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Date:   Thu, 22 Feb 2018 14:46:00 +0100
From:   Jan Kara <jack@suse.cz>
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Shakeel Butt <shakeelb@google.com>,
        Christopher Lameter <cl@linux.com>, Jan Kara <jack@suse.cz>,
        Amir Goldstein <amir73il@gmail.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Greg Thelen <gthelen@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Mel Gorman <mgorman@suse.de>, Vlastimil Babka <vbabka@suse.cz>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        Cgroups <cgroups@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v2 0/3] Directed kmem charging
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On Wed 21-02-18 12:54:26, Andrew Morton wrote:
> On Wed, 21 Feb 2018 09:18:35 -0800 Shakeel Butt <shakeelb@google.com> wrote:
> 
> > On Wed, Feb 21, 2018 at 8:09 AM, Christopher Lameter <cl@linux.com> wrote:
> > > Another way to solve this is to switch the user context right?
> > >
> > > Isnt it possible to avoid these patches if do the allocation in another
> > > task context instead?
> > >
> > 
> > Sorry, can you please explain what you mean by 'switch the user
> > context'. Is there any example in kernel which does something similar?
> > 
> > Another way is by adding a field 'remote_memcg_to_charge' in
> > task_struct and set it before the allocation and in memcontrol.c,
> > first check if current->remote_memcg_to_charge is set otherwise use
> > the memcg of current. Also if we provide a wrapper to do that for the
> > user, there will be a lot less plumbing.
> > 
> > Please let me know if you prefer this approach.
> 
> That would be a lot simpler.  Passing function arguments via
> task_struct is a bit dirty but is sometimes sooo effective.  You
> should've seen how much mess task_struct.journal_info avoided!  And
> reclaim_state.

Agreed, although from time to time people try to be too creative e.g. with
journal_info and surprising bugs come out of that :).

> And one always wonders whether we should do a local save/restore before
> modifying the task_struct field, so it nests.
> 
> What do others think?

Sounds nice to me.

> Maybe we can rename task_struct.reclaim_state to `struct task_mm_state
> *task_mm_state", add remote_memcg_to_charge to struct task_mm_state and
> avoid bloating the task_struct?

Yeah, even better, but then we really need to make sure these things stack
properly.

								Honza

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