mm: page_alloc: take the ALLOC_NO_WATERMARK check out of the fast path

ALLOC_NO_WATERMARK is set in a few cases.  Always by kswapd, always for
__GFP_MEMALLOC, sometimes for swap-over-nfs, tasks etc.  Each of these
cases are relatively rare events but the ALLOC_NO_WATERMARK check is an
unlikely branch in the fast path.  This patch moves the check out of the
fast path and after it has been determined that the watermarks have not
been met.  This helps the common fast path at the cost of making the slow
path slower and hitting kswapd with a performance cost.  It's a reasonable
tradeoff.

Signed-off-by: Mel Gorman <mgorman@suse.de>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Jan Kara <jack@suse.cz>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Hugh Dickins <hughd@google.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Theodore Ts'o <tytso@mit.edu>
Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -1937,9 +1937,6 @@ zonelist_scan:
 			(alloc_flags & ALLOC_CPUSET) &&
 			!cpuset_zone_allowed_softwall(zone, gfp_mask))
 				continue;
-		BUILD_BUG_ON(ALLOC_NO_WATERMARKS < NR_WMARK);
-		if (unlikely(alloc_flags & ALLOC_NO_WATERMARKS))
-			goto try_this_zone;
 		/*
 		 * Distribute pages in proportion to the individual
 		 * zone size to ensure fair page aging.  The zone a
@@ -1986,6 +1983,11 @@ zonelist_scan:
 				       classzone_idx, alloc_flags)) {
 			int ret;
 
+			/* Checked here to keep the fast path fast */
+			BUILD_BUG_ON(ALLOC_NO_WATERMARKS < NR_WMARK);
+			if (alloc_flags & ALLOC_NO_WATERMARKS)
+				goto try_this_zone;
+
 			if (IS_ENABLED(CONFIG_NUMA) &&
 					!did_zlc_setup && nr_online_nodes > 1) {
 				/*