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

[Vlastimil Babka]

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.

Vlastimil

> Cheers,
> Longman
>