memcg: use css_tryget in memcg

From:KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>

css_tryget() newly is added and we can know css is alive or not and get
refcnt of css in very safe way.  ("alive" here means "rmdir/destroy" is
not called.)

This patch replaces css_get() to css_tryget(), where I cannot explain
why css_get() is safe. And removes memcg->obsolete flag.

Reviewed-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Paul Menage <menage@google.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
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>

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 4f9a9c5..b311f19 100644 (file)
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -162,7 +162,6 @@ struct mem_cgroup {
         */
        bool use_hierarchy;
        unsigned long   last_oom_jiffies;
-       int             obsolete;
        atomic_t        refcnt;
 
        unsigned int    swappiness;
@@ -283,6 +282,31 @@ struct mem_cgroup *mem_cgroup_from_task(struct task_struct *p)
                                struct mem_cgroup, css);
 }
 
+static struct mem_cgroup *try_get_mem_cgroup_from_mm(struct mm_struct *mm)
+{
+       struct mem_cgroup *mem = NULL;
+       /*
+        * Because we have no locks, mm->owner's may be being moved to other
+        * cgroup. We use css_tryget() here even if this looks
+        * pessimistic (rather than adding locks here).
+        */
+       rcu_read_lock();
+       do {
+               mem = mem_cgroup_from_task(rcu_dereference(mm->owner));
+               if (unlikely(!mem))
+                       break;
+       } while (!css_tryget(&mem->css));
+       rcu_read_unlock();
+       return mem;
+}
+
+static bool mem_cgroup_is_obsolete(struct mem_cgroup *mem)
+{
+       if (!mem)
+               return true;
+       return css_is_removed(&mem->css);
+}
+
 /*
  * Following LRU functions are allowed to be used without PCG_LOCK.
  * Operations are called by routine of global LRU independently from memcg.
@@ -622,8 +646,9 @@ mem_cgroup_get_first_node(struct mem_cgroup *root_mem)
 {
        struct cgroup *cgroup;
        struct mem_cgroup *ret;
-       bool obsolete = (root_mem->last_scanned_child &&
-                               root_mem->last_scanned_child->obsolete);
+       bool obsolete;
+
+       obsolete = mem_cgroup_is_obsolete(root_mem->last_scanned_child);
 
        /*
         * Scan all children under the mem_cgroup mem
@@ -636,7 +661,7 @@ mem_cgroup_get_first_node(struct mem_cgroup *root_mem)
 
        if (!root_mem->last_scanned_child || obsolete) {
 
-               if (obsolete)
+               if (obsolete && root_mem->last_scanned_child)
                        mem_cgroup_put(root_mem->last_scanned_child);
 
                cgroup = list_first_entry(&root_mem->css.cgroup->children,
@@ -711,7 +736,7 @@ static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *root_mem,
        next_mem = mem_cgroup_get_first_node(root_mem);
 
        while (next_mem != root_mem) {
-               if (next_mem->obsolete) {
+               if (mem_cgroup_is_obsolete(next_mem)) {
                        mem_cgroup_put(next_mem);
                        cgroup_lock();
                        next_mem = mem_cgroup_get_first_node(root_mem);
@@ -769,23 +794,17 @@ static int __mem_cgroup_try_charge(struct mm_struct *mm,
         * thread group leader migrates. It's possible that mm is not
         * set, if so charge the init_mm (happens for pagecache usage).
         */
-       if (likely(!*memcg)) {
-               rcu_read_lock();
-               mem = mem_cgroup_from_task(rcu_dereference(mm->owner));
-               if (unlikely(!mem)) {
-                       rcu_read_unlock();
-                       return 0;
-               }
-               /*
-                * For every charge from the cgroup, increment reference count
-                */
-               css_get(&mem->css);
+       mem = *memcg;
+       if (likely(!mem)) {
+               mem = try_get_mem_cgroup_from_mm(mm);
                *memcg = mem;
-               rcu_read_unlock();
        } else {
-               mem = *memcg;
                css_get(&mem->css);
        }
+       if (unlikely(!mem))
+               return 0;
+
+       VM_BUG_ON(mem_cgroup_is_obsolete(mem));
 
        while (1) {
                int ret;
@@ -1072,12 +1091,19 @@ int mem_cgroup_cache_charge(struct page *page, struct mm_struct *mm,
                                MEM_CGROUP_CHARGE_TYPE_SHMEM, NULL);
 }
 
+/*
+ * While swap-in, try_charge -> commit or cancel, the page is locked.
+ * And when try_charge() successfully returns, one refcnt to memcg without
+ * struct page_cgroup is aquired. This refcnt will be cumsumed by
+ * "commit()" or removed by "cancel()"
+ */
 int mem_cgroup_try_charge_swapin(struct mm_struct *mm,
                                 struct page *page,
                                 gfp_t mask, struct mem_cgroup **ptr)
 {
        struct mem_cgroup *mem;
        swp_entry_t     ent;
+       int ret;
 
        if (mem_cgroup_disabled())
                return 0;
@@ -1096,10 +1122,15 @@ int mem_cgroup_try_charge_swapin(struct mm_struct *mm,
        ent.val = page_private(page);
 
        mem = lookup_swap_cgroup(ent);
-       if (!mem || mem->obsolete)
+       if (!mem)
+               goto charge_cur_mm;
+       if (!css_tryget(&mem->css))
                goto charge_cur_mm;
        *ptr = mem;
-       return __mem_cgroup_try_charge(NULL, mask, ptr, true);
+       ret = __mem_cgroup_try_charge(NULL, mask, ptr, true);
+       /* drop extra refcnt from tryget */
+       css_put(&mem->css);
+       return ret;
 charge_cur_mm:
        if (unlikely(!mm))
                mm = &init_mm;
@@ -1130,13 +1161,18 @@ int mem_cgroup_cache_charge_swapin(struct page *page,
                ent.val = page_private(page);
                if (do_swap_account) {
                        mem = lookup_swap_cgroup(ent);
-                       if (mem && mem->obsolete)
-                               mem = NULL;
-                       if (mem)
-                               mm = NULL;
+                       if (mem) {
+                               if (css_tryget(&mem->css))
+                                       mm = NULL; /* charge to recorded */
+                               else
+                                       mem = NULL; /* charge to current */
+                       }
                }
                ret = mem_cgroup_charge_common(page, mm, mask,
                                MEM_CGROUP_CHARGE_TYPE_SHMEM, mem);
+               /* drop extra refcnt from tryget */
+               if (mem)
+                       css_put(&mem->css);
 
                if (!ret && do_swap_account) {
                        /* avoid double counting */
@@ -1178,7 +1214,6 @@ void mem_cgroup_commit_charge_swapin(struct page *page, struct mem_cgroup *ptr)
                struct mem_cgroup *memcg;
                memcg = swap_cgroup_record(ent, NULL);
                if (memcg) {
-                       /* If memcg is obsolete, memcg can be != ptr */
                        res_counter_uncharge(&memcg->memsw, PAGE_SIZE);
                        mem_cgroup_put(memcg);
                }
@@ -1421,14 +1456,9 @@ int mem_cgroup_shrink_usage(struct mm_struct *mm, gfp_t gfp_mask)
        if (!mm)
                return 0;
 
-       rcu_read_lock();
-       mem = mem_cgroup_from_task(rcu_dereference(mm->owner));
-       if (unlikely(!mem)) {
-               rcu_read_unlock();
+       mem = try_get_mem_cgroup_from_mm(mm);
+       if (unlikely(!mem))
                return 0;
-       }
-       css_get(&mem->css);
-       rcu_read_unlock();
 
        do {
                progress = mem_cgroup_hierarchical_reclaim(mem, gfp_mask, true);
@@ -2086,9 +2116,6 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
  * the number of reference from swap_cgroup and free mem_cgroup when
  * it goes down to 0.
  *
- * When mem_cgroup is destroyed, mem->obsolete will be set to 0 and
- * entry which points to this memcg will be ignore at swapin.
- *
  * Removal of cgroup itself succeeds regardless of refs from swap.
  */
 
@@ -2174,7 +2201,6 @@ static void mem_cgroup_pre_destroy(struct cgroup_subsys *ss,
                                        struct cgroup *cont)
 {
        struct mem_cgroup *mem = mem_cgroup_from_cont(cont);
-       mem->obsolete = 1;
        mem_cgroup_force_empty(mem, false);
 }