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Commit 05257a1a authored by Vladimir Davydov's avatar Vladimir Davydov Committed by Linus Torvalds
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memcg: add rwsem to synchronize against memcg_caches arrays relocation 



We need a stable value of memcg_nr_cache_ids in kmem_cache_create()
(memcg_alloc_cache_params() wants it for root caches), where we only
hold the slab_mutex and no memcg-related locks.  As a result, we have to
update memcg_nr_cache_ids under the slab_mutex, which we can only take
on the slab's side (see memcg_update_array_size).  This looks awkward
and will become even worse when per-memcg list_lru is introduced, which
also wants stable access to memcg_nr_cache_ids.

To get rid of this dependency between the memcg_nr_cache_ids and the
slab_mutex, this patch introduces a special rwsem.  The rwsem is held
for writing during memcg_caches arrays relocation and memcg_nr_cache_ids
updates.  Therefore one can take it for reading to get a stable access
to memcg_caches arrays and/or memcg_nr_cache_ids.

Currently the semaphore is taken for reading only from
kmem_cache_create, right before taking the slab_mutex, so right now
there's no much point in using rwsem instead of mutex.  However, once
list_lru is made per-memcg it will allow list_lru initializations to
proceed concurrently.

Signed-off-by: default avatarVladimir Davydov <vdavydov@parallels.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Greg Thelen <gthelen@google.com>
Cc: Glauber Costa <glommer@gmail.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christoph Lameter <cl@linux.com>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent dbcf73e2

include/linux/memcontrol.h

+10 −2
Original line number Diff line number Diff line
@@ -399,6 +399,8 @@ static inline void sock_release_memcg(struct sock *sk) 
extern struct static_key memcg_kmem_enabled_key;



extern int memcg_nr_cache_ids;

extern void memcg_get_cache_ids(void);

extern void memcg_put_cache_ids(void);



/*

 * Helper macro to loop through all memcg-specific caches. Callers must still
@@ -434,8 +436,6 @@ void __memcg_kmem_uncharge_pages(struct page *page, int order); 


int memcg_cache_id(struct mem_cgroup *memcg);



void memcg_update_array_size(int num_groups);



struct kmem_cache *__memcg_kmem_get_cache(struct kmem_cache *cachep);

void __memcg_kmem_put_cache(struct kmem_cache *cachep);
@@ -569,6 +569,14 @@ static inline int memcg_cache_id(struct mem_cgroup *memcg) 
	return -1;

}



static inline void memcg_get_cache_ids(void)

{

}



static inline void memcg_put_cache_ids(void)

{

}



static inline struct kmem_cache *

memcg_kmem_get_cache(struct kmem_cache *cachep, gfp_t gfp)

{

mm/memcontrol.c

+19 −10
Original line number Diff line number Diff line
@@ -544,6 +544,19 @@ static void disarm_sock_keys(struct mem_cgroup *memcg) 
static DEFINE_IDA(memcg_cache_ida);

int memcg_nr_cache_ids;



/* Protects memcg_nr_cache_ids */

static DECLARE_RWSEM(memcg_cache_ids_sem);



void memcg_get_cache_ids(void)

{

	down_read(&memcg_cache_ids_sem);

}



void memcg_put_cache_ids(void)

{

	up_read(&memcg_cache_ids_sem);

}



/*

 * MIN_SIZE is different than 1, because we would like to avoid going through

 * the alloc/free process all the time. In a small machine, 4 kmem-limited
@@ -2549,6 +2562,7 @@ static int memcg_alloc_cache_id(void) 
	 * There's no space for the new id in memcg_caches arrays,

	 * so we have to grow them.

	 */

	down_write(&memcg_cache_ids_sem);



	size = 2 * (id + 1);

	if (size < MEMCG_CACHES_MIN_SIZE)
@@ -2557,6 +2571,11 @@ static int memcg_alloc_cache_id(void) 
		size = MEMCG_CACHES_MAX_SIZE;



	err = memcg_update_all_caches(size);

	if (!err)

		memcg_nr_cache_ids = size;



	up_write(&memcg_cache_ids_sem);



	if (err) {

		ida_simple_remove(&memcg_cache_ida, id);

		return err;
@@ -2569,16 +2588,6 @@ static void memcg_free_cache_id(int id) 
	ida_simple_remove(&memcg_cache_ida, id);

}



/*

 * We should update the current array size iff all caches updates succeed. This

 * can only be done from the slab side. The slab mutex needs to be held when

 * calling this.

 */

void memcg_update_array_size(int num)

{

	memcg_nr_cache_ids = num;

}



struct memcg_kmem_cache_create_work {

	struct mem_cgroup *memcg;

	struct kmem_cache *cachep;

mm/slab_common.c

+4 −5
Original line number Diff line number Diff line
@@ -169,8 +169,8 @@ int memcg_update_all_caches(int num_memcgs) 
{

	struct kmem_cache *s;

	int ret = 0;

	mutex_lock(&slab_mutex);



	mutex_lock(&slab_mutex);

	list_for_each_entry(s, &slab_caches, list) {

		if (!is_root_cache(s))

			continue;
@@ -181,11 +181,8 @@ int memcg_update_all_caches(int num_memcgs) 
		 * up to this point in an updated state.

		 */

		if (ret)

			goto out;

			break;

	}



	memcg_update_array_size(num_memcgs);

out:

	mutex_unlock(&slab_mutex);

	return ret;

}
@@ -369,6 +366,7 @@ kmem_cache_create(const char *name, size_t size, size_t align, 


	get_online_cpus();

	get_online_mems();

	memcg_get_cache_ids();



	mutex_lock(&slab_mutex);
@@ -407,6 +405,7 @@ kmem_cache_create(const char *name, size_t size, size_t align, 
out_unlock:

	mutex_unlock(&slab_mutex);



	memcg_put_cache_ids();

	put_online_mems();

	put_online_cpus();