Re: kmemleaks reports a lot of cases around memcg_create_kmem_cache

[Vladimir Davydov <vdavydov.dev-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On Thu, Jun 29, 2017 at 11:04:01AM -0700, Andrei Vagin wrote:
> Hello,
> 
> We run CRIU tests on the linus' tree and found that kmemleak reports
> unreferenced objects which are allocated from memcg_create_kmem_cache:
> 
> unreferenced object 0xffff9f79442cd980 (size 112):
>   comm "kworker/1:4", pid 15416, jiffies 4307432421 (age 28687.562s)
>   hex dump (first 32 bytes):
>     00 00 00 00 ad 4e ad de ff ff ff ff 00 00 00 00  .....N..........
>     ff ff ff ff ff ff ff ff b8 39 1b 97 ff ff ff ff  .........9......
>   backtrace:
>     [<ffffffff9591d28a>] kmemleak_alloc+0x4a/0xa0
>     [<ffffffff95276198>] kmem_cache_alloc_node+0x168/0x2a0
>     [<ffffffff95279f28>] __kmem_cache_create+0x2b8/0x5c0
>     [<ffffffff9522ff57>] create_cache+0xb7/0x1e0
>     [<ffffffff952305f8>] memcg_create_kmem_cache+0x118/0x160
>     [<ffffffff9528eaf0>] memcg_kmem_cache_create_func+0x20/0x110
>     [<ffffffff950cd6c5>] process_one_work+0x205/0x5d0
>     [<ffffffff950cdade>] worker_thread+0x4e/0x3a0
>     [<ffffffff950d5169>] kthread+0x109/0x140
>     [<ffffffff9592b8fa>] ret_from_fork+0x2a/0x40
>     [<ffffffffffffffff>] 0xffffffffffffffff
> unreferenced object 0xffff9f798a79f540 (size 32):
>   comm "kworker/1:4", pid 15416, jiffies 4307432429 (age 28687.554s)
>   hex dump (first 32 bytes):
>     6b 6d 61 6c 6c 6f 63 2d 31 36 28 31 35 39 39 3a  kmalloc-16(1599:
>     6e 65 77 72 6f 6f 74 29 00 23 6b c0 ff ff ff ff  newroot).#k.....
>   backtrace:
>     [<ffffffff9591d28a>] kmemleak_alloc+0x4a/0xa0
>     [<ffffffff9527a378>] __kmalloc_track_caller+0x148/0x2c0
>     [<ffffffff95499466>] kvasprintf+0x66/0xd0
>     [<ffffffff954995a9>] kasprintf+0x49/0x70
>     [<ffffffff952305c6>] memcg_create_kmem_cache+0xe6/0x160
>     [<ffffffff9528eaf0>] memcg_kmem_cache_create_func+0x20/0x110
>     [<ffffffff950cd6c5>] process_one_work+0x205/0x5d0
>     [<ffffffff950cdade>] worker_thread+0x4e/0x3a0
>     [<ffffffff950d5169>] kthread+0x109/0x140
>     [<ffffffff9592b8fa>] ret_from_fork+0x2a/0x40
>     [<ffffffffffffffff>] 0xffffffffffffffff
> unreferenced object 0xffff9f79b6136840 (size 416):
>   comm "kworker/1:4", pid 15416, jiffies 4307432429 (age 28687.573s)
>   hex dump (first 32 bytes):
>     40 fb 80 c2 3e 33 00 00 00 00 00 40 00 00 00 00  @...>3.....@....
>     00 00 00 00 00 00 00 00 10 00 00 00 10 00 00 00  ................
>   backtrace:
>     [<ffffffff9591d28a>] kmemleak_alloc+0x4a/0xa0
>     [<ffffffff95275bc8>] kmem_cache_alloc+0x128/0x280
>     [<ffffffff9522fedb>] create_cache+0x3b/0x1e0
>     [<ffffffff952305f8>] memcg_create_kmem_cache+0x118/0x160
>     [<ffffffff9528eaf0>] memcg_kmem_cache_create_func+0x20/0x110
>     [<ffffffff950cd6c5>] process_one_work+0x205/0x5d0
>     [<ffffffff950cdade>] worker_thread+0x4e/0x3a0
>     [<ffffffff950d5169>] kthread+0x109/0x140
>     [<ffffffff9592b8fa>] ret_from_fork+0x2a/0x40
>     [<ffffffffffffffff>] 0xffffffffffffffff
> unreferenced object 0xffff9f798cac8000 (size 1024):
>   comm "kworker/1:4", pid 15416, jiffies 4307432429 (age 28687.573s)
>   hex dump (first 32 bytes):
>     10 00 00 00 70 09 00 00 20 09 00 00 00 09 00 00  ....p... .......
>     80 02 00 00 b0 03 00 00 30 06 00 00 50 02 00 00  ........0...P...
>   backtrace:
>     [<ffffffff9591d28a>] kmemleak_alloc+0x4a/0xa0
>     [<ffffffff952766b8>] __kmalloc+0x158/0x2c0
>     [<ffffffff95230a5f>] cache_random_seq_create+0x6f/0x130
>     [<ffffffff952714da>] init_cache_random_seq+0x3a/0x90
>     [<ffffffff95279d70>] __kmem_cache_create+0x100/0x5c0
>     [<ffffffff9522ff57>] create_cache+0xb7/0x1e0
>     [<ffffffff952305f8>] memcg_create_kmem_cache+0x118/0x160
>     [<ffffffff9528eaf0>] memcg_kmem_cache_create_func+0x20/0x110
>     [<ffffffff950cd6c5>] process_one_work+0x205/0x5d0
>     [<ffffffff950cdade>] worker_thread+0x4e/0x3a0
>     [<ffffffff950d5169>] kthread+0x109/0x140
>     [<ffffffff9592b8fa>] ret_from_fork+0x2a/0x40
>     [<ffffffffffffffff>] 0xffffffffffffffff
> unreferenced object 0xffff9f79442cd800 (size 112):
> 
> [root@zdtm linux]# git describe HEAD
> v4.12-rc7-26-gb216759
> 
> [root@zdtm linux]# uname -a
> Linux zdtm.openvz.org 4.12.0-rc7+ #9 SMP Thu Jun 29 08:28:18 CEST 2017
> x86_64 x86_64 x86_64 GNU/Linux

Could you check if the patch below fixes the issue?
--
From: Vladimir Davydov <vdavydov.dev-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Subject: [PATCH] slub: fix per memcg cache leak on css offline

To avoid a possible deadlock, sysfs_slab_remove() schedules an
asynchronous work to delete sysfs entries corresponding to the kmem
cache. To ensure the cache isn't freed before the work function is
called, it takes a reference to the cache kobject. The reference is
supposed to be released by the work function. However, the work function
(sysfs_slab_remove_workfn()) does nothing in case the cache sysfs entry
has already been deleted, leaking the kobject and the corresponding
cache. This may happen on a per memcg cache destruction, because sysfs
entries of a per memcg cache are deleted on memcg offline if the cache
is empty (see __kmemcg_cache_deactivate()).

The kmemleak report looks like this:

  unreferenced object 0xffff9f798a79f540 (size 32):
    comm "kworker/1:4", pid 15416, jiffies 4307432429 (age 28687.554s)
    hex dump (first 32 bytes):
      6b 6d 61 6c 6c 6f 63 2d 31 36 28 31 35 39 39 3a  kmalloc-16(1599:
      6e 65 77 72 6f 6f 74 29 00 23 6b c0 ff ff ff ff  newroot).#k.....
    backtrace:
      [<ffffffff9591d28a>] kmemleak_alloc+0x4a/0xa0
      [<ffffffff9527a378>] __kmalloc_track_caller+0x148/0x2c0
      [<ffffffff95499466>] kvasprintf+0x66/0xd0
      [<ffffffff954995a9>] kasprintf+0x49/0x70
      [<ffffffff952305c6>] memcg_create_kmem_cache+0xe6/0x160
      [<ffffffff9528eaf0>] memcg_kmem_cache_create_func+0x20/0x110
      [<ffffffff950cd6c5>] process_one_work+0x205/0x5d0
      [<ffffffff950cdade>] worker_thread+0x4e/0x3a0
      [<ffffffff950d5169>] kthread+0x109/0x140
      [<ffffffff9592b8fa>] ret_from_fork+0x2a/0x40
      [<ffffffffffffffff>] 0xffffffffffffffff
  unreferenced object 0xffff9f79b6136840 (size 416):
    comm "kworker/1:4", pid 15416, jiffies 4307432429 (age 28687.573s)
    hex dump (first 32 bytes):
      40 fb 80 c2 3e 33 00 00 00 00 00 40 00 00 00 00  @...>3.....@....
      00 00 00 00 00 00 00 00 10 00 00 00 10 00 00 00  ................
    backtrace:
      [<ffffffff9591d28a>] kmemleak_alloc+0x4a/0xa0
      [<ffffffff95275bc8>] kmem_cache_alloc+0x128/0x280
      [<ffffffff9522fedb>] create_cache+0x3b/0x1e0
      [<ffffffff952305f8>] memcg_create_kmem_cache+0x118/0x160
      [<ffffffff9528eaf0>] memcg_kmem_cache_create_func+0x20/0x110
      [<ffffffff950cd6c5>] process_one_work+0x205/0x5d0
      [<ffffffff950cdade>] worker_thread+0x4e/0x3a0
      [<ffffffff950d5169>] kthread+0x109/0x140
      [<ffffffff9592b8fa>] ret_from_fork+0x2a/0x40
      [<ffffffffffffffff>] 0xffffffffffffffff

Fix the leak by adding the missing call to kobject_put() to
sysfs_slab_remove_workfn().

Reported-by: Andrei Vagin <avagin-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Signed-off-by: Vladimir Davydov <vdavydov.dev-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>
Fixes: 3b7b314053d02 ("slub: make sysfs file removal asynchronous")

diff --git a/mm/slub.c b/mm/slub.c
index 8addc535bcdc..a0f3c56611c6 100644
--- a/mm/slub.c
+++ b/mm/slub.c
@@ -5637,13 +5637,14 @@ static void sysfs_slab_remove_workfn(struct work_struct *work)
 		 * A cache is never shut down before deactivation is
 		 * complete, so no need to worry about synchronization.
 		 */
-		return;
+		goto out;
 
 #ifdef CONFIG_MEMCG
 	kset_unregister(s->memcg_kset);
 #endif
 	kobject_uevent(&s->kobj, KOBJ_REMOVE);
 	kobject_del(&s->kobj);
+out:
 	kobject_put(&s->kobj);
 }