[PATCH] mm: extend reuse_swap_page range as much as possible

[zhouxianrong]

From: zhouxianrong <zhouxianrong@xxxxxxxxxx>

origanlly reuse_swap_page requires that the sum of page's
mapcount and swapcount less than or equal to one.
in this case we can reuse this page and avoid COW currently.

now reuse_swap_page requires only that page's mapcount
less than or equal to one and the page is not dirty in
swap cache. in this case we do not care its swap count.

the page without dirty in swap cache means that it has
been written to swap device successfully for reclaim before
and then read again on a swap fault. in this case the page
can be reused even though its swap count is greater than one
and postpone the COW on other successive accesses to the swap
cache page later rather than now.

i did this patch test in kernel 4.4.23 with arm64 and none huge
memory. it work fine.

Signed-off-by: zhouxianrong <zhouxianrong@xxxxxxxxxx>
---
 mm/swapfile.c |    9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/mm/swapfile.c b/mm/swapfile.c
index bf91dc9..c21cf07 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -1543,22 +1543,27 @@ static int page_trans_huge_map_swapcount(struct page *page, int *total_mapcount,
 bool reuse_swap_page(struct page *page, int *total_map_swapcount)
 {
 	int count, total_mapcount, total_swapcount;
+	int dirty;
 
 	VM_BUG_ON_PAGE(!PageLocked(page), page);
 	if (unlikely(PageKsm(page)))
 		return false;
+	dirty = PageDirty(page);
 	count = page_trans_huge_map_swapcount(page, &total_mapcount,
 					      &total_swapcount);
 	if (total_map_swapcount)
 		*total_map_swapcount = total_mapcount + total_swapcount;
-	if (count == 1 && PageSwapCache(page) &&
+	if ((total_mapcount <= 1 && !dirty) ||
+		(count == 1 && PageSwapCache(page) &&
 	    (likely(!PageTransCompound(page)) ||
 	     /* The remaining swap count will be freed soon */
-	     total_swapcount == page_swapcount(page))) {
+	     total_swapcount == page_swapcount(page)))) {
 		if (!PageWriteback(page)) {
 			page = compound_head(page);
 			delete_from_swap_cache(page);
 			SetPageDirty(page);
+			if (!dirty)
+				return true;
 		} else {
 			swp_entry_t entry;
 			struct swap_info_struct *p;
-- 
1.7.9.5