Re: [PATCH v3 2/2] mm: memcg/slab: Create a new set of kmalloc-cg-<n> caches

[Shakeel Butt]

On Wed, May 5, 2021 at 8:47 AM Waiman Long <longman@xxxxxxxxxx> 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 non-accounted 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 non-accounted
> 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.
>
> 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 <vbabka@xxxxxxx>
> Signed-off-by: Waiman Long <longman@xxxxxxxxxx>

One nit below and after incorporating Vlastimil's suggestions:

Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>

> ---
>  include/linux/slab.h | 42 ++++++++++++++++++++++++++++++++++--------
>  mm/slab_common.c     | 23 +++++++++++++++--------
>  2 files changed, 49 insertions(+), 16 deletions(-)
>
> diff --git a/include/linux/slab.h b/include/linux/slab.h
> index 0c97d788762c..f2d9ebc34f5c 100644
> --- a/include/linux/slab.h
> +++ b/include/linux/slab.h
> @@ -305,9 +305,16 @@ static inline void __check_heap_object(const void *ptr, unsigned long n,
>  /*
>   * Whenever changing this, take care of that kmalloc_type() and
>   * create_kmalloc_caches() still work as intended.
> + *
> + * KMALLOC_NORMAL is for non-accounted objects only whereas KMALLOC_CGROUP
> + * is for accounted objects only.

I think you can say "KMALLOC_CGROUP is for accounted and unreclaimable
objects only".