On Wed 28-07-21 07:10:26, Shakeel Butt wrote:

+Roman

On Wed, Jul 28, 2021 at 6:23 AM Michal Hocko [off-list ref] wrote:

quoted

On Wed 28-07-21 17:13:48, Wang Hai wrote:

quoted

When I use kfree_rcu() to free a large memory allocated by
kmalloc_node(), the following dump occurs.

BUG: kernel NULL pointer dereference, address: 0000000000000020
[...]
Oops: 0000 [#1] SMP
[...]
Workqueue: events kfree_rcu_work
RIP: 0010:__obj_to_index include/linux/slub_def.h:182 [inline]
RIP: 0010:obj_to_index include/linux/slub_def.h:191 [inline]
RIP: 0010:memcg_slab_free_hook+0x120/0x260 mm/slab.h:363
[...]
Call Trace:
 kmem_cache_free_bulk+0x58/0x630 mm/slub.c:3293
 kfree_bulk include/linux/slab.h:413 [inline]
 kfree_rcu_work+0x1ab/0x200 kernel/rcu/tree.c:3300
 process_one_work+0x207/0x530 kernel/workqueue.c:2276
 worker_thread+0x320/0x610 kernel/workqueue.c:2422
 kthread+0x13d/0x160 kernel/kthread.c:313
 ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:294

When kmalloc_node() a large memory, page is allocated, not slab,
so when freeing memory via kfree_rcu(), this large memory should not
be used by memcg_slab_free_hook(), because memcg_slab_free_hook() is
is used for slab.

So in this case, there is no need to do anything with this large
page in memcg_slab_free_hook(), just skip it.

Fixes: 270c6a71460e ("mm: memcontrol/slab: Use helpers to access slab page's memcg_data")

Are you sure that this commit is really breaking the code. Unless I have
missed something there shouldn't be any real change wrt. large
allocations here. page_has_obj_cgroups is just a different name for what
what page_objcgs is giving us.

Actually they are different. For MEMCG_DATA_KMEM page,
page_has_obj_cgroups() will return false while page_objcgs() on
non-VM_DEBUG kernels will return "struct obj_cgroup *" instead of
"struct obj_cgroup **".

Right. Thanks for the clarification. I have missed that subtle
difference.
-- 
Michal Hocko
SUSE Labs