[Devel] [PATCH RHEL7 COMMIT] mm/slab: use monotonic serial number for naming per memcg caches

[Konstantin Khorenko]

The commit is pushed to "branch-rh7-3.10.0-933.vz7.80.x-ovz" and will appear at https://src.openvz.org/scm/ovz/vzkernel.git
after rh7-3.10.0-933.vz7.80.1
------>
commit fbef4327d9b0693d1e65aa9aad18a69783dbeffe
Author: Konstantin Khorenko <khorenko at virtuozzo.com>
Date:   Mon Oct 15 13:09:15 2018 +0300

    mm/slab: use monotonic serial number for naming per memcg caches
    
    Currently, we use mem_cgroup->kmemcg_id to guarantee kmem_cache->name
    uniqueness. Unfortunately kmemcg_id release happens on cgroup offline
    when kmem caches are  still alive and their sysfs entries are not removed.
    
    Thus it's possible to start/suspend/resume/stop a Container and get into
    situation when mem cgroup is offline (thus mem_cgroup::kmemcg_id is
    released), but not freed and corresponding slab caches are alive along
    with their sysfs entries like:
      /sys/kernel/slab/filp/cgroup/filp($KMEMCG_ID:$MEMCG_NAME)
    
    If we start a Container after that, new memory cgroup is created and
    gets same kmemcg_id, new kmem_cache is created and sysfs_slab_add()
    fails due to duplicate name provided which triggers a warning:
    
      WARNING: CPU: 0 PID: 31346 at lib/kobject.c:239
        kobject_add_internal+0x6ef/0x910
      kobject_add_internal failed for kmalloc-512(3:100) with -EEXIST, don't
        try to register things with the same name in the same directory.
    
    In older kernels we use css_id(mem_cgroup_css(memcg)), but now cssid is
    dropped.
    In mainstream monotonic counter css->serial_nr is used instead,
    so let's add monotonic serial_nr to kmem_cache->name as well.
    No need to store is anywhere as kmem_cache has a link to memcg.
    
    https://jira.sw.ru/browse/PSBM-89047
    
    Signed-off-by: Konstantin Khorenko <khorenko at virtuozzo.com>
    
    v2: Holding slab_mutex makes atomic variable redundant.
        No need to store serial number assigned to a kmem_cache,
        because kmem_cache has a link to memcg.
    
    Signed-off-by: Konstantin Khorenko <khorenko at virtuozzo.com>
---
 mm/slab_common.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/mm/slab_common.c b/mm/slab_common.c
index 1b2a7d208596..b08f48541f4a 100644
--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -421,6 +421,7 @@ static void do_kmem_cache_release(struct list_head *release,
 void memcg_create_kmem_cache(struct mem_cgroup *memcg,
 			     struct kmem_cache *root_cache)
 {
+	static u64 serial_nr_cursor = 0;
 	static char memcg_name_buf[NAME_MAX + 1]; /* protected by slab_mutex */
 	struct memcg_cache_array *arr;
 	struct kmem_cache *s = NULL;
@@ -453,8 +454,14 @@ void memcg_create_kmem_cache(struct mem_cgroup *memcg,
 	strlcpy(memcg_name_buf, cgroup_name(mem_cgroup_css(memcg)->cgroup),
 		NAME_MAX + 1);
 	rcu_read_unlock();
-	cache_name = kasprintf(GFP_KERNEL, "%s(%d:%s)", root_cache->name,
-			       idx, memcg_name_buf);
+
+	/*
+	 * Monotonically increasing serial number in the name guarantees
+	 * kmem_cache name uniqueness even if a mem cgroup goes offline,
+	 * releases kmemcg_id and new mem cgroup gets same kmemcg_id.
+	 */
+	cache_name = kasprintf(GFP_KERNEL, "%s(%llu:%s)", root_cache->name,
+			       serial_nr_cursor++, memcg_name_buf);
 	if (!cache_name)
 		goto out_unlock;