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    Subject[PATCHv4 4/7] mm: break up swap_writepage() for frontswap backends
    Date
    swap_writepage() is currently where frontswap hooks into the swap
    write path to capture pages with the frontswap_store() function.
    However, if a frontswap backend wants to "resume" the writeback of
    a page to the swap device, it can't call swap_writepage() as
    the page will simply reenter the backend.

    This patch separates swap_writepage() into a top and bottom half, the
    bottom half named __swap_writepage() to allow a frontswap backend,
    like zswap, to resume writeback beyond the frontswap_store() hook.

    __add_to_swap_cache() is also made non-static so that the page for
    which writeback is to be resumed can be added to the swap cache.

    Signed-off-by: Seth Jennings <sjenning@linux.vnet.ibm.com>
    Acked-by: Minchan Kim <minchan@kernel.org>
    ---
    include/linux/swap.h | 2 ++
    mm/page_io.c | 16 +++++++++++++---
    mm/swap_state.c | 2 +-
    3 files changed, 16 insertions(+), 4 deletions(-)

    diff --git a/include/linux/swap.h b/include/linux/swap.h
    index 68df9c1..fc8920d 100644
    --- a/include/linux/swap.h
    +++ b/include/linux/swap.h
    @@ -321,6 +321,7 @@ static inline void mem_cgroup_uncharge_swap(swp_entry_t ent)
    /* linux/mm/page_io.c */
    extern int swap_readpage(struct page *);
    extern int swap_writepage(struct page *page, struct writeback_control *wbc);
    +extern int __swap_writepage(struct page *page, struct writeback_control *wbc);
    extern int swap_set_page_dirty(struct page *page);
    extern void end_swap_bio_read(struct bio *bio, int err);

    @@ -335,6 +336,7 @@ extern struct address_space swapper_space;
    extern void show_swap_cache_info(void);
    extern int add_to_swap(struct page *);
    extern int add_to_swap_cache(struct page *, swp_entry_t, gfp_t);
    +extern int __add_to_swap_cache(struct page *page, swp_entry_t entry);
    extern void __delete_from_swap_cache(struct page *);
    extern void delete_from_swap_cache(struct page *);
    extern void free_page_and_swap_cache(struct page *);
    diff --git a/mm/page_io.c b/mm/page_io.c
    index 78eee32..1cb382d 100644
    --- a/mm/page_io.c
    +++ b/mm/page_io.c
    @@ -179,15 +179,15 @@ bad_bmap:
    goto out;
    }

    +int __swap_writepage(struct page *page, struct writeback_control *wbc);
    +
    /*
    * We may have stale swap cache pages in memory: notice
    * them here and get rid of the unnecessary final write.
    */
    int swap_writepage(struct page *page, struct writeback_control *wbc)
    {
    - struct bio *bio;
    - int ret = 0, rw = WRITE;
    - struct swap_info_struct *sis = page_swap_info(page);
    + int ret = 0;

    if (try_to_free_swap(page)) {
    unlock_page(page);
    @@ -199,6 +199,16 @@ int swap_writepage(struct page *page, struct writeback_control *wbc)
    end_page_writeback(page);
    goto out;
    }
    + ret = __swap_writepage(page, wbc);
    +out:
    + return ret;
    +}
    +
    +int __swap_writepage(struct page *page, struct writeback_control *wbc)
    +{
    + struct bio *bio;
    + int ret = 0, rw = WRITE;
    + struct swap_info_struct *sis = page_swap_info(page);

    if (sis->flags & SWP_FILE) {
    struct kiocb kiocb;
    diff --git a/mm/swap_state.c b/mm/swap_state.c
    index 0cb36fb..7eded9c 100644
    --- a/mm/swap_state.c
    +++ b/mm/swap_state.c
    @@ -67,7 +67,7 @@ void show_swap_cache_info(void)
    * __add_to_swap_cache resembles add_to_page_cache_locked on swapper_space,
    * but sets SwapCache flag and private instead of mapping and index.
    */
    -static int __add_to_swap_cache(struct page *page, swp_entry_t entry)
    +int __add_to_swap_cache(struct page *page, swp_entry_t entry)
    {
    int error;

    --
    1.8.1.1


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