linux-kernel - Re: [PATCH v3 2/3] mm: Charge active memcg when no mm is set

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Message-ID: <20200224213750.GA3773@dschatzberg-fedora-PC0Y6AEN.dhcp.thefacebook.com>
Date:   Mon, 24 Feb 2020 16:37:50 -0500
From:   Dan Schatzberg <schatzberg.dan@...il.com>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Jens Axboe <axboe@...nel.dk>, Tejun Heo <tj@...nel.org>,
        Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Chris Down <chris@...isdown.name>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "open list:BLOCK LAYER" <linux-block@...r.kernel.org>,
        open list <linux-kernel@...r.kernel.org>,
        "open list:CONTROL GROUP (CGROUP)" <cgroups@...r.kernel.org>,
        "open list:CONTROL GROUP - MEMORY RESOURCE CONTROLLER (MEMCG)" 
        <linux-mm@...ck.org>
Subject: Re: [PATCH v3 2/3] mm: Charge active memcg when no mm is set

On Sun, Feb 23, 2020 at 05:11:12PM -0800, Hugh Dickins wrote:
> On Sun, 23 Feb 2020, Hugh Dickins wrote:
> > On Thu, 20 Feb 2020, Dan Schatzberg wrote:
> > 
> > > memalloc_use_memcg() worked for kernel allocations but was silently
> > > ignored for user pages.
> > > 
> > > This patch establishes a precedence order for who gets charged:
> > > 
> > > 1. If there is a memcg associated with the page already, that memcg is
> > >    charged. This happens during swapin.
> > > 
> > > 2. If an explicit mm is passed, mm->memcg is charged. This happens
> > >    during page faults, which can be triggered in remote VMs (eg gup).
> > > 
> > > 3. Otherwise consult the current process context. If it has configured
> > >    a current->active_memcg, use that. Otherwise, current->mm->memcg.
> > > 
> > > Previously, if a NULL mm was passed to mem_cgroup_try_charge (case 3) it
> > > would always charge the root cgroup. Now it looks up the current
> > > active_memcg first (falling back to charging the root cgroup if not
> > > set).
> > > 
> > > Signed-off-by: Dan Schatzberg <schatzberg.dan@...il.com>
> > > Acked-by: Johannes Weiner <hannes@...xchg.org>
> > > Acked-by: Tejun Heo <tj@...nel.org>
> > 
> > Acked-by: Hugh Dickins <hughd@...gle.com>
> > 
> > Yes, internally we have some further not-yet-upstreamed complications
> > here (mainly, the "memcg=" mount option for all charges on a tmpfs to
> > be charged to that memcg); but what you're doing here does not obstruct
> > adding that later, they fit in well with the hierarchy that you (and
> > Johannes) mapped out above, and it's really an improvement for shmem
> > not to be referring to current there - thanks.
> 
> I acked slightly too soon. There are two other uses of "try_charge" in
> mm/shmem.c: we can be confident that the userfaultfd one knows what mm
> it's dealing with, but the shmem_swapin_page() instance has a similar
> use of current->mm, that you also want to adjust to NULL, don't you?
> 
> Hugh

Yes, you're right. I'll change shmem_swapin_page as well