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

[Waiman Long <llong@redhat.com>]

On 5/3/21 1:21 PM, Waiman Long wrote:
> On 5/3/21 12:24 PM, Shakeel Butt wrote:
>> 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.
>>
> OK, I will try to go this route then if there is no objection from 
> others.
>
> From slabinfo, the objs/slab numbers range from 4-512. That means we 
> need kmalloc-cg-{32,64,128,256,512,1k,2k,4k}. A init function to set 
> up the new kmemcaches and an allocation function that use the proper 
> kmemcaches to allocate from. 

I think I had misinterpreted the kmalloc-* setup. In this case, the 
kmalloc-cg-* should have the same set of sizes as kmalloc-*.

Cheers,
Longman