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

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Date:   Wed, 15 Nov 2017 10:31:31 +0100
From:   Jan Kara <jack@...e.cz>
To:     Yang Shi <yang.s@...baba-inc.com>
Cc:     Michal Hocko <mhocko@...nel.org>, Jan Kara <jack@...e.cz>,
        amir73il@...il.com, linux-fsdevel@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] fs: fsnotify: account fsnotify metadata to kmemcg

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...

								Honza
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR