On 5/5/21 5:41 PM, Vlastimil Babka wrote:

On 5/5/21 10:06 PM, Waiman Long wrote:

quoted

There are currently two problems in the way the objcg pointer array
(memcg_data) in the page structure is being allocated and freed.

On its allocation, it is possible that the allocated objcg pointer
array comes from the same slab that requires memory accounting. If this
happens, the slab will never become empty again as there is at least
one object left (the obj_cgroup array) in the slab.

When it is freed, the objcg pointer array object may be the last one
in its slab and hence causes kfree() to be called again. With the
right workload, the slab cache may be set up in a way that allows the
recursive kfree() calling loop to nest deep enough to cause a kernel
stack overflow and panic the system.

One way to solve this problem is to split the kmalloc-<n> caches
(KMALLOC_NORMAL) into two separate sets - a new set of kmalloc-<n>
(KMALLOC_NORMAL) caches for unaccounted objects only and a new set of
kmalloc-cg-<n> (KMALLOC_CGROUP) caches for accounted objects only. All
the other caches can still allow a mix of accounted and unaccounted
objects.

With this change, all the objcg pointer array objects will come from
KMALLOC_NORMAL caches which won't have their objcg pointer arrays. So
both the recursive kfree() problem and non-freeable slab problem are
gone.

Since both the KMALLOC_NORMAL and KMALLOC_CGROUP caches no longer have
mixed accounted and unaccounted objects, this will slightly reduce the
number of objcg pointer arrays that need to be allocated and save a bit
of memory. On the other hand, creating a new set of kmalloc caches does
have the effect of reducing cache utilization. So it is properly a wash.

The new KMALLOC_CGROUP is added between KMALLOC_NORMAL and
KMALLOC_RECLAIM so that the first for loop in create_kmalloc_caches()
will include the newly added caches without change.

Suggested-by: Vlastimil Babka <redacted>
Signed-off-by: Waiman Long <redacted>
Reviewed-by: Shakeel Butt <redacted>

A last nitpick: the new caches -cg should perhaps not be created when
cgroup_memory_nokmem == true because kmemcg was disabled by the respective boot
param.

It is a nice to have feature. However, the nokmem kernel parameter isn't 
used that often. The cgroup_memory_nokmem variable is private to 
memcontrol.c and is not directly accessible. I will take a look on that, 
but it will be a follow-on patch. I am not planning to change the 
current patchset unless there are other issues coming up.

Cheers,
Longman