[PATCHv4 4/7] mm: break up swap_writepage() for frontswap backends

[Seth Jennings <sjenning@xxxxxxxxxxxxxxxxxx>]

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@xxxxxxxxxxxxxxxxxx>
Acked-by: Minchan Kim <minchan@xxxxxxxxxx>
---
 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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