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

[Shakeel Butt <shakeelb@google.com>]

On Mon, May 3, 2021 at 8:32 AM Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 5/3/21 4:20 PM, Waiman Long wrote:
> > 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.
>
> Well if we go that way, there might be additional benefits:
>
> depending of gfp flags, kmalloc() would allocate from:
>
> kmalloc-* caches that never have kmemcg objects, thus can be used for the objcg
> pointer arrays
> kmalloc-cg-* caches that have only kmemcg unreclaimable objects
> kmalloc-rcl-* and dma-kmalloc-* can stay with on-demand
> memcg_alloc_page_obj_cgroups()
>
> This way we fully solve the issues that this patchset solves. In addition we get
> better separation between kmemcg and !kmemcg thus save memory - no allocation of
> the array as soon as a single object appears in slab. For "kmalloc-8" we now
> have 8 bytes for the useful data and 8 bytes for the obj_cgroup  pointer.
>

Yes this seems like a better approach.