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

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Date:   Tue, 14 Nov 2017 10:39:09 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Yang Shi <yang.s@...baba-inc.com>
Cc:     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 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.
I haven't checked deeply but this doesn't look trivial to me.
-- 
Michal Hocko
SUSE Labs