mm: break up swap_writepage() for frontswap backends

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>
Signed-off-by: Bob Liu <bob.liu@oracle.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Reviewed-by: Dan Magenheimer <dan.magenheimer@oracle.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

2 files changed, 12 insertions, 4 deletions

diff --git a/mm/page_io.c b/mm/page_io.c
index 78eee32ee486..8e6bcf176cfb 100644
--- a/mm/page_io.c
+++ b/mm/page_io.c
@@ -185,9 +185,7 @@ bad_bmap:
  */
 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 +197,16 @@ 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 7efcf1525921..fe43fd5578cf 100644
--- a/mm/swap_state.c
+++ b/mm/swap_state.c
@@ -78,7 +78,7 @@ 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;
         struct address_space *address_space;