[PATCH] fix can_share_swap_page() when !CONFIG_SWAP

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.
Signed-off-by: Hua Zhong <hzhong@gmail.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Hugh Dickins <hugh@veritas.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

include/linux/swap.h

@@ -296,7 +296,7 @@ static inline void disable_swap_token(void)
 #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*/