mm: memcontrol: simplify move precharge function

The move precharge function does some baroque things: it tries raw
res_counter charging of the entire amount first, and then falls back to
a loop of one-by-one charges, with checks for pending signals and
cond_resched() batching.

Just use mem_cgroup_try_charge() without __GFP_WAIT for the first bulk
charge attempt.  In the one-by-one loop, remove the signal check (this
is already checked in try_charge), and simply call cond_resched() after
every charge - it's not that expensive.
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

@@ -6385,56 +6385,38 @@ static void mem_cgroup_css_reset(struct cgroup_subsys_state *css)
 
 #ifdef CONFIG_MMU
 /* Handlers for move charge at task migration. */
-#define PRECHARGE_COUNT_AT_ONCE	256
 static int mem_cgroup_do_precharge(unsigned long count)
 {
 	int ret = 0;
-	int batch_count = PRECHARGE_COUNT_AT_ONCE;
-	struct mem_cgroup *memcg = mc.to;
 
-	if (mem_cgroup_is_root(memcg)) {
+	if (mem_cgroup_is_root(mc.to)) {
 		mc.precharge += count;
 		/* we don't need css_get for root */
 		return ret;
 	}
-	/* try to charge at once */
-	if (count > 1) {
-		struct res_counter *dummy;
-		/*
-		 * "memcg" cannot be under rmdir() because we've already checked
-		 * by cgroup_lock_live_cgroup() that it is not removed and we
-		 * are still under the same cgroup_mutex. So we can postpone
-		 * css_get().
-		 */
-		if (res_counter_charge(&memcg->res, PAGE_SIZE * count, &dummy))
-			goto one_by_one;
-		if (do_swap_account && res_counter_charge(&memcg->memsw,
-						PAGE_SIZE * count, &dummy)) {
-			res_counter_uncharge(&memcg->res, PAGE_SIZE * count);
-			goto one_by_one;
-		}
+
+	/* Try a single bulk charge without reclaim first */
+	ret = mem_cgroup_try_charge(mc.to, GFP_KERNEL & ~__GFP_WAIT, count);
+	if (!ret) {
 		mc.precharge += count;
 		return ret;
 	}
-one_by_one:
-	/* fall back to one by one charge */
+
+	/* Try charges one by one with reclaim */
 	while (count--) {
-		if (signal_pending(current)) {
-			ret = -EINTR;
-			break;
-		}
-		if (!batch_count--) {
-			batch_count = PRECHARGE_COUNT_AT_ONCE;
-			cond_resched();
-		}
-		ret = mem_cgroup_try_charge(memcg,
+		ret = mem_cgroup_try_charge(mc.to,
 					    GFP_KERNEL & ~__GFP_NORETRY, 1);
+		/*
+		 * In case of failure, any residual charges against
+		 * mc.to will be dropped by mem_cgroup_clear_mc()
+		 * later on.
+		 */
 		if (ret)
-			/* mem_cgroup_clear_mc() will do uncharge later */
 			return ret;
 		mc.precharge++;
+		cond_resched();
 	}
-	return ret;
+	return 0;
 }
 
 /**