Re: [PATCH v2] fs: fsnotify: account fsnotify metadata to kmemcg

[Shakeel Butt]

On Wed, Nov 15, 2017 at 1:31 AM, Jan Kara <jack@xxxxxxx> wrote:
> On Wed 15-11-17 01:32:16, Yang Shi wrote:
>>
>>
>> On 11/14/17 1:39 AM, Michal Hocko wrote:
>> >On Tue 14-11-17 03:10:22, Yang Shi wrote:
>> >>
>> >>
>> >>On 11/9/17 5:54 AM, Michal Hocko wrote:
>> >>>[Sorry for the late reply]
>> >>>
>> >>>On Tue 31-10-17 11:12:38, Jan Kara wrote:
>> >>>>On Tue 31-10-17 00:39:58, Yang Shi wrote:
>> >>>[...]
>> >>>>>I do agree it is not fair and not neat to account to producer rather than
>> >>>>>misbehaving consumer, but current memcg design looks not support such use
>> >>>>>case. And, the other question is do we know who is the listener if it
>> >>>>>doesn't read the events?
>> >>>>
>> >>>>So you never know who will read from the notification file descriptor but
>> >>>>you can simply account that to the process that created the notification
>> >>>>group and that is IMO the right process to account to.
>> >>>
>> >>>Yes, if the creator is de-facto owner which defines the lifetime of
>> >>>those objects then this should be a target of the charge.
>> >>>
>> >>>>I agree that current SLAB memcg accounting does not allow to account to a
>> >>>>different memcg than the one of the running process. However I *think* it
>> >>>>should be possible to add such interface. Michal?
>> >>>
>> >>>We do have memcg_kmem_charge_memcg but that would require some plumbing
>> >>>to hook it into the specific allocation path. I suspect it uses kmalloc,
>> >>>right?
>> >>
>> >>Yes.
>> >>
>> >>I took a look at the implementation and the callsites of
>> >>memcg_kmem_charge_memcg(). It looks it is called by:
>> >>
>> >>* charge kmem to memcg, but it is charged to the allocator's memcg
>> >>* allocate new slab page, charge to memcg_params.memcg
>> >>
>> >>I think this is the plumbing you mentioned, right?
>> >
>> >Maybe I have misunderstood, but you are using slab allocator. So you
>> >would need to force it to use a different charging context than current.
>>
>> Yes.
>>
>> >I haven't checked deeply but this doesn't look trivial to me.
>>
>> I agree. This is also what I explained to Jan and Amir in earlier
>> discussion.
>
> And I also agree. But the fact that it is not trivial does not mean that it
> should not be done...
>

I am currently working on directed or remote memcg charging for a
different usecase and I think that would be helpful here as well.

I have two questions though:

1) Is fsnotify_group the right structure to hold the reference to
target mem_cgroup for charging?
2) Remote charging can trigger an OOM in the target memcg. In this
usecase, I think, there should be security concerns if the events
producer can trigger OOM in the memcg of the monitor. We can either
change these allocations to use __GFP_NORETRY or some new gfp flag to
not trigger oom-killer. So, is this valid concern or am I
over-thinking?

thanks,
Shakeel