mm: memcontrol: rearrange charging fast path

The charging path currently starts out with OOM condition checks when
OOM is the rarest possible case.

Rearrange this code to run OOM/task dying checks only after trying the
percpu charge and the res_counter charge and bail out before entering
reclaim.  Attempting a charge does not hurt an (oom-)killed task as much
as every charge attempt having to check OOM conditions.  Also, only
check __GFP_NOFAIL when the charge would actually fail.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Hugh Dickins <hughd@google.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vladimir Davydov <vdavydov@parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -2575,22 +2575,6 @@ static int mem_cgroup_try_charge(struct mem_cgroup *memcg,
 
 	if (mem_cgroup_is_root(memcg))
 		goto done;
-	/*
-	 * Unlike in global OOM situations, memcg is not in a physical
-	 * memory shortage.  Allow dying and OOM-killed tasks to
-	 * bypass the last charges so that they can exit quickly and
-	 * free their memory.
-	 */
-	if (unlikely(test_thread_flag(TIF_MEMDIE) ||
-		     fatal_signal_pending(current) ||
-		     current->flags & PF_EXITING))
-		goto bypass;
-
-	if (unlikely(task_in_memcg_oom(current)))
-		goto nomem;
-
-	if (gfp_mask & __GFP_NOFAIL)
-		oom = false;
 retry:
 	if (consume_stock(memcg, nr_pages))
 		goto done;
@@ -2612,6 +2596,20 @@ retry:
 		goto retry;
 	}
 
+	/*
+	 * Unlike in global OOM situations, memcg is not in a physical
+	 * memory shortage.  Allow dying and OOM-killed tasks to
+	 * bypass the last charges so that they can exit quickly and
+	 * free their memory.
+	 */
+	if (unlikely(test_thread_flag(TIF_MEMDIE) ||
+		     fatal_signal_pending(current) ||
+		     current->flags & PF_EXITING))
+		goto bypass;
+
+	if (unlikely(task_in_memcg_oom(current)))
+		goto nomem;
+
 	if (!(gfp_mask & __GFP_WAIT))
 		goto nomem;
 
@@ -2640,6 +2638,9 @@ retry:
 	if (mem_cgroup_wait_acct_move(mem_over_limit))
 		goto retry;
 
+	if (gfp_mask & __GFP_NOFAIL)
+		goto bypass;
+
 	if (fatal_signal_pending(current))
 		goto bypass;