mm, compaction: don't recheck watermarks after COMPACT_SUCCESS

Joonsoo has reminded me that in a later patch changing watermark checks
throughout compaction I forgot to update checks in
try_to_compact_pages() and compactd_do_work().  Closer inspection
however shows that they are redundant now in the success case, because
compact_zone() now reliably reports this with COMPACT_SUCCESS.  So
effectively the checks just repeat (a subset) of checks that have just
passed.  So instead of checking watermarks again, just test the return
value.

Note it's also possible that compaction would declare failure e.g.
because its find_suitable_fallback() is more strict than simple
watermark check, and then the watermark check we are removing would then
still succeed.  After this patch this is not possible and it's arguably
better, because for long-term fragmentation avoidance we should rather
try a different zone than allocate with the unsuitable fallback.  If
compaction of all zones fail and the allocation is important enough, it
will retry and succeed anyway.

Also remove the stray "bool success" variable from kcompactd_do_work().

Link: http://lkml.kernel.org/r/20160810091226.6709-5-vbabka@suse.cz
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Reported-by: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Tested-by: Lorenzo Stoakes <lstoakes@gmail.com>
Acked-by: Michal Hocko <mhocko@kernel.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: David Rientjes <rientjes@google.com>
Cc: Rik van Riel <riel@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/compaction.c

@@ -1698,9 +1698,8 @@ enum compact_result try_to_compact_pages(gfp_t gfp_mask, unsigned int order,
 					alloc_flags, ac_classzone_idx(ac));
 		rc = max(status, rc);
 
-		/* If a normal allocation would succeed, stop compacting */
-		if (zone_watermark_ok(zone, order, low_wmark_pages(zone),
-					ac_classzone_idx(ac), alloc_flags)) {
+		/* The allocation should succeed, stop compacting */
+		if (status == COMPACT_SUCCESS) {
 			/*
 			 * We think the allocation will succeed in this zone,
 			 * but it is not certain, hence the false. The caller
@@ -1873,8 +1872,6 @@ static void kcompactd_do_work(pg_data_t *pgdat)
 		.ignore_skip_hint = true,
 
 	};
-	bool success = false;
-
 	trace_mm_compaction_kcompactd_wake(pgdat->node_id, cc.order,
 							cc.classzone_idx);
 	count_vm_event(KCOMPACTD_WAKE);
@@ -1903,9 +1900,7 @@ static void kcompactd_do_work(pg_data_t *pgdat)
 			return;
 		status = compact_zone(zone, &cc);
 
-		if (zone_watermark_ok(zone, cc.order, low_wmark_pages(zone),
-						cc.classzone_idx, 0)) {
-			success = true;
+		if (status == COMPACT_SUCCESS) {
 			compaction_defer_reset(zone, cc.order, false);
 		} else if (status == COMPACT_PARTIAL_SKIPPED || status == COMPACT_COMPLETE) {
 			/*