memcg: fix shrinking memory to return -EBUSY by fixing retry algorithm

As pointed out, shrinking memcg's limit should return -EBUSY after
reasonable retries.  This patch tries to fix the current behavior of
shrink_usage.

Before looking into "shrink should return -EBUSY" problem, we should fix
hierarchical reclaim code.  It compares current usage and current limit,
but it only makes sense when the kernel reclaims memory because hit
limits.  This is also a problem.

What this patch does are.

  1. add new argument "shrink" to hierarchical reclaim. If "shrink==true",
     hierarchical reclaim returns immediately and the caller checks the kernel
     should shrink more or not.
     (At shrinking memory, usage is always smaller than limit. So check for
      usage < limit is useless.)

  2. For adjusting to above change, 2 changes in "shrink"'s retry path.
     2-a. retry_count depends on # of children because the kernel visits
	  the children under hierarchy one by one.
     2-b. rather than checking return value of hierarchical_reclaim's progress,
	  compares usage-before-shrink and usage-after-shrink.
	  If usage-before-shrink <= usage-after-shrink, retry_count is
	  decremented.

Reported-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Paul Menage <menage@google.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 59 insertions, 12 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 33fc0302e29e..6f6a575e77ad 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -702,6 +702,23 @@ static unsigned int get_swappiness(struct mem_cgroup *memcg)
         return swappiness;
 }
 
+static int mem_cgroup_count_children_cb(struct mem_cgroup *mem, void *data)
+{
+        int *val = data;
+        (*val)++;
+        return 0;
+}
+/*
+ * This function returns the number of memcg under hierarchy tree. Returns
+ * 1(self count) if no children.
+ */
+static int mem_cgroup_count_children(struct mem_cgroup *mem)
+{
+        int num = 0;
+        mem_cgroup_walk_tree(mem, &num, mem_cgroup_count_children_cb);
+        return num;
+}
+
 /*
  * Visit the first child (need not be the first child as per the ordering
  * of the cgroup list, since we track last_scanned_child) of @mem and use
@@ -750,9 +767,11 @@ mem_cgroup_select_victim(struct mem_cgroup *root_mem)
  *
  * We give up and return to the caller when we visit root_mem twice.
  * (other groups can be removed while we're walking....)
+ *
+ * If shrink==true, for avoiding to free too much, this returns immedieately.
  */
 static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *root_mem,
-                                                gfp_t gfp_mask, bool noswap)
+                                   gfp_t gfp_mask, bool noswap, bool shrink)
 {
         struct mem_cgroup *victim;
         int ret, total = 0;
@@ -771,6 +790,13 @@ static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *root_mem,
                 ret = try_to_free_mem_cgroup_pages(victim, gfp_mask, noswap,
                                                    get_swappiness(victim));
                 css_put(&victim->css);
+                /*
+                 * At shrinking usage, we can't check we should stop here or
+                 * reclaim more. It's depends on callers. last_scanned_child
+                 * will work enough for keeping fairness under tree.
+                 */
+                if (shrink)
+                        return ret;
                 total += ret;
                 if (mem_cgroup_check_under_limit(root_mem))
                         return 1 + total;
@@ -856,7 +882,7 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
                         goto nomem;
 
                 ret = mem_cgroup_hierarchical_reclaim(mem_over_limit, gfp_mask,
-                                                        noswap);
+                                                        noswap, false);
                 if (ret)
                         continue;
 
@@ -1489,7 +1515,8 @@ int mem_cgroup_shrink_usage(struct page *page,
                 return 0;
 
         do {
-                progress = mem_cgroup_hierarchical_reclaim(mem, gfp_mask, true);
+                progress = mem_cgroup_hierarchical_reclaim(mem,
+                                        gfp_mask, true, false);
                 progress += mem_cgroup_check_under_limit(mem);
         } while (!progress && --retry);
 
@@ -1504,11 +1531,21 @@ static DEFINE_MUTEX(set_limit_mutex);
 static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
                                 unsigned long long val)
 {
-
-        int retry_count = MEM_CGROUP_RECLAIM_RETRIES;
+        int retry_count;
         int progress;
         u64 memswlimit;
         int ret = 0;
+        int children = mem_cgroup_count_children(memcg);
+        u64 curusage, oldusage;
+
+        /*
+         * For keeping hierarchical_reclaim simple, how long we should retry
+         * is depends on callers. We set our retry-count to be function
+         * of # of children which we should visit in this loop.
+         */
+        retry_count = MEM_CGROUP_RECLAIM_RETRIES * children;
+
+        oldusage = res_counter_read_u64(&memcg->res, RES_USAGE);
 
         while (retry_count) {
                 if (signal_pending(current)) {
@@ -1534,8 +1571,13 @@ static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
                         break;
 
                 progress = mem_cgroup_hierarchical_reclaim(memcg, GFP_KERNEL,
-                                                           false);
-                if (!progress)                  retry_count--;
+                                                   false, true);
+                curusage = res_counter_read_u64(&memcg->res, RES_USAGE);
+                /* Usage is reduced ? */
+                if (curusage >= oldusage)
+                        retry_count--;
+                else
+                        oldusage = curusage;
         }
 
         return ret;
@@ -1544,13 +1586,16 @@ static int mem_cgroup_resize_limit(struct mem_cgroup *memcg,
 int mem_cgroup_resize_memsw_limit(struct mem_cgroup *memcg,
                                 unsigned long long val)
 {
-        int retry_count = MEM_CGROUP_RECLAIM_RETRIES;
+        int retry_count;
         u64 memlimit, oldusage, curusage;
-        int ret;
+        int children = mem_cgroup_count_children(memcg);
+        int ret = -EBUSY;
 
         if (!do_swap_account)
                 return -EINVAL;
-
+        /* see mem_cgroup_resize_res_limit */
+        retry_count = children * MEM_CGROUP_RECLAIM_RETRIES;
+        oldusage = res_counter_read_u64(&memcg->memsw, RES_USAGE);
         while (retry_count) {
                 if (signal_pending(current)) {
                         ret = -EINTR;
@@ -1574,11 +1619,13 @@ int mem_cgroup_resize_memsw_limit(struct mem_cgroup *memcg,
                 if (!ret)
                         break;
 
-                oldusage = res_counter_read_u64(&memcg->memsw, RES_USAGE);
-                mem_cgroup_hierarchical_reclaim(memcg, GFP_KERNEL, true);
+                mem_cgroup_hierarchical_reclaim(memcg, GFP_KERNEL, true, true);
                 curusage = res_counter_read_u64(&memcg->memsw, RES_USAGE);
+                /* Usage is reduced ? */
                 if (curusage >= oldusage)
                         retry_count--;
+                else
+                        oldusage = curusage;
         }
         return ret;
 }