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>

1 files changed, 11 insertions, 3 deletions

diff --git a/mm/page_io.c b/mm/page_io.c
index 78eee32ee48..8e6bcf176cf 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;