 0;
    @@ -611,6 +621,21 @@ static inline pgoff_t page_index(struct
    }

    /*
    + * Return the file index of the page. Regular pagecache pages use ->index
    + * whereas swapcache pages use swp_offset(->private)
    + */
    +static inline pgoff_t page_file_index(struct page *page)
    +{
    +#ifdef CONFIG_SWAP_FILE
    + if (unlikely(PageSwapCache(page))) {
    + swp_entry_t swap = { .val = page_private(page) };
    + return swp_offset(swap);
    + }
    +#endif
    + return page->index;
    +}
    +
    +/*
    * The atomic page->_mapcount, like _count, starts from -1:
    * so that transitions both from it and to it can be tracked,
    * using atomic_inc_and_test and atomic_add_negative(-1).
    Index: linux-2.6-git/include/linux/pagemap.h
    ===================================================================
    --- linux-2.6-git.orig/include/linux/pagemap.h 2007-02-21 12:14:54.000000000 +0100
    +++ linux-2.6-git/include/linux/pagemap.h 2007-02-21 12:15:07.000000000 +0100
    @@ -120,7 +120,7 @@ extern void __remove_from_page_cache(str
    */
    static inline loff_t page_offset(struct page *page)
    {
    - return ((loff_t)page->index) << PAGE_CACHE_SHIFT;
    + return ((loff_t)page_file_index(page)) << PAGE_CACHE_SHIFT;
    }

    static inline pgoff_t linear_page_index(struct vm_area_struct *vma,
    --

    -
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