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    Subject[PATCH 25/30] mm: methods for teaching filesystems about PG_swapcache pages
    In order to teach filesystems to handle swap cache pages, three new page
    functions are introduced:

    pgoff_t page_file_index(struct page *);
    loff_t page_file_offset(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_offset() - uses page_file_index(), so that it will give the expected
    result, even 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.

    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    ---
    include/linux/mm.h | 25 +++++++++++++++++++++++++
    include/linux/pagemap.h | 5 +++++
    mm/swapfile.c | 19 +++++++++++++++++++
    3 files changed, 49 insertions(+)

    Index: linux-2.6/include/linux/mm.h
    ===================================================================
    --- linux-2.6.orig/include/linux/mm.h
    +++ linux-2.6/include/linux/mm.h
    @@ -598,6 +598,17 @@ static inline struct address_space *page
    return mapping;
    }

    +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;
    +}
    +
    static inline int PageAnon(struct page *page)
    {
    return ((unsigned long)page->mapping & PAGE_MAPPING_ANON) != 0;
    @@ -614,6 +625,20 @@ static inline pgoff_t page_index(struct
    return page->index;
    }

    +extern pgoff_t __page_file_index(struct page *page);
    +
    +/*
    + * 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)
    +{
    + if (unlikely(PageSwapCache(page)))
    + return __page_file_index(page);
    +
    + return page->index;
    +}
    +
    /*
    * The atomic page->_mapcount, like _count, starts from -1:
    * so that transitions both from it and to it can be tracked,
    Index: linux-2.6/include/linux/pagemap.h
    ===================================================================
    --- linux-2.6.orig/include/linux/pagemap.h
    +++ linux-2.6/include/linux/pagemap.h
    @@ -148,6 +148,11 @@ static inline loff_t page_offset(struct
    return ((loff_t)page->index) << PAGE_CACHE_SHIFT;
    }

    +static inline loff_t page_file_offset(struct page *page)
    +{
    + return ((loff_t)page_file_index(page)) << PAGE_CACHE_SHIFT;
    +}
    +
    static inline pgoff_t linear_page_index(struct vm_area_struct *vma,
    unsigned long address)
    {
    Index: linux-2.6/mm/swapfile.c
    ===================================================================
    --- linux-2.6.orig/mm/swapfile.c
    +++ linux-2.6/mm/swapfile.c
    @@ -1828,6 +1828,25 @@ struct swap_info_struct *page_swap_info(
    }

    /*
    + * out-of-line __page_file_ methods to avoid include hell.
    + */
    +
    +struct address_space *__page_file_mapping(struct page *page)
    +{
    + VM_BUG_ON(!PageSwapCache(page));
    + return page_swap_info(page)->swap_file->f_mapping;
    +}
    +EXPORT_SYMBOL_GPL(__page_file_mapping);
    +
    +pgoff_t __page_file_index(struct page *page)
    +{
    + swp_entry_t swap = { .val = page_private(page) };
    + VM_BUG_ON(!PageSwapCache(page));
    + return swp_offset(swap);
    +}
    +EXPORT_SYMBOL_GPL(__page_file_index);
    +
    +/*
    * swap_lock prevents swap_map being freed. Don't grab an extra
    * reference on the swaphandle, it doesn't matter if it becomes unused.
    */
    --



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