From mboxrd@z Thu Jan  1 00:00:00 1970
From: Johannes Weiner <hannes-druUgvl0LCNAfugRpC6u6w@public.gmane.org>
Subject: Re: [PATCH 18/18] mm: memcontrol: update page->mem_cgroup stability
 rules
Date: Tue, 21 Apr 2020 10:34:35 -0400
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 <20200421091014.2180-1-hdanton@sina.com>
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To: Hillf Danton <hdanton-k+cT0dCbe1g@public.gmane.org>
Cc: Joonsoo Kim <js1304-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, Alex Shi <alex.shi-KPsoFbNs7GizrGE5bRqYAgC/G2K4zDHf@public.gmane.org>, Shakeel Butt <shakeelb-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Hugh Dickins <hughd-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org>, Michal Hocko <mhocko-IBi9RG/b67k@public.gmane.org>, "Kirill A. Shutemov" <kirill-oKw7cIdHH8eLwutG50LtGA@public.gmane.org>, Roman Gushchin <guro-b10kYP2dOMg@public.gmane.org>, linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org, cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org, kernel-team-b10kYP2dOMg@public.gmane.org

On Tue, Apr 21, 2020 at 05:10:14PM +0800, Hillf Danton wrote:
> 
> On Mon, 20 Apr 2020 18:11:26 -0400 Johannes Weiner wrote:
> > 
> > The previous patches have simplified the access rules around
> > page->mem_cgroup somewhat:
> > 
> > 1. We never change page->mem_cgroup while the page is isolated by
> >    somebody else. This was by far the biggest exception to our rules
> >    and it didn't stop at lock_page() or lock_page_memcg().
> > 
> > 2. We charge pages before they get put into page tables now, so the
> >    somewhat fishy rule about "can be in page table as long as it's
> >    still locked" is now gone and boiled down to having an exclusive
> >    reference to the page.
> > 
> > Document the new rules. Any of the following will stabilize the
> > page->mem_cgroup association:
> > 
> > - the page lock
> > - LRU isolation
> > - lock_page_memcg()
> > - exclusive access to the page
> 
> Then rule-1 makes rule-3 no longer needed in mem_cgroup_move_account()?

Well, mem_cgroup_move_account() is the write side. It's the function
that changes page->mem_cgroup. So it needs to take all these locks in
order for the readside / fastpath to be okay with any one of them.