[PATCH 4.19 034/120] mm: slab: fix kmem_cache_create failed when sysfs node not destroyed

Greg Kroah-Hartman
2021-07-26T16:01:58Z

From: Nanyong Sun <sunnanyong@huawei.com>

The commit d38a2b7a9c93 ("mm: memcg/slab: fix memory leak at non-root
kmem_cache destroy") introduced a problem: If one thread destroy a
kmem_cache A and another thread concurrently create a kmem_cache B,
which is mergeable with A and has same size with A, the B may fail to
create due to the duplicate sysfs node.
The scenario in detail:
1) Thread 1 uses kmem_cache_destroy() to destroy kmem_cache A which is
mergeable, it decreases A's refcount and if refcount is 0, then call
memcg_set_kmem_cache_dying() which set A->memcg_params.dying = true,
then unlock the slab_mutex and call flush_memcg_workqueue(), it may cost
a while.
Note: now the sysfs node(like '/kernel/slab/:0000248') of A is still
present, it will be deleted in shutdown_cache() which will be called
after flush_memcg_workqueue() is done and lock the slab_mutex again.
2) Now if thread 2 is coming, it use kmem_cache_create() to create B, which
is mergeable with A(their size is same), it gain the lock of slab_mutex,
then call __kmem_cache_alias() trying to find a mergeable node, because
of the below added code in commit d38a2b7a9c93 ("mm: memcg/slab: fix
memory leak at non-root kmem_cache destroy"), B is not mergeable with
A whose memcg_params.dying is true.

int slab_unmergeable(struct kmem_cache *s)
if (s->refcount < 0)
return 1;

/*
* Skip the dying kmem_cache.
*/
if (s->memcg_params.dying)
return 1;

return 0;
}

So B has to create its own sysfs node by calling:
create_cache->
__kmem_cache_create->
sysfs_slab_add->
kobject_init_and_add
Because B is mergeable itself, its filename of sysfs node is based on its size,
like '/kernel/slab/:0000248', which is duplicate with A, and the sysfs
node of A is still present now, so kobject_init_and_add() will return
fail and result in kmem_cache_create() fail.

Concurrently modprobe and rmmod the two modules below can reproduce the issue
quickly: nf_conntrack_expect, se_sess_cache. See call trace in the end.

LTS versions of v4.19.y and v5.4.y have this problem, whereas linux versions after
v5.9 do not have this problem because the patchset: ("The new cgroup slab memory
controller") almost refactored memcg slab.

A potential solution(this patch belongs): Just let the dying kmem_cache be mergeable,
the slab_mutex lock can prevent the race between alias kmem_cache creating thread
and root kmem_cache destroying thread. In the destroying thread, after
flush_memcg_workqueue() is done, judge the refcount again, if someone
reference it again during un-lock time, we don't need to destroy the kmem_cache
completely, we can reuse it.

Another potential solution: revert the commit d38a2b7a9c93 ("mm: memcg/slab:
fix memory leak at non-root kmem_cache destroy"), compare to the fail of
kmem_cache_create, the memory leak in special scenario seems less harmful.

Call trace:
sysfs: cannot create duplicate filename '/kernel/slab/:0000248'
Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
Call trace:
dump_backtrace+0x0/0x198
show_stack+0x24/0x30
dump_stack+0xb0/0x100
sysfs_warn_dup+0x6c/0x88
sysfs_create_dir_ns+0x104/0x120
kobject_add_internal+0xd0/0x378
kobject_init_and_add+0x90/0xd8
sysfs_slab_add+0x16c/0x2d0
__kmem_cache_create+0x16c/0x1d8
create_cache+0xbc/0x1f8
kmem_cache_create_usercopy+0x1a0/0x230
kmem_cache_create+0x50/0x68
init_se_kmem_caches+0x38/0x258 [target_core_mod]
target_core_init_configfs+0x8c/0x390 [target_core_mod]
do_one_initcall+0x54/0x230
do_init_module+0x64/0x1ec
load_module+0x150c/0x16f0
__se_sys_finit_module+0xf0/0x108
__arm64_sys_finit_module+0x24/0x30
el0_svc_common+0x80/0x1c0
el0_svc_handler+0x78/0xe0
el0_svc+0x10/0x260
kobject_add_internal failed for :0000248 with -EEXIST, don't try to register things with the same name in the same directory.
kmem_cache_create(se_sess_cache) failed with error -17
Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015
Call trace:
dump_backtrace+0x0/0x198
show_stack+0x24/0x30
dump_stack+0xb0/0x100
kmem_cache_create_usercopy+0xa8/0x230
kmem_cache_create+0x50/0x68
init_se_kmem_caches+0x38/0x258 [target_core_mod]
target_core_init_configfs+0x8c/0x390 [target_core_mod]
do_one_initcall+0x54/0x230
do_init_module+0x64/0x1ec
load_module+0x150c/0x16f0
__se_sys_finit_module+0xf0/0x108
__arm64_sys_finit_module+0x24/0x30
el0_svc_common+0x80/0x1c0
el0_svc_handler+0x78/0xe0
el0_svc+0x10/0x260

Fixes: d38a2b7a9c93 ("mm: memcg/slab: fix memory leak at non-root kmem_cache destroy")
Signed-off-by: Nanyong Sun <sunnanyong@huawei.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
mm/slab_common.c | 18 ++++++++++--------
1 file changed, 10 insertions(+), 8 deletions(-)

--- a/mm/slab_common.c
+++ b/mm/slab_common.c
@@ -311,14 +311,6 @@ int slab_unmergeable(struct kmem_cache *
if (s->refcount < 0)
return 1;

-#ifdef CONFIG_MEMCG_KMEM
- /*
- * Skip the dying kmem_cache.
- */
- if (s->memcg_params.dying)
- return 1;
-#endif
-
return 0;
}

@@ -918,6 +910,16 @@ void kmem_cache_destroy(struct kmem_cach
get_online_mems();

mutex_lock(&slab_mutex);
+
+ /*
+ * Another thread referenced it again
+ */
+ if (READ_ONCE(s->refcount)) {
+ spin_lock_irq(&memcg_kmem_wq_lock);
+ s->memcg_params.dying = false;
+ spin_unlock_irq(&memcg_kmem_wq_lock);
+ goto out_unlock;
+ }
#endif

err = shutdown_memcg_caches(s);