cgroups: make cftype.unregister_event() void-returning

Since we are unable to handle an error returned by
cftype.unregister_event() properly, let's make the callback
void-returning.

mem_cgroup_unregister_event() has been rewritten to be a "never fail"
function.  On mem_cgroup_usage_register_event() we save old buffer for
thresholds array and reuse it in mem_cgroup_usage_unregister_event() to
avoid allocation.

Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Phil Carmody <ext-phil.2.carmody@nokia.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Paul Menage <menage@google.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 42 insertions, 26 deletions

diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
index 8f78073d7caa..0c621604baa1 100644
--- a/include/linux/cgroup.h
+++ b/include/linux/cgroup.h
@@ -397,7 +397,7 @@ struct cftype {
          * This callback must be implemented, if you want provide
          * notification functionality.
          */
-        int (*unregister_event)(struct cgroup *cgrp, struct cftype *cft,
+        void (*unregister_event)(struct cgroup *cgrp, struct cftype *cft,
                         struct eventfd_ctx *eventfd);
 };
 
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 291775021b2e..422cb19f156e 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2994,7 +2994,6 @@ static void cgroup_event_remove(struct work_struct *work)
                         remove);
         struct cgroup *cgrp = event->cgrp;
 
-        /* TODO: check return code */
         event->cft->unregister_event(cgrp, event->cft, event->eventfd);
 
         eventfd_ctx_put(event->eventfd);
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index df1234c0dac3..a4172a861b30 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -226,9 +226,19 @@ struct mem_cgroup {
         /* thresholds for memory usage. RCU-protected */
         struct mem_cgroup_threshold_ary *thresholds;
 
+        /*
+         * Preallocated buffer to be used in mem_cgroup_unregister_event()
+         * to make it "never fail".
+         * It must be able to store at least thresholds->size - 1 entries.
+         */
+        struct mem_cgroup_threshold_ary *__thresholds;
+
         /* thresholds for mem+swap usage. RCU-protected */
         struct mem_cgroup_threshold_ary *memsw_thresholds;
 
+        /* the same as __thresholds, but for memsw_thresholds */
+        struct mem_cgroup_threshold_ary *__memsw_thresholds;
+
         /* For oom notifier event fd */
         struct list_head oom_notify;
 
@@ -3604,17 +3614,27 @@ static int mem_cgroup_usage_register_event(struct cgroup *cgrp,
         else
                 rcu_assign_pointer(memcg->memsw_thresholds, thresholds_new);
 
-        /* To be sure that nobody uses thresholds before freeing it */
+        /* To be sure that nobody uses thresholds */
         synchronize_rcu();
 
-        kfree(thresholds);
+        /*
+         * Free old preallocated buffer and use thresholds as new
+         * preallocated buffer.
+         */
+        if (type == _MEM) {
+                kfree(memcg->__thresholds);
+                memcg->__thresholds = thresholds;
+        } else {
+                kfree(memcg->__memsw_thresholds);
+                memcg->__memsw_thresholds = thresholds;
+        }
 unlock:
         mutex_unlock(&memcg->thresholds_lock);
 
         return ret;
 }
 
-static int mem_cgroup_usage_unregister_event(struct cgroup *cgrp,
+static void mem_cgroup_usage_unregister_event(struct cgroup *cgrp,
         struct cftype *cft, struct eventfd_ctx *eventfd)
 {
         struct mem_cgroup *memcg = mem_cgroup_from_cont(cgrp);
@@ -3622,7 +3642,7 @@ static int mem_cgroup_usage_unregister_event(struct cgroup *cgrp,
         int type = MEMFILE_TYPE(cft->private);
         u64 usage;
         int size = 0;
-        int i, j, ret = 0;
+        int i, j;
 
         mutex_lock(&memcg->thresholds_lock);
         if (type == _MEM)
@@ -3649,20 +3669,19 @@ static int mem_cgroup_usage_unregister_event(struct cgroup *cgrp,
                         size++;
         }
 
+        /* Use preallocated buffer for new array of thresholds */
+        if (type == _MEM)
+                thresholds_new = memcg->__thresholds;
+        else
+                thresholds_new = memcg->__memsw_thresholds;
+
         /* Set thresholds array to NULL if we don't have thresholds */
         if (!size) {
+                kfree(thresholds_new);
                 thresholds_new = NULL;
-                goto assign;
+                goto swap_buffers;
         }
 
-        /* Allocate memory for new array of thresholds */
-        thresholds_new = kmalloc(sizeof(*thresholds_new) +
-                        size * sizeof(struct mem_cgroup_threshold),
-                        GFP_KERNEL);
-        if (!thresholds_new) {
-                ret = -ENOMEM;
-                goto unlock;
-        }
         thresholds_new->size = size;
 
         /* Copy thresholds and find current threshold */
@@ -3683,20 +3702,20 @@ static int mem_cgroup_usage_unregister_event(struct cgroup *cgrp,
                 j++;
         }
 
-assign:
-        if (type == _MEM)
+swap_buffers:
+        /* Swap thresholds array and preallocated buffer */
+        if (type == _MEM) {
+                memcg->__thresholds = thresholds;
                 rcu_assign_pointer(memcg->thresholds, thresholds_new);
-        else
+        } else {
+                memcg->__memsw_thresholds = thresholds;
                 rcu_assign_pointer(memcg->memsw_thresholds, thresholds_new);
+        }
 
-        /* To be sure that nobody uses thresholds before freeing it */
+        /* To be sure that nobody uses thresholds */
         synchronize_rcu();
 
-        kfree(thresholds);
-unlock:
         mutex_unlock(&memcg->thresholds_lock);
-
-        return ret;
 }
 
 static int mem_cgroup_oom_register_event(struct cgroup *cgrp,
@@ -3724,7 +3743,7 @@ static int mem_cgroup_oom_register_event(struct cgroup *cgrp,
         return 0;
 }
 
-static int mem_cgroup_oom_unregister_event(struct cgroup *cgrp,
+static void mem_cgroup_oom_unregister_event(struct cgroup *cgrp,
         struct cftype *cft, struct eventfd_ctx *eventfd)
 {
         struct mem_cgroup *mem = mem_cgroup_from_cont(cgrp);
@@ -3743,8 +3762,6 @@ static int mem_cgroup_oom_unregister_event(struct cgroup *cgrp,
         }
 
         mutex_unlock(&memcg_oom_mutex);
-
-        return 0;
 }
 
 static int mem_cgroup_oom_control_read(struct cgroup *cgrp,