[PATCH] fix can_share_swap_page() when !CONFIG_SWAP

[Hua Zhong]

Hi,

can_share_swap_page() is used to check if the page has the last reference. This avoids allocating a new page for COW if it's the last page.

However, if CONFIG_SWAP is not set, can_share_swap_page() is defined as 0, thus always causes a copy for the last COW page. The below simple patch fixes it.

I'm not sure if it's the best fix. Maybe we should rename can_share_swap_page() and move it out of swapfile.c. Comments?

Signed-off-by: Hua Zhong <hzhong@xxxxxxxxx>

diff --git a/include/linux/swap.h b/include/linux/swap.h
index 5b1fdf1..f03c247 100644
--- a/include/linux/swap.h
+++ b/include/linux/swap.h
@@ -296,7 +296,7 @@ #define swap_free(swp)				/*NOTHING*/
 #define read_swap_cache_async(swp,vma,addr)	NULL
 #define lookup_swap_cache(swp)			NULL
 #define valid_swaphandles(swp, off)		0
-#define can_share_swap_page(p)			0
+#define can_share_swap_page(p)			(page_mapcount(p) == 1)
 #define move_to_swap_cache(p, swp)		1
 #define move_from_swap_cache(p, i, m)		1
 #define __delete_from_swap_cache(p)		/*NOTHING*/
-
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