mm: vmscan: replace shrink_node() loop with a retry jump

Most of the function body is inside a loop, which imposes an additional
indentation and scoping level that makes the code a bit hard to follow and
modify.

The looping only happens in case of reclaim-compaction, which isn't the
common case.  So rather than adding yet another function level to the
reclaim path and have every reclaim invocation go through a level that
only exists for one specific cornercase, use a retry goto.

Link: http://lkml.kernel.org/r/20191022144803.302233-6-hannes@cmpxchg.orgSigned-off-by: NJohannes Weiner <hannes@cmpxchg.org>
Reviewed-by: NRoman Gushchin <guro@fb.com>
Reviewed-by: NShakeel Butt <shakeelb@google.com>
Cc: Michal Hocko <mhocko@suse.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

mm/vmscan.c

@@ -2729,13 +2729,11 @@ static bool pgdat_memcg_congested(pg_data_t *pgdat, struct mem_cgroup *memcg)
 static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 {
 	struct reclaim_state *reclaim_state = current->reclaim_state;
+	struct mem_cgroup *root = sc->target_mem_cgroup;
 	unsigned long nr_reclaimed, nr_scanned;
 	bool reclaimable = false;
-
-	do {
-		struct mem_cgroup *root = sc->target_mem_cgroup;
 	struct mem_cgroup *memcg;
-
+again:
 	memset(&sc->nr, 0, sizeof(sc->nr));
 
 	nr_reclaimed = sc->nr_reclaimed;
@@ -2865,8 +2863,9 @@ static bool shrink_node(pg_data_t *pgdat, struct scan_control *sc)
 	    current_may_throttle() && pgdat_memcg_congested(pgdat, root))
 		wait_iff_congested(BLK_RW_ASYNC, HZ/10);
 
-	} while (should_continue_reclaim(pgdat, sc->nr_reclaimed - nr_reclaimed,
-					 sc));
+	if (should_continue_reclaim(pgdat, sc->nr_reclaimed - nr_reclaimed,
+				    sc))
+		goto again;
 
 	/*
 	 * Kswapd gives up on balancing particular nodes after too