Re: [PATCH 2/2] mm: memcg/slab: Don't create unfreeable slab

[Waiman Long]

On 5/3/21 8:22 AM, Vlastimil Babka wrote:
> On 5/2/21 8:07 PM, Waiman Long wrote:
> > The obj_cgroup array (memcg_data) embedded in the page structure is
> > allocated at the first instance an accounted memory allocation happens.
> > With the right size object, it is possible that the allocated obj_cgroup
> > 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.
> >
> > With instructmentation code added to detect this situation, I got 76
> > hits on the kmalloc-192 slab when booting up a test kernel on a VM.
> > So this can really happen.
> >
> > To avoid the creation of these unfreeable slabs, a check is added to
> > memcg_alloc_page_obj_cgroups() to detect that and double the size
> > of the array in case it happens to make sure that it comes from a
> > different kmemcache.
> >
> > This change, however, does not completely eliminate the presence
> > of unfreeable slabs which can still happen if a circular obj_cgroup
> > array dependency is formed.
> Hm this looks like only a half fix then.
> I'm afraid the proper fix is for kmemcg to create own set of caches for the
> arrays. It would also solve the recursive kfree() issue.

Right, this is a possible solution. However, the objcg pointers array 
should need that much memory. Creating its own set of kmemcaches may 
seem like an overkill.

Cheers,
Longman