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Date:   Wed, 27 Jun 2018 09:31:45 +0200
From:   Jan Kara <jack@suse.cz>
To:     Shakeel Butt <shakeelb@google.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Amir Goldstein <amir73il@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Michal Hocko <mhocko@kernel.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Jan Kara <jack@suse.com>, Greg Thelen <gthelen@google.com>,
        Roman Gushchin <guro@fb.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        LKML <linux-kernel@vger.kernel.org>,
        Cgroups <cgroups@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>, Jan Kara <jack@suse.cz>
Subject: Re: [PATCH 1/2] fs: fsnotify: account fsnotify metadata to kmemcg
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On Tue 26-06-18 12:07:57, Shakeel Butt wrote:
> On Tue, Jun 26, 2018 at 11:55 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> >
> > On Tue, Jun 26, 2018 at 11:00:53AM -0700, Shakeel Butt wrote:
> > > On Mon, Jun 25, 2018 at 10:49 PM Amir Goldstein <amir73il@gmail.com> wrote:
> > > >
> > > ...
> > > >
> > > > The verb 'unuse' takes an argument memcg and 'uses' it - too weird.
> > > > You can use 'override'/'revert' verbs like override_creds or just call
> > > > memalloc_use_memcg(old_memcg) since there is no reference taken
> > > > anyway in use_memcg and no reference released in unuse_memcg.
> > > >
> > > > Otherwise looks good to me.
> > > >
> > >
> > > Thanks for your feedback. Just using memalloc_use_memcg(old_memcg) and
> > > ignoring the return seems more simple. I will wait for feedback from
> > > other before changing anything.
> >
> > We're not nesting calls to memalloc_use_memcg(), right? So we don't
> > have to return old_memcg and don't have to pass anything to unuse, it
> > can always set current->active_memcg to NULL.
> 
> For buffer_head, the allocation is done with GFP_NOFS. So, I think
> there is no chance of nesting. The fsnotify uses GFP_KERNEL but based
> on my limited understanding of fsnotify, there should not be any
> nesting i.e. the allocation triggering reclaim which trigger fsnotify
> events. Though I would like Amir or Jan to confirm there is no nesting
> possible.

You are correct. Fsnotify events are generated only as a result of some
syscall, not due to reclaim or stuff like that.

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