memcg: use for_each_mem_cgroup

In memory cgroup management, we sometimes have to walk through
subhierarchy of cgroup to gather informaiton, or lock something, etc.

Now, to do that, mem_cgroup_walk_tree() function is provided.  It calls
given callback function per cgroup found.  But the bad thing is that it
has to pass a fixed style function and argument, "void*" and it adds much
type casting to memcontrol.c.

To make the code clean, this patch replaces walk_tree() with

  for_each_mem_cgroup_tree(iter, root)

An iterator style call.  The good point is that iterator call doesn't have
to assume what kind of function is called under it.  A bad point is that
it may cause reference-count leak if a caller use "break" from the loop by
mistake.

I think the benefit is larger.  The modified code seems straigtforward and
easy to read because we don't have misterious callbacks and pointer cast.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 83 insertions, 87 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index dd845d25827a..5e7a14d117c7 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -660,40 +660,57 @@ static struct mem_cgroup *try_get_mem_cgroup_from_mm(struct mm_struct *mm)
         return mem;
 }
 
-/*
- * Call callback function against all cgroup under hierarchy tree.
- */
-static int mem_cgroup_walk_tree(struct mem_cgroup *root, void *data,
-                          int (*func)(struct mem_cgroup *, void *))
+/* The caller has to guarantee "mem" exists before calling this */
+static struct mem_cgroup *mem_cgroup_start_loop(struct mem_cgroup *mem)
 {
-        int found, ret, nextid;
+        if (mem && css_tryget(&mem->css))
+                return mem;
+        return NULL;
+}
+
+static struct mem_cgroup *mem_cgroup_get_next(struct mem_cgroup *iter,
+                                        struct mem_cgroup *root,
+                                        bool cond)
+{
+        int nextid = css_id(&iter->css) + 1;
+        int found;
+        int hierarchy_used;
         struct cgroup_subsys_state *css;
-        struct mem_cgroup *mem;
 
-        if (!root->use_hierarchy)
-                return (*func)(root, data);
+        hierarchy_used = iter->use_hierarchy;
 
-        nextid = 1;
-        do {
-                ret = 0;
-                mem = NULL;
+        css_put(&iter->css);
+        if (!cond || !hierarchy_used)
+                return NULL;
 
+        do {
+                iter = NULL;
                 rcu_read_lock();
-                css = css_get_next(&mem_cgroup_subsys, nextid, &root->css,
-                                   &found);
+
+                css = css_get_next(&mem_cgroup_subsys, nextid,
+                                &root->css, &found);
                 if (css && css_tryget(css))
-                        mem = container_of(css, struct mem_cgroup, css);
+                        iter = container_of(css, struct mem_cgroup, css);
                 rcu_read_unlock();
-
-                if (mem) {
-                        ret = (*func)(mem, data);
-                        css_put(&mem->css);
-                }
+                /* If css is NULL, no more cgroups will be found */
                 nextid = found + 1;
-        } while (!ret && css);
+        } while (css && !iter);
 
-        return ret;
+        return iter;
 }
+/*
+ * for_eacn_mem_cgroup_tree() for visiting all cgroup under tree. Please
+ * be careful that "break" loop is not allowed. We have reference count.
+ * Instead of that modify "cond" to be false and "continue" to exit the loop.
+ */
+#define for_each_mem_cgroup_tree_cond(iter, root, cond) \
+        for (iter = mem_cgroup_start_loop(root);\
+             iter != NULL;\
+             iter = mem_cgroup_get_next(iter, root, cond))
+
+#define for_each_mem_cgroup_tree(iter, root) \
+        for_each_mem_cgroup_tree_cond(iter, root, true)
+
 
 static inline bool mem_cgroup_is_root(struct mem_cgroup *mem)
 {
@@ -1132,13 +1149,6 @@ static bool mem_cgroup_wait_acct_move(struct mem_cgroup *mem)
         return false;
 }
 
-static int mem_cgroup_count_children_cb(struct mem_cgroup *mem, void *data)
-{
-        int *val = data;
-        (*val)++;
-        return 0;
-}
-
 /**
  * mem_cgroup_print_oom_info: Called from OOM with tasklist_lock held in read mode.
  * @memcg: The memory cgroup that went over limit
@@ -1213,7 +1223,10 @@ done:
 static int mem_cgroup_count_children(struct mem_cgroup *mem)
 {
         int num = 0;
-        mem_cgroup_walk_tree(mem, &num, mem_cgroup_count_children_cb);
+        struct mem_cgroup *iter;
+
+        for_each_mem_cgroup_tree(iter, mem)
+                num++;
         return num;
 }
 
@@ -1362,49 +1375,39 @@ static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *root_mem,
         return total;
 }
 
-static int mem_cgroup_oom_lock_cb(struct mem_cgroup *mem, void *data)
-{
-        int *val = (int *)data;
-        int x;
-        /*
-         * Logically, we can stop scanning immediately when we find
-         * a memcg is already locked. But condidering unlock ops and
-         * creation/removal of memcg, scan-all is simple operation.
-         */
-        x = atomic_inc_return(&mem->oom_lock);
-        *val = max(x, *val);
-        return 0;
-}
 /*
  * Check OOM-Killer is already running under our hierarchy.
  * If someone is running, return false.
  */
 static bool mem_cgroup_oom_lock(struct mem_cgroup *mem)
 {
-        int lock_count = 0;
+        int x, lock_count = 0;
+        struct mem_cgroup *iter;
 
-        mem_cgroup_walk_tree(mem, &lock_count, mem_cgroup_oom_lock_cb);
+        for_each_mem_cgroup_tree(iter, mem) {
+                x = atomic_inc_return(&iter->oom_lock);
+                lock_count = max(x, lock_count);
+        }
 
         if (lock_count == 1)
                 return true;
         return false;
 }
 
-static int mem_cgroup_oom_unlock_cb(struct mem_cgroup *mem, void *data)
+static int mem_cgroup_oom_unlock(struct mem_cgroup *mem)
 {
+        struct mem_cgroup *iter;
+
         /*
          * When a new child is created while the hierarchy is under oom,
          * mem_cgroup_oom_lock() may not be called. We have to use
          * atomic_add_unless() here.
          */
-        atomic_add_unless(&mem->oom_lock, -1, 0);
+        for_each_mem_cgroup_tree(iter, mem)
+                atomic_add_unless(&iter->oom_lock, -1, 0);
         return 0;
 }
 
-static void mem_cgroup_oom_unlock(struct mem_cgroup *mem)
-{
-        mem_cgroup_walk_tree(mem, NULL, mem_cgroup_oom_unlock_cb);
-}
 
 static DEFINE_MUTEX(memcg_oom_mutex);
 static DECLARE_WAIT_QUEUE_HEAD(memcg_oom_waitq);
@@ -3207,33 +3210,25 @@ static int mem_cgroup_hierarchy_write(struct cgroup *cont, struct cftype *cft,
         return retval;
 }
 
-struct mem_cgroup_idx_data {
-        s64 val;
-        enum mem_cgroup_stat_index idx;
-};
 
-static int
-mem_cgroup_get_idx_stat(struct mem_cgroup *mem, void *data)
+static u64 mem_cgroup_get_recursive_idx_stat(struct mem_cgroup *mem,
+                                enum mem_cgroup_stat_index idx)
 {
-        struct mem_cgroup_idx_data *d = data;
-        d->val += mem_cgroup_read_stat(mem, d->idx);
-        return 0;
-}
+        struct mem_cgroup *iter;
+        s64 val = 0;
 
-static void
-mem_cgroup_get_recursive_idx_stat(struct mem_cgroup *mem,
-                                enum mem_cgroup_stat_index idx, s64 *val)
-{
-        struct mem_cgroup_idx_data d;
-        d.idx = idx;
-        d.val = 0;
-        mem_cgroup_walk_tree(mem, &d, mem_cgroup_get_idx_stat);
-        *val = d.val;
+        /* each per cpu's value can be minus.Then, use s64 */
+        for_each_mem_cgroup_tree(iter, mem)
+                val += mem_cgroup_read_stat(iter, idx);
+
+        if (val < 0) /* race ? */
+                val = 0;
+        return val;
 }
 
 static inline u64 mem_cgroup_usage(struct mem_cgroup *mem, bool swap)
 {
-        u64 idx_val, val;
+        u64 val;
 
         if (!mem_cgroup_is_root(mem)) {
                 if (!swap)
@@ -3242,16 +3237,12 @@ static inline u64 mem_cgroup_usage(struct mem_cgroup *mem, bool swap)
                         return res_counter_read_u64(&mem->memsw, RES_USAGE);
         }
 
-        mem_cgroup_get_recursive_idx_stat(mem, MEM_CGROUP_STAT_CACHE, &idx_val);
-        val = idx_val;
-        mem_cgroup_get_recursive_idx_stat(mem, MEM_CGROUP_STAT_RSS, &idx_val);
-        val += idx_val;
+        val = mem_cgroup_get_recursive_idx_stat(mem, MEM_CGROUP_STAT_CACHE);
+        val += mem_cgroup_get_recursive_idx_stat(mem, MEM_CGROUP_STAT_RSS);
 
-        if (swap) {
-                mem_cgroup_get_recursive_idx_stat(mem,
-                                MEM_CGROUP_STAT_SWAPOUT, &idx_val);
-                val += idx_val;
-        }
+        if (swap)
+                val += mem_cgroup_get_recursive_idx_stat(mem,
+                                MEM_CGROUP_STAT_SWAPOUT);
 
         return val << PAGE_SHIFT;
 }
@@ -3459,9 +3450,9 @@ struct {
 };
 
 
-static int mem_cgroup_get_local_stat(struct mem_cgroup *mem, void *data)
+static void
+mem_cgroup_get_local_stat(struct mem_cgroup *mem, struct mcs_total_stat *s)
 {
-        struct mcs_total_stat *s = data;
         s64 val;
 
         /* per cpu stat */
@@ -3491,13 +3482,15 @@ static int mem_cgroup_get_local_stat(struct mem_cgroup *mem, void *data)
         s->stat[MCS_ACTIVE_FILE] += val * PAGE_SIZE;
         val = mem_cgroup_get_local_zonestat(mem, LRU_UNEVICTABLE);
         s->stat[MCS_UNEVICTABLE] += val * PAGE_SIZE;
-        return 0;
 }
 
 static void
 mem_cgroup_get_total_stat(struct mem_cgroup *mem, struct mcs_total_stat *s)
 {
-        mem_cgroup_walk_tree(mem, s, mem_cgroup_get_local_stat);
+        struct mem_cgroup *iter;
+
+        for_each_mem_cgroup_tree(iter, mem)
+                mem_cgroup_get_local_stat(iter, s);
 }
 
 static int mem_control_stat_show(struct cgroup *cont, struct cftype *cft,
@@ -3674,7 +3667,7 @@ static int compare_thresholds(const void *a, const void *b)
         return _a->threshold - _b->threshold;
 }
 
-static int mem_cgroup_oom_notify_cb(struct mem_cgroup *mem, void *data)
+static int mem_cgroup_oom_notify_cb(struct mem_cgroup *mem)
 {
         struct mem_cgroup_eventfd_list *ev;
 
@@ -3685,7 +3678,10 @@ static int mem_cgroup_oom_notify_cb(struct mem_cgroup *mem, void *data)
 
 static void mem_cgroup_oom_notify(struct mem_cgroup *mem)
 {
-        mem_cgroup_walk_tree(mem, NULL, mem_cgroup_oom_notify_cb);
+        struct mem_cgroup *iter;
+
+        for_each_mem_cgroup_tree(iter, mem)
+                mem_cgroup_oom_notify_cb(iter);
 }
 
 static int mem_cgroup_usage_register_event(struct cgroup *cgrp,