Re: [PATCH nft] nft: memcg accounting for dynamically allocated objects

[Vasily Averin]

On 4/1/22 22:31, Florian Westphal wrote:
> Vasily Averin <vasily.averin@xxxxxxxxx> wrote:
>>> Same problem as connlimit, can be called from packet path.
>>> Basically all GFP_ATOMIC are suspicious.
>>>
>>> Not sure how to resolve this, similar mechanics in iptables world (e.g.
>>> connlimit or SET target) don't use memcg accounting.
>>>
>>> Perhaps for now resend with only the GFP_KERNEL parts converted?
>>> Those are safe.
>>
>> It is safe for packet path too, _ACCOUNT allocation will not be able to find memcg
>> in case of "!in_task()" context.
>> On the other hand any additional checks on such path will affect performance.
> 
> I'm not sure this works with ksoftirqd serving network stack?

Please take look at memcg_kmem_bypass() called from 
memcg_slab_pre_alloc_hook -> get_obj_cgroup_from_current

By default memcg accounting does not work for any kernel threads.
If required thread can use set_active_memcg() but at present it is not widely used.

>> Could you please estimate how often is this code used in the case of nft vs packet path?
> 
> It depends on user configuration.
> Update from packet path is used for things like port knocking or other
> dyanamic filter lists, or somehing like Limiting connections to x-per-address/subnet and so on.

Ok, I think we can skip accounting in such cases at the moment.
I doubt it can be misused and consume significant piece of host memory.
So I'm going to resend the patch w/o accounting in all .clone callbacks. 

>> If the opposite is the case, then I can add __GFP_ACCOUNT flag depending on in_task() check.
> 
> But what task/memcg is used for the accounting in that case?

Thanks to Roman for the explanation in concurrent thread.

Thank you,
	Vasily Averin