memcg: fix behavior under memory.limit equals to memsw.limit

A user can set memcg.limit_in_bytes == memcg.memsw.limit_in_bytes when the
user just want to limit the total size of applications, in other words,
not very interested in memory usage itself.  In this case, swap-out will
be done only by global-LRU.

But, under current implementation, memory.limit_in_bytes is checked at
first and try_to_free_page() may do swap-out.  But, that swap-out is
useless for memsw.limit_in_bytes and the thread may hit limit again.

This patch tries to fix the current behavior at memory.limit ==
memsw.limit case.  And documentation is updated to explain the behavior of
this special case.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Dhaval Giani <dhaval@linux.vnet.ibm.com>
Cc: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Documentation/cgroups/memory.txt

@@ -152,14 +152,19 @@ When swap is accounted, following files are added.
 
 usage of mem+swap is limited by memsw.limit_in_bytes.
 
-Note: why 'mem+swap' rather than swap.
+* why 'mem+swap' rather than swap.
 The global LRU(kswapd) can swap out arbitrary pages. Swap-out means
 to move account from memory to swap...there is no change in usage of
-mem+swap.
-
-In other words, when we want to limit the usage of swap without affecting
-global LRU, mem+swap limit is better than just limiting swap from OS point
-of view.
+mem+swap. In other words, when we want to limit the usage of swap without
+affecting global LRU, mem+swap limit is better than just limiting swap from
+OS point of view.
+
+* What happens when a cgroup hits memory.memsw.limit_in_bytes
+When a cgroup his memory.memsw.limit_in_bytes, it's useless to do swap-out
+in this cgroup. Then, swap-out will not be done by cgroup routine and file
+caches are dropped. But as mentioned above, global LRU can do swapout memory
+from it for sanity of the system's memory management state. You can't forbid
+it by cgroup.
 
 2.5 Reclaim
 

mm/memcontrol.c

@@ -177,6 +177,9 @@ struct mem_cgroup {
 
 	unsigned int	swappiness;
 
+	/* set when res.limit == memsw.limit */
+	bool		memsw_is_minimum;
+
 	/*
 	 * statistics. This must be placed at the end of memcg.
 	 */
@@ -847,6 +850,10 @@ static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *root_mem,
 	int ret, total = 0;
 	int loop = 0;
 
+	/* If memsw_is_minimum==1, swap-out is of-no-use. */
+	if (root_mem->memsw_is_minimum)
+		noswap = true;
+
 	while (loop < 2) {
 		victim = mem_cgroup_select_victim(root_mem);
 		if (victim == root_mem)
@@ -1752,6 +1759,12 @@ static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
 			break;
 		}
 		ret = res_counter_set_limit(&memcg->res, val);
+		if (!ret) {
+			if (memswlimit == val)
+				memcg->memsw_is_minimum = true;
+			else
+				memcg->memsw_is_minimum = false;
+		}
 		mutex_unlock(&set_limit_mutex);
 
 		if (!ret)
@@ -1799,6 +1812,12 @@ static int mem_cgroup_resize_memsw_limit(struct mem_cgroup *memcg,
 			break;
 		}
 		ret = res_counter_set_limit(&memcg->memsw, val);
+		if (!ret) {
+			if (memlimit == val)
+				memcg->memsw_is_minimum = true;
+			else
+				memcg->memsw_is_minimum = false;
+		}
 		mutex_unlock(&set_limit_mutex);
 
 		if (!ret)