linux-kernel - Re: [PATCH v2 2/3] mm: Charge active memcg when no mm is set

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Date:   Wed, 19 Feb 2020 14:37:11 -0500
From:   Johannes Weiner <hannes@...xchg.org>
To:     Chris Down <chris@...isdown.name>
Cc:     Dan Schatzberg <dschatzberg@...com>, Jens Axboe <axboe@...nel.dk>,
        Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        Michal Hocko <mhocko@...nel.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Hugh Dickins <hughd@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>,
        Shakeel Butt <shakeelb@...gle.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 v2 2/3] mm: Charge active memcg when no mm is set

On Fri, Feb 07, 2020 at 09:18:07PM +0000, Chris Down wrote:
> > @@ -6856,8 +6857,12 @@ int mem_cgroup_try_charge(struct page *page, struct mm_struct *mm,
> > 		}
> > 	}
> > 
> > -	if (!memcg)
> > -		memcg = get_mem_cgroup_from_mm(mm);
> > +	if (!memcg) {
> > +		if (!mm)
> > +			memcg = get_mem_cgroup_from_current();
> > +		else
> > +			memcg = get_mem_cgroup_from_mm(mm);
> > +	}
> 
> Just to do due diligence, did we double check whether this results in any
> unintentional shift in accounting for those passing in both mm and memcg as
> NULL with no current->active_memcg set, since previously we never even tried
> to consult current->mm and always used root_mem_cgroup in
> get_mem_cgroup_from_mm?

Excellent question on a subtle issue.

But nobody actually passes NULL. They either pass current->mm (or a
destination mm) in syscalls, or vma->vm_mm in page faults.

The only times we end up with NULL is when kernel threads do something
and have !current->mm. We redirect those to root_mem_cgroup.

So this patch doesn't change those semantics.