Re: [PATCH v4 2/3] mm: memcg/slab: Create a new set of kmalloc-cg-<n> caches

[Waiman Long]

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 <vbabka@xxxxxxx>
> > Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
> > Reviewed-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> 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