[PATCH RESEND 00/11] kmemcg-fixes

[PATCH RESEND 00/11] kmemcg-fixes

[Vladimir Davydov]

Hi,

This patch-set fixes several bugs here and there in the implementation
of kmem accounting for memory cgroups and hopefully makes the code look
a bit clearer.

Links to discussion threads that led to this patch-set:
http://www.spinics.net/lists/cgroups/msg09512.html
http://www.spinics.net/lists/cgroups/msg09695.html
http://www.spinics.net/lists/cgroups/msg09796.html

Any comments are highly appreciated.

Thanks.

Vladimir Davydov (11):
  slab: cleanup kmem_cache_create_memcg() error handling
  memcg, slab: kmem_cache_create_memcg(): fix memleak on fail path
  memcg, slab: cleanup memcg cache initialization/destruction
  memcg, slab: fix barrier usage when accessing memcg_caches
  memcg: fix possible NULL deref while traversing memcg_slab_caches
    list
  memcg, slab: fix races in per-memcg cache creation/destruction
  memcg: get rid of kmem_cache_dup
  slab: do not panic if we fail to create memcg cache
  memcg, slab: RCU protect memcg_params for root caches
  memcg: remove KMEM_ACCOUNTED_ACTIVATED flag
  memcg: rework memcg_update_kmem_limit synchronization

 include/linux/memcontrol.h |   23 +--
 include/linux/slab.h       |    9 +-
 mm/memcontrol.c            |  405 +++++++++++++++++++++-----------------------
 mm/slab.h                  |   26 ++-
 mm/slab_common.c           |   90 ++++++----
 5 files changed, 292 insertions(+), 261 deletions(-)

-- 
1.7.10.4

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[PATCH RESEND 01/11] slab: cleanup kmem_cache_create_memcg() error handling

[Vladimir Davydov]

Currently kmem_cache_create_memcg() backoffs on failure inside
conditionals, without using gotos. This results in the rollback code
duplication, which makes the function look cumbersome even though on
error we should only free the allocated cache. Since in the next patch I
am going to add yet another rollback function call on error path there,
let's employ labels instead of conditionals for undoing any changes on
failure to keep things clean.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Reviewed-by: Pekka Enberg <penberg@kernel.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Glauber Costa <glommer@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 mm/slab_common.c |   65 ++++++++++++++++++++++++++----------------------------
 1 file changed, 31 insertions(+), 34 deletions(-)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 0b7bb39..f70df3e 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -171,13 +171,14 @@ kmem_cache_create_memcg(struct mem_cgroup *memcg, const char *name, size_t size,
 			struct kmem_cache *parent_cache)
 {
 	struct kmem_cache *s = NULL;
-	int err = 0;
+	int err;
 
 	get_online_cpus();
 	mutex_lock(&slab_mutex);
 
-	if (!kmem_cache_sanity_check(memcg, name, size) == 0)
-		goto out_locked;
+	err = kmem_cache_sanity_check(memcg, name, size);
+	if (err)
+		goto out_unlock;
 
 	/*
 	 * Some allocators will constraint the set of valid flags to a subset
@@ -189,45 +190,38 @@ kmem_cache_create_memcg(struct mem_cgroup *memcg, const char *name, size_t size,
 
 	s = __kmem_cache_alias(memcg, name, size, align, flags, ctor);
 	if (s)
-		goto out_locked;
+		goto out_unlock;
 
+	err = -ENOMEM;
 	s = kmem_cache_zalloc(kmem_cache, GFP_KERNEL);
-	if (s) {
-		s->object_size = s->size = size;
-		s->align = calculate_alignment(flags, align, size);
-		s->ctor = ctor;
+	if (!s)
+		goto out_unlock;
 
-		if (memcg_register_cache(memcg, s, parent_cache)) {
-			kmem_cache_free(kmem_cache, s);
-			err = -ENOMEM;
-			goto out_locked;
-		}
+	s->object_size = s->size = size;
+	s->align = calculate_alignment(flags, align, size);
+	s->ctor = ctor;
 
-		s->name = kstrdup(name, GFP_KERNEL);
-		if (!s->name) {
-			kmem_cache_free(kmem_cache, s);
-			err = -ENOMEM;
-			goto out_locked;
-		}
+	s->name = kstrdup(name, GFP_KERNEL);
+	if (!s->name)
+		goto out_free_cache;
 
-		err = __kmem_cache_create(s, flags);
-		if (!err) {
-			s->refcount = 1;
-			list_add(&s->list, &slab_caches);
-			memcg_cache_list_add(memcg, s);
-		} else {
-			kfree(s->name);
-			kmem_cache_free(kmem_cache, s);
-		}
-	} else
-		err = -ENOMEM;
+	err = memcg_register_cache(memcg, s, parent_cache);
+	if (err)
+		goto out_free_cache;
+
+	err = __kmem_cache_create(s, flags);
+	if (err)
+		goto out_free_cache;
+
+	s->refcount = 1;
+	list_add(&s->list, &slab_caches);
+	memcg_cache_list_add(memcg, s);
 
-out_locked:
+out_unlock:
 	mutex_unlock(&slab_mutex);
 	put_online_cpus();
 
 	if (err) {
-
 		if (flags & SLAB_PANIC)
 			panic("kmem_cache_create: Failed to create slab '%s'. Error %d\n",
 				name, err);
@@ -236,11 +230,14 @@ out_locked:
 				name, err);
 			dump_stack();
 		}
-
 		return NULL;
 	}
-
 	return s;
+
+out_free_cache:
+	kfree(s->name);
+	kmem_cache_free(kmem_cache, s);
+	goto out_unlock;
 }
 
 struct kmem_cache *
-- 
1.7.10.4

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[PATCH RESEND 02/11] memcg, slab: kmem_cache_create_memcg(): fix memleak on fail path

[Vladimir Davydov]

We do not free the cache's memcg_params if __kmem_cache_create fails.
Fix this.

Plus, rename memcg_register_cache() to memcg_alloc_cache_params(),
because it actually does not register the cache anywhere, but simply
initialize kmem_cache::memcg_params.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Glauber Costa <glommer@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 include/linux/memcontrol.h |   14 +++++++++-----
 mm/memcontrol.c            |   11 ++++++++---
 mm/slab_common.c           |    3 ++-
 3 files changed, 19 insertions(+), 9 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index b3e7a66..5e6541f 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -497,8 +497,9 @@ void __memcg_kmem_commit_charge(struct page *page,
 void __memcg_kmem_uncharge_pages(struct page *page, int order);
 
 int memcg_cache_id(struct mem_cgroup *memcg);
-int memcg_register_cache(struct mem_cgroup *memcg, struct kmem_cache *s,
-			 struct kmem_cache *root_cache);
+int memcg_alloc_cache_params(struct mem_cgroup *memcg, struct kmem_cache *s,
+			     struct kmem_cache *root_cache);
+void memcg_free_cache_params(struct kmem_cache *s);
 void memcg_release_cache(struct kmem_cache *cachep);
 void memcg_cache_list_add(struct mem_cgroup *memcg, struct kmem_cache *cachep);
 
@@ -640,13 +641,16 @@ static inline int memcg_cache_id(struct mem_cgroup *memcg)
 	return -1;
 }
 
-static inline int
-memcg_register_cache(struct mem_cgroup *memcg, struct kmem_cache *s,
-		     struct kmem_cache *root_cache)
+static inline int memcg_alloc_cache_params(struct mem_cgroup *memcg,
+		struct kmem_cache *s, struct kmem_cache *root_cache)
 {
 	return 0;
 }
 
+static inline void memcg_free_cache_params(struct kmem_cache *s);
+{
+}
+
 static inline void memcg_release_cache(struct kmem_cache *cachep)
 {
 }
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index bf5e894..8c47910 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3195,8 +3195,8 @@ int memcg_update_cache_size(struct kmem_cache *s, int num_groups)
 	return 0;
 }
 
-int memcg_register_cache(struct mem_cgroup *memcg, struct kmem_cache *s,
-			 struct kmem_cache *root_cache)
+int memcg_alloc_cache_params(struct mem_cgroup *memcg, struct kmem_cache *s,
+			     struct kmem_cache *root_cache)
 {
 	size_t size;
 
@@ -3224,6 +3224,11 @@ int memcg_register_cache(struct mem_cgroup *memcg, struct kmem_cache *s,
 	return 0;
 }
 
+void memcg_free_cache_params(struct kmem_cache *s)
+{
+	kfree(s->memcg_params);
+}
+
 void memcg_release_cache(struct kmem_cache *s)
 {
 	struct kmem_cache *root;
@@ -3252,7 +3257,7 @@ void memcg_release_cache(struct kmem_cache *s)
 
 	css_put(&memcg->css);
 out:
-	kfree(s->memcg_params);
+	memcg_free_cache_params(s);
 }
 
 /*
diff --git a/mm/slab_common.c b/mm/slab_common.c
index f70df3e..70f9e24 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -205,7 +205,7 @@ kmem_cache_create_memcg(struct mem_cgroup *memcg, const char *name, size_t size,
 	if (!s->name)
 		goto out_free_cache;
 
-	err = memcg_register_cache(memcg, s, parent_cache);
+	err = memcg_alloc_cache_params(memcg, s, parent_cache);
 	if (err)
 		goto out_free_cache;
 
@@ -235,6 +235,7 @@ out_unlock:
 	return s;
 
 out_free_cache:
+	memcg_free_cache_params(s);
 	kfree(s->name);
 	kmem_cache_free(kmem_cache, s);
 	goto out_unlock;
-- 
1.7.10.4

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[PATCH RESEND 03/11] memcg, slab: cleanup memcg cache initialization/destruction

[Vladimir Davydov]

Currently, we have rather a messy function set relating to per-memcg
kmem cache initialization/destruction.

Per-memcg caches are created in memcg_create_kmem_cache(). This function
calls kmem_cache_create_memcg() to allocate and initialize a kmem cache
and then "registers" the new cache in the memcg_params::memcg_caches
array of the parent cache.

During its work-flow, kmem_cache_create_memcg() executes the following
memcg-related functions:

 - memcg_alloc_cache_params(), to initialize memcg_params of the newly
   created cache;
 - memcg_cache_list_add(), to add the new cache to the memcg_slab_caches
   list.

On the other hand, kmem_cache_destroy() called on a cache destruction
only calls memcg_release_cache(), which does all the work: it cleans the
reference to the cache in its parent's memcg_params::memcg_caches,
removes the cache from the memcg_slab_caches list, and frees
memcg_params.

Such an inconsistency between destruction and initialization paths make
the code difficult to read, so let's clean this up a bit.

This patch moves all the code relating to registration of per-memcg
caches (adding to memcg list, setting the pointer to a cache from its
parent) to the newly created memcg_register_cache() and
memcg_unregister_cache() functions making the initialization and
destruction paths look symmetrical.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Glauber Costa <glommer@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 include/linux/memcontrol.h |    9 +++----
 mm/memcontrol.c            |   64 +++++++++++++++++++++-----------------------
 mm/slab_common.c           |    5 ++--
 3 files changed, 37 insertions(+), 41 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 5e6541f..6202406 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -500,8 +500,8 @@ int memcg_cache_id(struct mem_cgroup *memcg);
 int memcg_alloc_cache_params(struct mem_cgroup *memcg, struct kmem_cache *s,
 			     struct kmem_cache *root_cache);
 void memcg_free_cache_params(struct kmem_cache *s);
-void memcg_release_cache(struct kmem_cache *cachep);
-void memcg_cache_list_add(struct mem_cgroup *memcg, struct kmem_cache *cachep);
+void memcg_register_cache(struct kmem_cache *s);
+void memcg_unregister_cache(struct kmem_cache *s);
 
 int memcg_update_cache_size(struct kmem_cache *s, int num_groups);
 void memcg_update_array_size(int num_groups);
@@ -651,12 +651,11 @@ static inline void memcg_free_cache_params(struct kmem_cache *s);
 {
 }
 
-static inline void memcg_release_cache(struct kmem_cache *cachep)
+static inline void memcg_register_cache(struct kmem_cache *s)
 {
 }
 
-static inline void memcg_cache_list_add(struct mem_cgroup *memcg,
-					struct kmem_cache *s)
+static inline void memcg_unregister_cache(struct kmem_cache *s)
 {
 }
 
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 8c47910..f8eb994 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3059,16 +3059,6 @@ static void memcg_uncharge_kmem(struct mem_cgroup *memcg, u64 size)
 		css_put(&memcg->css);
 }
 
-void memcg_cache_list_add(struct mem_cgroup *memcg, struct kmem_cache *cachep)
-{
-	if (!memcg)
-		return;
-
-	mutex_lock(&memcg->slab_caches_mutex);
-	list_add(&cachep->memcg_params->list, &memcg->memcg_slab_caches);
-	mutex_unlock(&memcg->slab_caches_mutex);
-}
-
 /*
  * helper for acessing a memcg's index. It will be used as an index in the
  * child cache array in kmem_cache, and also to derive its name. This function
@@ -3229,21 +3219,41 @@ void memcg_free_cache_params(struct kmem_cache *s)
 	kfree(s->memcg_params);
 }
 
-void memcg_release_cache(struct kmem_cache *s)
+void memcg_register_cache(struct kmem_cache *s)
 {
 	struct kmem_cache *root;
 	struct mem_cgroup *memcg;
 	int id;
 
+	if (is_root_cache(s))
+		return;
+
+	root = s->memcg_params->root_cache;
+	memcg = s->memcg_params->memcg;
+	id = memcg_cache_id(memcg);
+
+	css_get(&memcg->css);
+
+	mutex_lock(&memcg->slab_caches_mutex);
+	list_add(&s->memcg_params->list, &memcg->memcg_slab_caches);
+	mutex_unlock(&memcg->slab_caches_mutex);
+
+	root->memcg_params->memcg_caches[id] = s;
 	/*
-	 * This happens, for instance, when a root cache goes away before we
-	 * add any memcg.
+	 * the readers won't lock, make sure everybody sees the updated value,
+	 * so they won't put stuff in the queue again for no reason
 	 */
-	if (!s->memcg_params)
-		return;
+	wmb();
+}
 
-	if (s->memcg_params->is_root_cache)
-		goto out;
+void memcg_unregister_cache(struct kmem_cache *s)
+{
+	struct kmem_cache *root;
+	struct mem_cgroup *memcg;
+	int id;
+
+	if (is_root_cache(s))
+		return;
 
 	memcg = s->memcg_params->memcg;
 	id  = memcg_cache_id(memcg);
@@ -3256,8 +3266,6 @@ void memcg_release_cache(struct kmem_cache *s)
 	mutex_unlock(&memcg->slab_caches_mutex);
 
 	css_put(&memcg->css);
-out:
-	memcg_free_cache_params(s);
 }
 
 /*
@@ -3415,26 +3423,13 @@ static struct kmem_cache *memcg_create_kmem_cache(struct mem_cgroup *memcg,
 
 	mutex_lock(&memcg_cache_mutex);
 	new_cachep = cache_from_memcg_idx(cachep, idx);
-	if (new_cachep) {
-		css_put(&memcg->css);
+	if (new_cachep)
 		goto out;
-	}
 
 	new_cachep = kmem_cache_dup(memcg, cachep);
-	if (new_cachep == NULL) {
+	if (new_cachep == NULL)
 		new_cachep = cachep;
-		css_put(&memcg->css);
-		goto out;
-	}
-
-	atomic_set(&new_cachep->memcg_params->nr_pages , 0);
 
-	cachep->memcg_params->memcg_caches[idx] = new_cachep;
-	/*
-	 * the readers won't lock, make sure everybody sees the updated value,
-	 * so they won't put stuff in the queue again for no reason
-	 */
-	wmb();
 out:
 	mutex_unlock(&memcg_cache_mutex);
 	return new_cachep;
@@ -3514,6 +3509,7 @@ static void memcg_create_cache_work_func(struct work_struct *w)
 
 	cw = container_of(w, struct create_work, work);
 	memcg_create_kmem_cache(cw->memcg, cw->cachep);
+	css_put(&cw->memcg->css);
 	kfree(cw);
 }
 
diff --git a/mm/slab_common.c b/mm/slab_common.c
index 70f9e24..db24ec4 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -215,7 +215,7 @@ kmem_cache_create_memcg(struct mem_cgroup *memcg, const char *name, size_t size,
 
 	s->refcount = 1;
 	list_add(&s->list, &slab_caches);
-	memcg_cache_list_add(memcg, s);
+	memcg_register_cache(s);
 
 out_unlock:
 	mutex_unlock(&slab_mutex);
@@ -265,7 +265,8 @@ void kmem_cache_destroy(struct kmem_cache *s)
 			if (s->flags & SLAB_DESTROY_BY_RCU)
 				rcu_barrier();
 
-			memcg_release_cache(s);
+			memcg_unregister_cache(s);
+			memcg_free_cache_params(s);
 			kfree(s->name);
 			kmem_cache_free(kmem_cache, s);
 		} else {
-- 
1.7.10.4

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[PATCH RESEND 04/11] memcg, slab: fix barrier usage when accessing memcg_caches

[Vladimir Davydov]

Each root kmem_cache has pointers to per-memcg caches stored in its
memcg_params::memcg_caches array. Whenever we want to allocate a slab
for a memcg, we access this array to get per-memcg cache to allocate
from (see memcg_kmem_get_cache()). The access must be lock-free for
performance reasons, so we should use barriers to assert the kmem_cache
is up-to-date.

First, we should place a write barrier immediately before setting the
pointer to it in the memcg_caches array in order to make sure nobody
will see a partially initialized object. Second, we should issue a read
barrier before dereferencing the pointer to conform to the write
barrier.

However, currently the barrier usage looks rather strange. We have a
write barrier *after* setting the pointer and a read barrier *before*
reading the pointer, which is incorrect. This patch fixes this.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Glauber Costa <glommer@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 mm/memcontrol.c |   24 ++++++++++--------------
 mm/slab.h       |   12 +++++++++++-
 2 files changed, 21 insertions(+), 15 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index f8eb994..999e7d4 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3238,12 +3238,14 @@ void memcg_register_cache(struct kmem_cache *s)
 	list_add(&s->memcg_params->list, &memcg->memcg_slab_caches);
 	mutex_unlock(&memcg->slab_caches_mutex);
 
-	root->memcg_params->memcg_caches[id] = s;
 	/*
-	 * the readers won't lock, make sure everybody sees the updated value,
-	 * so they won't put stuff in the queue again for no reason
+	 * Since readers won't lock (see cache_from_memcg_idx()), we need a
+	 * barrier here to ensure nobody will see the kmem_cache partially
+	 * initialized.
 	 */
-	wmb();
+	smp_wmb();
+
+	root->memcg_params->memcg_caches[id] = s;
 }
 
 void memcg_unregister_cache(struct kmem_cache *s)
@@ -3569,7 +3571,7 @@ struct kmem_cache *__memcg_kmem_get_cache(struct kmem_cache *cachep,
 					  gfp_t gfp)
 {
 	struct mem_cgroup *memcg;
-	int idx;
+	struct kmem_cache *memcg_cachep;
 
 	VM_BUG_ON(!cachep->memcg_params);
 	VM_BUG_ON(!cachep->memcg_params->is_root_cache);
@@ -3583,15 +3585,9 @@ struct kmem_cache *__memcg_kmem_get_cache(struct kmem_cache *cachep,
 	if (!memcg_can_account_kmem(memcg))
 		goto out;
 
-	idx = memcg_cache_id(memcg);
-
-	/*
-	 * barrier to mare sure we're always seeing the up to date value.  The
-	 * code updating memcg_caches will issue a write barrier to match this.
-	 */
-	read_barrier_depends();
-	if (likely(cache_from_memcg_idx(cachep, idx))) {
-		cachep = cache_from_memcg_idx(cachep, idx);
+	memcg_cachep = cache_from_memcg_idx(cachep, memcg_cache_id(memcg));
+	if (likely(memcg_cachep)) {
+		cachep = memcg_cachep;
 		goto out;
 	}
 
diff --git a/mm/slab.h b/mm/slab.h
index 0859c42..72d1f9d 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -163,9 +163,19 @@ static inline const char *cache_name(struct kmem_cache *s)
 static inline struct kmem_cache *
 cache_from_memcg_idx(struct kmem_cache *s, int idx)
 {
+	struct kmem_cache *cachep;
+
 	if (!s->memcg_params)
 		return NULL;
-	return s->memcg_params->memcg_caches[idx];
+	cachep = s->memcg_params->memcg_caches[idx];
+
+	/*
+	 * Make sure we will access the up-to-date value. The code updating
+	 * memcg_caches issues a write barrier to match this (see
+	 * memcg_register_cache()).
+	 */
+	smp_read_barrier_depends();
+	return cachep;
 }
 
 static inline struct kmem_cache *memcg_root_cache(struct kmem_cache *s)
-- 
1.7.10.4

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[PATCH RESEND 05/11] memcg: fix possible NULL deref while traversing memcg_slab_caches list

[Vladimir Davydov]

All caches of the same memory cgroup are linked in the memcg_slab_caches
list via kmem_cache::memcg_params::list. This list is traversed, for
example, when we read memory.kmem.slabinfo. Since the list actually
consists of memcg_cache_params objects, we have to convert an element of
the list to a kmem_cache object using memcg_params_to_cache(), which
obtains the pointer to the cache from the memcg_params::memcg_caches
array of the corresponding root cache. That said the pointer to a
kmem_cache in its parent's memcg_params must be initialized before
adding the cache to the list, and cleared only after it has been
unlinked. Currently it is vice-versa, which can result in a NULL ptr
dereference while traversing the memcg_slab_caches list. This patch
restores the correct order.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Glauber Costa <glommer@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 mm/memcontrol.c |   25 ++++++++++++++++++-------
 1 file changed, 18 insertions(+), 7 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 999e7d4..d918626 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3234,9 +3234,6 @@ void memcg_register_cache(struct kmem_cache *s)
 
 	css_get(&memcg->css);
 
-	mutex_lock(&memcg->slab_caches_mutex);
-	list_add(&s->memcg_params->list, &memcg->memcg_slab_caches);
-	mutex_unlock(&memcg->slab_caches_mutex);
 
 	/*
 	 * Since readers won't lock (see cache_from_memcg_idx()), we need a
@@ -3245,7 +3242,16 @@ void memcg_register_cache(struct kmem_cache *s)
 	 */
 	smp_wmb();
 
+	/*
+	 * Initialize the pointer to this cache in its parent's memcg_params
+	 * before adding it to the memcg_slab_caches list, otherwise we can
+	 * fail to convert memcg_params_to_cache() while traversing the list.
+	 */
 	root->memcg_params->memcg_caches[id] = s;
+
+	mutex_lock(&memcg->slab_caches_mutex);
+	list_add(&s->memcg_params->list, &memcg->memcg_slab_caches);
+	mutex_unlock(&memcg->slab_caches_mutex);
 }
 
 void memcg_unregister_cache(struct kmem_cache *s)
@@ -3257,16 +3263,21 @@ void memcg_unregister_cache(struct kmem_cache *s)
 	if (is_root_cache(s))
 		return;
 
-	memcg = s->memcg_params->memcg;
-	id  = memcg_cache_id(memcg);
-
 	root = s->memcg_params->root_cache;
-	root->memcg_params->memcg_caches[id] = NULL;
+	memcg = s->memcg_params->memcg;
+	id = memcg_cache_id(memcg);
 
 	mutex_lock(&memcg->slab_caches_mutex);
 	list_del(&s->memcg_params->list);
 	mutex_unlock(&memcg->slab_caches_mutex);
 
+	/*
+	 * Clear the pointer to this cache in its parent's memcg_params only
+	 * after removing it from the memcg_slab_caches list, otherwise we can
+	 * fail to convert memcg_params_to_cache() while traversing the list.
+	 */
+	root->memcg_params->memcg_caches[id] = NULL;
+
 	css_put(&memcg->css);
 }
 
-- 
1.7.10.4

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[PATCH RESEND 06/11] memcg, slab: fix races in per-memcg cache creation/destruction

[Vladimir Davydov]

We obtain a per-memcg cache from a root kmem_cache by dereferencing an
entry of the root cache's memcg_params::memcg_caches array. If we find
no cache for a memcg there on allocation, we initiate the memcg cache
creation (see memcg_kmem_get_cache()). The cache creation proceeds
asynchronously in memcg_create_kmem_cache() in order to avoid lock
clashes, so there can be several threads trying to create the same
kmem_cache concurrently, but only one of them may succeed. However, due
to a race in the code, it is not always true. The point is that the
memcg_caches array can be relocated when we activate kmem accounting for
a memcg (see memcg_update_all_caches(), memcg_update_cache_size()). If
memcg_update_cache_size() and memcg_create_kmem_cache() proceed
concurrently as described below, we can leak a kmem_cache.

Asume two threads schedule creation of the same kmem_cache. One of them
successfully creates it. Another one should fail then, but if
memcg_create_kmem_cache() interleaves with memcg_update_cache_size() as
follows, it won't:

  memcg_create_kmem_cache()             memcg_update_cache_size()
  (called w/o mutexes held)             (called with slab_mutex,
                                         set_limit_mutex held)
  -------------------------             -------------------------

  mutex_lock(&memcg_cache_mutex)

                                        s->memcg_params=kzalloc(...)

  new_cachep=cache_from_memcg_idx(cachep,idx)
  // new_cachep==NULL => proceed to creation

                                        s->memcg_params->memcg_caches[i]
                                            =cur_params->memcg_caches[i]

  // kmem_cache_create_memcg takes slab_mutex
  // so we will hang around until
  // memcg_update_cache_size finishes, but
  // nothing will prevent it from succeeding so
  // memcg_caches[idx] will be overwritten in
  // memcg_register_cache!

  new_cachep = kmem_cache_create_memcg(...)
  mutex_unlock(&memcg_cache_mutex)

Let's fix this by moving the check for existence of the memcg cache to
kmem_cache_create_memcg() to be called under the slab_mutex and make it
return NULL if so.

A similar race is possible when destroying a memcg cache (see
kmem_cache_destroy()). Since memcg_unregister_cache(), which clears the
pointer in the memcg_caches array, is called w/o protection, we can race
with memcg_update_cache_size() and omit clearing the pointer. Therefore
memcg_unregister_cache() should be moved before we release the
slab_mutex.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Glauber Costa <glommer@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 mm/memcontrol.c  |   23 ++++++++++++++---------
 mm/slab_common.c |   14 +++++++++++++-
 2 files changed, 27 insertions(+), 10 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index d918626..56fc410 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3228,6 +3228,12 @@ void memcg_register_cache(struct kmem_cache *s)
 	if (is_root_cache(s))
 		return;
 
+	/*
+	 * Holding the slab_mutex assures nobody will touch the memcg_caches
+	 * array while we are modifying it.
+	 */
+	lockdep_assert_held(&slab_mutex);
+
 	root = s->memcg_params->root_cache;
 	memcg = s->memcg_params->memcg;
 	id = memcg_cache_id(memcg);
@@ -3247,6 +3253,7 @@ void memcg_register_cache(struct kmem_cache *s)
 	 * before adding it to the memcg_slab_caches list, otherwise we can
 	 * fail to convert memcg_params_to_cache() while traversing the list.
 	 */
+	VM_BUG_ON(root->memcg_params->memcg_caches[id]);
 	root->memcg_params->memcg_caches[id] = s;
 
 	mutex_lock(&memcg->slab_caches_mutex);
@@ -3263,6 +3270,12 @@ void memcg_unregister_cache(struct kmem_cache *s)
 	if (is_root_cache(s))
 		return;
 
+	/*
+	 * Holding the slab_mutex assures nobody will touch the memcg_caches
+	 * array while we are modifying it.
+	 */
+	lockdep_assert_held(&slab_mutex);
+
 	root = s->memcg_params->root_cache;
 	memcg = s->memcg_params->memcg;
 	id = memcg_cache_id(memcg);
@@ -3276,6 +3289,7 @@ void memcg_unregister_cache(struct kmem_cache *s)
 	 * after removing it from the memcg_slab_caches list, otherwise we can
 	 * fail to convert memcg_params_to_cache() while traversing the list.
 	 */
+	VM_BUG_ON(!root->memcg_params->memcg_caches[id]);
 	root->memcg_params->memcg_caches[id] = NULL;
 
 	css_put(&memcg->css);
@@ -3428,22 +3442,13 @@ static struct kmem_cache *memcg_create_kmem_cache(struct mem_cgroup *memcg,
 						  struct kmem_cache *cachep)
 {
 	struct kmem_cache *new_cachep;
-	int idx;
 
 	BUG_ON(!memcg_can_account_kmem(memcg));
 
-	idx = memcg_cache_id(memcg);
-
 	mutex_lock(&memcg_cache_mutex);
-	new_cachep = cache_from_memcg_idx(cachep, idx);
-	if (new_cachep)
-		goto out;
-
 	new_cachep = kmem_cache_dup(memcg, cachep);
 	if (new_cachep == NULL)
 		new_cachep = cachep;
-
-out:
 	mutex_unlock(&memcg_cache_mutex);
 	return new_cachep;
 }
diff --git a/mm/slab_common.c b/mm/slab_common.c
index db24ec4..f34707e 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -180,6 +180,18 @@ kmem_cache_create_memcg(struct mem_cgroup *memcg, const char *name, size_t size,
 	if (err)
 		goto out_unlock;
 
+	if (memcg) {
+		/*
+		 * Since per-memcg caches are created asynchronously on first
+		 * allocation (see memcg_kmem_get_cache()), several threads can
+		 * try to create the same cache, but only one of them may
+		 * succeed. Therefore if we get here and see the cache has
+		 * already been created, we silently return NULL.
+		 */
+		if (cache_from_memcg_idx(parent_cache, memcg_cache_id(memcg)))
+			goto out_unlock;
+	}
+
 	/*
 	 * Some allocators will constraint the set of valid flags to a subset
 	 * of all flags. We expect them to define CACHE_CREATE_MASK in this
@@ -261,11 +273,11 @@ void kmem_cache_destroy(struct kmem_cache *s)
 		list_del(&s->list);
 
 		if (!__kmem_cache_shutdown(s)) {
+			memcg_unregister_cache(s);
 			mutex_unlock(&slab_mutex);
 			if (s->flags & SLAB_DESTROY_BY_RCU)
 				rcu_barrier();
 
-			memcg_unregister_cache(s);
 			memcg_free_cache_params(s);
 			kfree(s->name);
 			kmem_cache_free(kmem_cache, s);
-- 
1.7.10.4

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[PATCH RESEND 07/11] memcg: get rid of kmem_cache_dup

[Vladimir Davydov]

kmem_cache_dup() is only called from memcg_create_kmem_cache(). The
latter, in fact, does nothing besides this, so let's fold
kmem_cache_dup() into memcg_create_kmem_cache().

This patch also makes the memcg_cache_mutex private to
memcg_create_kmem_cache(), because it is not used anywhere else.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Glauber Costa <glommer@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 mm/memcontrol.c |   39 ++++++++-------------------------------
 1 file changed, 8 insertions(+), 31 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 56fc410..ce25f77 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3391,27 +3391,16 @@ void mem_cgroup_destroy_cache(struct kmem_cache *cachep)
 	schedule_work(&cachep->memcg_params->destroy);
 }
 
-/*
- * This lock protects updaters, not readers. We want readers to be as fast as
- * they can, and they will either see NULL or a valid cache value. Our model
- * allow them to see NULL, in which case the root memcg will be selected.
- *
- * We need this lock because multiple allocations to the same cache from a non
- * will span more than one worker. Only one of them can create the cache.
- */
-static DEFINE_MUTEX(memcg_cache_mutex);
-
-/*
- * Called with memcg_cache_mutex held
- */
-static struct kmem_cache *kmem_cache_dup(struct mem_cgroup *memcg,
-					 struct kmem_cache *s)
+static struct kmem_cache *memcg_create_kmem_cache(struct mem_cgroup *memcg,
+						  struct kmem_cache *s)
 {
 	struct kmem_cache *new;
 	static char *tmp_name = NULL;
+	static DEFINE_MUTEX(mutex);	/* protects tmp_name */
 
-	lockdep_assert_held(&memcg_cache_mutex);
+	BUG_ON(!memcg_can_account_kmem(memcg));
 
+	mutex_lock(&mutex);
 	/*
 	 * kmem_cache_create_memcg duplicates the given name and
 	 * cgroup_name for this name requires RCU context.
@@ -3434,25 +3423,13 @@ static struct kmem_cache *kmem_cache_dup(struct mem_cgroup *memcg,
 
 	if (new)
 		new->allocflags |= __GFP_KMEMCG;
+	else
+		new = s;
 
+	mutex_unlock(&mutex);
 	return new;
 }
 
-static struct kmem_cache *memcg_create_kmem_cache(struct mem_cgroup *memcg,
-						  struct kmem_cache *cachep)
-{
-	struct kmem_cache *new_cachep;
-
-	BUG_ON(!memcg_can_account_kmem(memcg));
-
-	mutex_lock(&memcg_cache_mutex);
-	new_cachep = kmem_cache_dup(memcg, cachep);
-	if (new_cachep == NULL)
-		new_cachep = cachep;
-	mutex_unlock(&memcg_cache_mutex);
-	return new_cachep;
-}
-
 void kmem_cache_destroy_memcg_children(struct kmem_cache *s)
 {
 	struct kmem_cache *c;
-- 
1.7.10.4

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[PATCH RESEND 08/11] slab: do not panic if we fail to create memcg cache

[Vladimir Davydov]

There is no point in flooding logs with warnings or especially crashing
the system if we fail to create a cache for a memcg. In this case we
will be accounting the memcg allocation to the root cgroup until we
succeed to create its own cache, but it isn't that critical.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Glauber Costa <glommer@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 mm/slab_common.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index f34707e..8e40321 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -233,7 +233,14 @@ out_unlock:
 	mutex_unlock(&slab_mutex);
 	put_online_cpus();
 
-	if (err) {
+	/*
+	 * There is no point in flooding logs with warnings or especially
+	 * crashing the system if we fail to create a cache for a memcg. In
+	 * this case we will be accounting the memcg allocation to the root
+	 * cgroup until we succeed to create its own cache, but it isn't that
+	 * critical.
+	 */
+	if (err && !memcg) {
 		if (flags & SLAB_PANIC)
 			panic("kmem_cache_create: Failed to create slab '%s'. Error %d\n",
 				name, err);
-- 
1.7.10.4

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[PATCH RESEND 09/11] memcg, slab: RCU protect memcg_params for root caches

[Vladimir Davydov]

We relocate root cache's memcg_params whenever we need to grow the
memcg_caches array to accommodate all kmem-active memory cgroups.
Currently on relocation we free the old version immediately, which can
lead to use-after-free, because the memcg_caches array is accessed
lock-free (see cache_from_memcg_idx()). This patch fixes this by making
memcg_params RCU-protected for root caches.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Glauber Costa <glommer@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: Christoph Lameter <cl@linux.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 include/linux/slab.h |    9 +++++++--
 mm/memcontrol.c      |   15 ++++++++-------
 mm/slab.h            |   16 +++++++++++++++-
 3 files changed, 30 insertions(+), 10 deletions(-)

diff --git a/include/linux/slab.h b/include/linux/slab.h
index 1e2f4fe..a060142 100644
--- a/include/linux/slab.h
+++ b/include/linux/slab.h
@@ -513,7 +513,9 @@ static __always_inline void *kmalloc_node(size_t size, gfp_t flags, int node)
  *
  * Both the root cache and the child caches will have it. For the root cache,
  * this will hold a dynamically allocated array large enough to hold
- * information about the currently limited memcgs in the system.
+ * information about the currently limited memcgs in the system. To allow the
+ * array to be accessed without taking any locks, on relocation we free the old
+ * version only after a grace period.
  *
  * Child caches will hold extra metadata needed for its operation. Fields are:
  *
@@ -528,7 +530,10 @@ static __always_inline void *kmalloc_node(size_t size, gfp_t flags, int node)
 struct memcg_cache_params {
 	bool is_root_cache;
 	union {
-		struct kmem_cache *memcg_caches[0];
+		struct {
+			struct rcu_head rcu_head;
+			struct kmem_cache *memcg_caches[0];
+		};
 		struct {
 			struct mem_cgroup *memcg;
 			struct list_head list;
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index ce25f77..a7521c3 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -3142,18 +3142,17 @@ int memcg_update_cache_size(struct kmem_cache *s, int num_groups)
 
 	if (num_groups > memcg_limited_groups_array_size) {
 		int i;
+		struct memcg_cache_params *new_params;
 		ssize_t size = memcg_caches_array_size(num_groups);
 
 		size *= sizeof(void *);
 		size += offsetof(struct memcg_cache_params, memcg_caches);
 
-		s->memcg_params = kzalloc(size, GFP_KERNEL);
-		if (!s->memcg_params) {
-			s->memcg_params = cur_params;
+		new_params = kzalloc(size, GFP_KERNEL);
+		if (!new_params)
 			return -ENOMEM;
-		}
 
-		s->memcg_params->is_root_cache = true;
+		new_params->is_root_cache = true;
 
 		/*
 		 * There is the chance it will be bigger than
@@ -3167,7 +3166,7 @@ int memcg_update_cache_size(struct kmem_cache *s, int num_groups)
 		for (i = 0; i < memcg_limited_groups_array_size; i++) {
 			if (!cur_params->memcg_caches[i])
 				continue;
-			s->memcg_params->memcg_caches[i] =
+			new_params->memcg_caches[i] =
 						cur_params->memcg_caches[i];
 		}
 
@@ -3180,7 +3179,9 @@ int memcg_update_cache_size(struct kmem_cache *s, int num_groups)
 		 * bigger than the others. And all updates will reset this
 		 * anyway.
 		 */
-		kfree(cur_params);
+		rcu_assign_pointer(s->memcg_params, new_params);
+		if (cur_params)
+			kfree_rcu(cur_params, rcu_head);
 	}
 	return 0;
 }
diff --git a/mm/slab.h b/mm/slab.h
index 72d1f9d..8184a7c 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -160,14 +160,28 @@ static inline const char *cache_name(struct kmem_cache *s)
 	return s->name;
 }
 
+/*
+ * Note, we protect with RCU only the memcg_caches array, not per-memcg caches.
+ * That said the caller must assure the memcg's cache won't go away. Since once
+ * created a memcg's cache is destroyed only along with the root cache, it is
+ * true if we are going to allocate from the cache or hold a reference to the
+ * root cache by other means. Otherwise, we should hold either the slab_mutex
+ * or the memcg's slab_caches_mutex while calling this function and accessing
+ * the returned value.
+ */
 static inline struct kmem_cache *
 cache_from_memcg_idx(struct kmem_cache *s, int idx)
 {
 	struct kmem_cache *cachep;
+	struct memcg_cache_params *params;
 
 	if (!s->memcg_params)
 		return NULL;
-	cachep = s->memcg_params->memcg_caches[idx];
+
+	rcu_read_lock();
+	params = rcu_dereference(s->memcg_params);
+	cachep = params->memcg_caches[idx];
+	rcu_read_unlock();
 
 	/*
 	 * Make sure we will access the up-to-date value. The code updating
-- 
1.7.10.4

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[PATCH RESEND 10/11] memcg: remove KMEM_ACCOUNTED_ACTIVATED flag

[Vladimir Davydov]

Currently we have two state bits in mem_cgroup::kmem_account_flags
regarding kmem accounting activation, ACTIVATED and ACTIVE. We start
kmem accounting only if both flags are set (memcg_can_account_kmem()),
plus throughout the code there are several places where we check only
the ACTIVE flag, but we never check the ACTIVATED flag alone. These
flags are both set from memcg_update_kmem_limit() under the
set_limit_mutex, the ACTIVE flag always being set after ACTIVATED, and
they never get cleared. That said checking if both flags are set is
equivalent to checking only for the ACTIVE flag, and since there is no
ACTIVATED flag checks, we can safely remove the ACTIVATED flag, and
nothing will change.

Let's try to understand what was the reason for introducing these flags.
The purpose of the ACTIVE flag is clear - it states that kmem should be
accounting to the cgroup. The only requirement for it is that it should
be set after we have fully initialized kmem accounting bits for the
cgroup and patched all static branches relating to kmem accounting.
Since we always check if static branch is enabled before actually
considering if we should account (otherwise we wouldn't benefit from
static branching), this guarantees us that we won't skip a commit or
uncharge after a charge due to an unpatched static branch.

Now let's move on to the ACTIVATED bit. As I proved in the beginning of
this message, it is absolutely useless, and removing it will change
nothing. So what was the reason introducing it?

The ACTIVATED flag was introduced by commit a8964b9b ("memcg: use static
branches when code not in use") in order to guarantee that
static_key_slow_inc(&memcg_kmem_enabled_key) would be called only once
for each memory cgroup when its kmem accounting was activated. The point
was that at that time the memcg_update_kmem_limit() function's work-flow
looked like this:

        bool must_inc_static_branch = false;

        cgroup_lock();
        mutex_lock(&set_limit_mutex);
        if (!memcg->kmem_account_flags && val != RESOURCE_MAX) {
                /* The kmem limit is set for the first time */
                ret = res_counter_set_limit(&memcg->kmem, val);

                memcg_kmem_set_activated(memcg);
                must_inc_static_branch = true;
        } else
                ret = res_counter_set_limit(&memcg->kmem, val);
        mutex_unlock(&set_limit_mutex);
        cgroup_unlock();

        if (must_inc_static_branch) {
                /* We can't do this under cgroup_lock */
                static_key_slow_inc(&memcg_kmem_enabled_key);
                memcg_kmem_set_active(memcg);
        }

So that without the ACTIVATED flag we could race with other threads
trying to set the limit and increment the static branching ref-counter
more than once. Today we call the whole memcg_update_kmem_limit()
function under the set_limit_mutex and this race is impossible.

As now we understand why the ACTIVATED bit was introduced and why we
don't need it now, and know that removing it will change nothing anyway,
let's get rid of it.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Glauber Costa <glommer@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 mm/memcontrol.c |   28 ++--------------------------
 1 file changed, 2 insertions(+), 26 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index a7521c3..a5a1ae1 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -343,15 +343,10 @@ static size_t memcg_size(void)
 
 /* internal only representation about the status of kmem accounting. */
 enum {
-	KMEM_ACCOUNTED_ACTIVE = 0, /* accounted by this cgroup itself */
-	KMEM_ACCOUNTED_ACTIVATED, /* static key enabled. */
+	KMEM_ACCOUNTED_ACTIVE, /* accounted by this cgroup itself */
 	KMEM_ACCOUNTED_DEAD, /* dead memcg with pending kmem charges */
 };
 
-/* We account when limit is on, but only after call sites are patched */
-#define KMEM_ACCOUNTED_MASK \
-		((1 << KMEM_ACCOUNTED_ACTIVE) | (1 << KMEM_ACCOUNTED_ACTIVATED))
-
 #ifdef CONFIG_MEMCG_KMEM
 static inline void memcg_kmem_set_active(struct mem_cgroup *memcg)
 {
@@ -363,16 +358,6 @@ static bool memcg_kmem_is_active(struct mem_cgroup *memcg)
 	return test_bit(KMEM_ACCOUNTED_ACTIVE, &memcg->kmem_account_flags);
 }
 
-static void memcg_kmem_set_activated(struct mem_cgroup *memcg)
-{
-	set_bit(KMEM_ACCOUNTED_ACTIVATED, &memcg->kmem_account_flags);
-}
-
-static void memcg_kmem_clear_activated(struct mem_cgroup *memcg)
-{
-	clear_bit(KMEM_ACCOUNTED_ACTIVATED, &memcg->kmem_account_flags);
-}
-
 static void memcg_kmem_mark_dead(struct mem_cgroup *memcg)
 {
 	/*
@@ -2959,7 +2944,7 @@ static DEFINE_MUTEX(set_limit_mutex);
 static inline bool memcg_can_account_kmem(struct mem_cgroup *memcg)
 {
 	return !mem_cgroup_disabled() && !mem_cgroup_is_root(memcg) &&
-		(memcg->kmem_account_flags & KMEM_ACCOUNTED_MASK);
+		memcg_kmem_is_active(memcg);
 }
 
 /*
@@ -3084,19 +3069,10 @@ int memcg_update_cache_sizes(struct mem_cgroup *memcg)
 				0, MEMCG_CACHES_MAX_SIZE, GFP_KERNEL);
 	if (num < 0)
 		return num;
-	/*
-	 * After this point, kmem_accounted (that we test atomically in
-	 * the beginning of this conditional), is no longer 0. This
-	 * guarantees only one process will set the following boolean
-	 * to true. We don't need test_and_set because we're protected
-	 * by the set_limit_mutex anyway.
-	 */
-	memcg_kmem_set_activated(memcg);
 
 	ret = memcg_update_all_caches(num+1);
 	if (ret) {
 		ida_simple_remove(&kmem_limited_groups, num);
-		memcg_kmem_clear_activated(memcg);
 		return ret;
 	}
 
-- 
1.7.10.4

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[PATCH RESEND 11/11] memcg: rework memcg_update_kmem_limit synchronization

[Vladimir Davydov]

Currently we take both the memcg_create_mutex and the set_limit_mutex
when we enable kmem accounting for a memory cgroup, which makes kmem
activation events serialize with both memcg creations and other memcg
limit updates (memory.limit, memory.memsw.limit). However, there is no
point in such strict synchronization rules there.

First, the set_limit_mutex was introduced to keep the memory.limit and
memory.memsw.limit values in sync. Since memory.kmem.limit can be set
independently of them, it is better to introduce a separate mutex to
synchronize against concurrent kmem limit updates.

Second, we take the memcg_create_mutex in order to make sure all
children of this memcg will be kmem-active as well. For achieving that,
it is enough to hold this mutex only while checking if
memcg_has_children() though. This guarantees that if a child is added
after we checked that the memcg has no children, the newly added cgroup
will see its parent kmem-active (of course if the latter succeeded), and
call kmem activation for itself.

This patch simplifies the locking rules of memcg_update_kmem_limit()
according to these considerations.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Glauber Costa <glommer@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
---
 mm/memcontrol.c |  198 +++++++++++++++++++++++++++++--------------------------
 1 file changed, 106 insertions(+), 92 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index a5a1ae1..696707c 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2941,6 +2941,8 @@ static void __mem_cgroup_commit_charge(struct mem_cgroup *memcg,
 static DEFINE_MUTEX(set_limit_mutex);
 
 #ifdef CONFIG_MEMCG_KMEM
+static DEFINE_MUTEX(activate_kmem_mutex);
+
 static inline bool memcg_can_account_kmem(struct mem_cgroup *memcg)
 {
 	return !mem_cgroup_disabled() && !mem_cgroup_is_root(memcg) &&
@@ -3054,34 +3056,6 @@ int memcg_cache_id(struct mem_cgroup *memcg)
 	return memcg ? memcg->kmemcg_id : -1;
 }
 
-/*
- * This ends up being protected by the set_limit mutex, during normal
- * operation, because that is its main call site.
- *
- * But when we create a new cache, we can call this as well if its parent
- * is kmem-limited. That will have to hold set_limit_mutex as well.
- */
-int memcg_update_cache_sizes(struct mem_cgroup *memcg)
-{
-	int num, ret;
-
-	num = ida_simple_get(&kmem_limited_groups,
-				0, MEMCG_CACHES_MAX_SIZE, GFP_KERNEL);
-	if (num < 0)
-		return num;
-
-	ret = memcg_update_all_caches(num+1);
-	if (ret) {
-		ida_simple_remove(&kmem_limited_groups, num);
-		return ret;
-	}
-
-	memcg->kmemcg_id = num;
-	INIT_LIST_HEAD(&memcg->memcg_slab_caches);
-	mutex_init(&memcg->slab_caches_mutex);
-	return 0;
-}
-
 static size_t memcg_caches_array_size(int num_groups)
 {
 	ssize_t size;
@@ -3424,9 +3398,10 @@ void kmem_cache_destroy_memcg_children(struct kmem_cache *s)
 	 *
 	 * Still, we don't want anyone else freeing memcg_caches under our
 	 * noses, which can happen if a new memcg comes to life. As usual,
-	 * we'll take the set_limit_mutex to protect ourselves against this.
+	 * we'll take the activate_kmem_mutex to protect ourselves against
+	 * this.
 	 */
-	mutex_lock(&set_limit_mutex);
+	mutex_lock(&activate_kmem_mutex);
 	for_each_memcg_cache_index(i) {
 		c = cache_from_memcg_idx(s, i);
 		if (!c)
@@ -3449,7 +3424,7 @@ void kmem_cache_destroy_memcg_children(struct kmem_cache *s)
 		cancel_work_sync(&c->memcg_params->destroy);
 		kmem_cache_destroy(c);
 	}
-	mutex_unlock(&set_limit_mutex);
+	mutex_unlock(&activate_kmem_mutex);
 }
 
 struct create_work {
@@ -5116,11 +5091,23 @@ static ssize_t mem_cgroup_read(struct cgroup_subsys_state *css,
 	return simple_read_from_buffer(buf, nbytes, ppos, str, len);
 }
 
-static int memcg_update_kmem_limit(struct cgroup_subsys_state *css, u64 val)
-{
-	int ret = -EINVAL;
 #ifdef CONFIG_MEMCG_KMEM
-	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
+/* should be called with activate_kmem_mutex held */
+static int __memcg_activate_kmem(struct mem_cgroup *memcg,
+				 unsigned long long limit)
+{
+	int err = 0;
+	int memcg_id;
+
+	if (memcg_kmem_is_active(memcg))
+		return 0;
+
+	/*
+	 * We are going to allocate memory for data shared by all memory
+	 * cgroups so let's stop accounting here.
+	 */
+	memcg_stop_kmem_account();
+
 	/*
 	 * For simplicity, we won't allow this to be disabled.  It also can't
 	 * be changed if the cgroup has children already, or if tasks had
@@ -5134,72 +5121,101 @@ static int memcg_update_kmem_limit(struct cgroup_subsys_state *css, u64 val)
 	 * of course permitted.
 	 */
 	mutex_lock(&memcg_create_mutex);
-	mutex_lock(&set_limit_mutex);
-	if (!memcg->kmem_account_flags && val != RES_COUNTER_MAX) {
-		if (cgroup_task_count(css->cgroup) || memcg_has_children(memcg)) {
-			ret = -EBUSY;
-			goto out;
-		}
-		ret = res_counter_set_limit(&memcg->kmem, val);
-		VM_BUG_ON(ret);
+	if (cgroup_task_count(memcg->css.cgroup) || memcg_has_children(memcg))
+		err = -EBUSY;
+	mutex_unlock(&memcg_create_mutex);
+	if (err)
+		goto out;
 
-		ret = memcg_update_cache_sizes(memcg);
-		if (ret) {
-			res_counter_set_limit(&memcg->kmem, RES_COUNTER_MAX);
-			goto out;
-		}
-		static_key_slow_inc(&memcg_kmem_enabled_key);
-		/*
-		 * setting the active bit after the inc will guarantee no one
-		 * starts accounting before all call sites are patched
-		 */
-		memcg_kmem_set_active(memcg);
-	} else
-		ret = res_counter_set_limit(&memcg->kmem, val);
+	memcg_id = ida_simple_get(&kmem_limited_groups,
+				  0, MEMCG_CACHES_MAX_SIZE, GFP_KERNEL);
+	if (memcg_id < 0) {
+		err = memcg_id;
+		goto out;
+	}
+
+	/*
+	 * Make sure we have enough space for this cgroup in each root cache's
+	 * memcg_params.
+	 */
+	err = memcg_update_all_caches(memcg_id + 1);
+	if (err)
+		goto out_rmid;
+
+	memcg->kmemcg_id = memcg_id;
+	INIT_LIST_HEAD(&memcg->memcg_slab_caches);
+	mutex_init(&memcg->slab_caches_mutex);
+
+	/*
+	 * We couldn't have accounted to this cgroup, because it hasn't got the
+	 * active bit set yet, so this should succeed.
+	 */
+	err = res_counter_set_limit(&memcg->kmem, limit);
+	VM_BUG_ON(err);
+
+	static_key_slow_inc(&memcg_kmem_enabled_key);
+	/*
+	 * Setting the active bit after enabling static branching will
+	 * guarantee no one starts accounting before all call sites are
+	 * patched.
+	 */
+	memcg_kmem_set_active(memcg);
 out:
-	mutex_unlock(&set_limit_mutex);
-	mutex_unlock(&memcg_create_mutex);
-#endif
+	memcg_resume_kmem_account();
+	return err;
+
+out_rmid:
+	ida_simple_remove(&kmem_limited_groups, memcg_id);
+	goto out;
+}
+
+static int memcg_activate_kmem(struct mem_cgroup *memcg,
+			       unsigned long long limit)
+{
+	int ret;
+
+	mutex_lock(&activate_kmem_mutex);
+	ret = __memcg_activate_kmem(memcg, limit);
+	mutex_unlock(&activate_kmem_mutex);
+	return ret;
+}
+
+static int memcg_update_kmem_limit(struct mem_cgroup *memcg,
+				   unsigned long long val)
+{
+	int ret;
+
+	if (!memcg_kmem_is_active(memcg))
+		ret = memcg_activate_kmem(memcg, val);
+	else
+		ret = res_counter_set_limit(&memcg->kmem, val);
 	return ret;
 }
 
-#ifdef CONFIG_MEMCG_KMEM
 static int memcg_propagate_kmem(struct mem_cgroup *memcg)
 {
-	int ret = 0;
+	int ret;
 	struct mem_cgroup *parent = parent_mem_cgroup(memcg);
-	if (!parent)
-		goto out;
 
-	memcg->kmem_account_flags = parent->kmem_account_flags;
-	/*
-	 * When that happen, we need to disable the static branch only on those
-	 * memcgs that enabled it. To achieve this, we would be forced to
-	 * complicate the code by keeping track of which memcgs were the ones
-	 * that actually enabled limits, and which ones got it from its
-	 * parents.
-	 *
-	 * It is a lot simpler just to do static_key_slow_inc() on every child
-	 * that is accounted.
-	 */
-	if (!memcg_kmem_is_active(memcg))
-		goto out;
+	if (!parent)
+		return 0;
 
+	mutex_lock(&activate_kmem_mutex);
 	/*
-	 * __mem_cgroup_free() will issue static_key_slow_dec() because this
-	 * memcg is active already. If the later initialization fails then the
-	 * cgroup core triggers the cleanup so we do not have to do it here.
+	 * If the parent cgroup is not kmem-active now, it cannot be activated
+	 * after this point, because it has at least one child already.
 	 */
-	static_key_slow_inc(&memcg_kmem_enabled_key);
-
-	mutex_lock(&set_limit_mutex);
-	memcg_stop_kmem_account();
-	ret = memcg_update_cache_sizes(memcg);
-	memcg_resume_kmem_account();
-	mutex_unlock(&set_limit_mutex);
-out:
+	if (memcg_kmem_is_active(parent))
+		ret = __memcg_activate_kmem(memcg, RES_COUNTER_MAX);
+	mutex_unlock(&activate_kmem_mutex);
 	return ret;
 }
+#else
+static int memcg_update_kmem_limit(struct mem_cgroup *memcg,
+				   unsigned long long val)
+{
+	return -EINVAL;
+}
 #endif /* CONFIG_MEMCG_KMEM */
 
 /*
@@ -5233,7 +5249,7 @@ static int mem_cgroup_write(struct cgroup_subsys_state *css, struct cftype *cft,
 		else if (type == _MEMSWAP)
 			ret = mem_cgroup_resize_memsw_limit(memcg, val);
 		else if (type == _KMEM)
-			ret = memcg_update_kmem_limit(css, val);
+			ret = memcg_update_kmem_limit(memcg, val);
 		else
 			return -EINVAL;
 		break;
@@ -6248,7 +6264,6 @@ mem_cgroup_css_online(struct cgroup_subsys_state *css)
 {
 	struct mem_cgroup *memcg = mem_cgroup_from_css(css);
 	struct mem_cgroup *parent = mem_cgroup_from_css(css_parent(css));
-	int error = 0;
 
 	if (css->cgroup->id > MEM_CGROUP_ID_MAX)
 		return -ENOSPC;
@@ -6283,10 +6298,9 @@ mem_cgroup_css_online(struct cgroup_subsys_state *css)
 		if (parent != root_mem_cgroup)
 			mem_cgroup_subsys.broken_hierarchy = true;
 	}
-
-	error = memcg_init_kmem(memcg, &mem_cgroup_subsys);
 	mutex_unlock(&memcg_create_mutex);
-	return error;
+
+	return memcg_init_kmem(memcg, &mem_cgroup_subsys);
 }
 
 /*
-- 
1.7.10.4

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