[PATCH] shrink_list(): check PageSwapCache() after add_to_swap()

From: Nikita Danilov <Nikita@Namesys.COM>

shrink_list() checks PageSwapCache() before calling add_to_swap(), this
means that anonymous page that is going to be added to the swap right
now these checks return false and:

 (*) it will be unaccounted for in nr_mapped, and

 (*) it won't be written to the swap if gfp_flags include __GFP_IO but
     not __GFP_FS.

(Both will happen only on the next round of scanning.)

Patch below just moves may_enter_fs initialization down. I am not sure
about (*nr_mapped) increase though.

mm/vmscan.c

@@ -271,8 +271,6 @@ shrink_list(struct list_head *page_list, unsigned int gfp_mask,
 			(*nr_mapped)++;
 
 		BUG_ON(PageActive(page));
-		may_enter_fs = (gfp_mask & __GFP_FS) ||
-				(PageSwapCache(page) && (gfp_mask & __GFP_IO));
 
 		if (PageWriteback(page))
 			goto keep_locked;
@@ -303,6 +301,9 @@ shrink_list(struct list_head *page_list, unsigned int gfp_mask,
 		}
 #endif /* CONFIG_SWAP */
 
+		may_enter_fs = (gfp_mask & __GFP_FS) ||
+				(PageSwapCache(page) && (gfp_mask & __GFP_IO));
+
 		/*
 		 * The page is mapped into the page tables of one or more
 		 * processes. Try to unmap it here.