MM: reclaim mustn't enter FS for swap-over-NFS

If swap-out is using filesystem operations (SWP_FS_OPS), then it is not
safe to enter the FS for reclaim.
So only down-grade the requirement for swap pages to __GFP_IO after
checking that SWP_FS_OPS are not being used.

Signed-off-by: NeilBrown <neilb@suse.de>

mm/vmscan.c

@@ -1513,8 +1513,14 @@ static unsigned int shrink_page_list(struct list_head *page_list,
 		if (!sc->may_unmap && page_mapped(page))
 			goto keep_locked;
 
+		/* ->flags can be updated non-atomicially (scan_swap_map_slots),
+		 * but that will never affect SWP_FS_OPS, so the data_race
+		 * is safe.
+		 */
 		may_enter_fs = (sc->gfp_mask & __GFP_FS) ||
-			(PageSwapCache(page) && (sc->gfp_mask & __GFP_IO));
+			(PageSwapCache(page) &&
+			 !data_race(page_swap_info(page)->flags & SWP_FS_OPS) &&
+			 (sc->gfp_mask & __GFP_IO));
 
 		/*
 		 * The number of dirty pages determines if a node is marked
@@ -1682,7 +1688,9 @@ static unsigned int shrink_page_list(struct list_head *page_list,
 						goto activate_locked_split;
 				}
 
-				may_enter_fs = true;
+				if ((sc->gfp_mask & __GFP_FS) ||
+				    !data_race(page_swap_info(page)->flags & SWP_FS_OPS))
+					may_enter_fs = true;
 
 				/* Adding to swap updated mapping */
 				mapping = page_mapping(page);