On 5/5/21 10:06 PM, Waiman Long wrote:
> 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 <[email protected]>
> Signed-off-by: Waiman Long <[email protected]>
> Reviewed-by: Shakeel Butt <[email protected]>
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.
On 5/5/21 5:41 PM, Vlastimil Babka wrote:
> On 5/5/21 10:06 PM, Waiman Long wrote:
>> 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 <[email protected]>
>> Signed-off-by: Waiman Long <[email protected]>
>> Reviewed-by: Shakeel Butt <[email protected]>
> 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