mm: page_alloc: rearrange watermark checking in get_page_from_freelist

Allocations that do not have to respect the watermarks are rare
high-priority events.  Reorder the code such that per-zone dirty limits
and future checks important only to regular page allocations are ignored
in these extraordinary situations.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mgorman@suse.de>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Paul Bolle <paul.bollee@gmail.com>
Tested-by: Zlatko Calusic <zcalusic@bitsync.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -1889,12 +1889,17 @@ zonelist_scan:
 	 */
 	for_each_zone_zonelist_nodemask(zone, z, zonelist,
 						high_zoneidx, nodemask) {
+		unsigned long mark;
+
 		if (IS_ENABLED(CONFIG_NUMA) && zlc_active &&
 			!zlc_zone_worth_trying(zonelist, z, allowednodes))
 				continue;
 		if ((alloc_flags & ALLOC_CPUSET) &&
 			!cpuset_zone_allowed_softwall(zone, gfp_mask))
 				continue;
+		BUILD_BUG_ON(ALLOC_NO_WATERMARKS < NR_WMARK);
+		if (alloc_flags & ALLOC_NO_WATERMARKS)
+			goto try_this_zone;
 		/*
 		 * When allocating a page cache page for writing, we
 		 * want to get it from a zone that is within its dirty
@@ -1925,16 +1930,11 @@ zonelist_scan:
 		    (gfp_mask & __GFP_WRITE) && !zone_dirty_ok(zone))
 			goto this_zone_full;
 
-		BUILD_BUG_ON(ALLOC_NO_WATERMARKS < NR_WMARK);
-		if (!(alloc_flags & ALLOC_NO_WATERMARKS)) {
-			unsigned long mark;
+		mark = zone->watermark[alloc_flags & ALLOC_WMARK_MASK];
+		if (!zone_watermark_ok(zone, order, mark,
+				       classzone_idx, alloc_flags)) {
 			int ret;
 
-			mark = zone->watermark[alloc_flags & ALLOC_WMARK_MASK];
-			if (zone_watermark_ok(zone, order, mark,
-				    classzone_idx, alloc_flags))
-				goto try_this_zone;
-
 			if (IS_ENABLED(CONFIG_NUMA) &&
 					!did_zlc_setup && nr_online_nodes > 1) {
 				/*