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Date:   Thu, 1 Oct 2020 13:27:13 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Michal =?iso-8859-1?Q?Koutn=FD?= <mkoutny@suse.com>
Cc:     Roman Gushchin <guro@fb.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Shakeel Butt <shakeelb@google.com>,
        Michal Hocko <mhocko@kernel.org>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, kernel-team@fb.com
Subject: Re: [PATCH v4 4/4] mm: convert page kmemcg type to a page memcg flag
Message-ID: <20201001172713.GA500308@cmpxchg.org>
References: <20201001002710.748166-1-guro@fb.com>
 <20201001002710.748166-5-guro@fb.com>
 <20201001170036.GA29848@blackbook>
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On Thu, Oct 01, 2020 at 07:00:36PM +0200, Michal Koutn? wrote:
> Hi.
> 
> On Wed, Sep 30, 2020 at 05:27:10PM -0700, Roman Gushchin <guro@fb.com> wrote:
> > @@ -369,8 +371,12 @@ enum page_memcg_data_flags {
> >   */
> >  static inline struct mem_cgroup *page_memcg(struct page *page)
> >  {
> > +	unsigned long memcg_data = page->memcg_data;
> > +
> >  	VM_BUG_ON_PAGE(PageSlab(page), page);
> > -	return (struct mem_cgroup *)page->memcg_data;
> > +	VM_BUG_ON_PAGE(memcg_data & MEMCG_DATA_OBJCGS, page);
> > +
> > +	return (struct mem_cgroup *)(memcg_data & ~MEMCG_DATA_FLAGS_MASK);
> >  }
> Shouldn't this change go also into page_memcg_rcu()? (I don't think the
> current single user (workingset_activation() would pass a non-slab
> kernel page but for consistency sake.)

+1

> Alternatively, I'm thinking why (in its single use) is there
> page_memcg_rcu() a separate function to page_memcg() (cross memcg page
> migration?).

It goes back to commit 55779ec759ccc3c12b917b3712a7716e1140c652.

The activation code is the only path where page migration is not
excluded. Because unlike with page state statistics, we don't really
mind a race when counting an activation event.

I do think there is a bug, though: mem_cgroup_move_account() should
use WRITE_ONCE() on page->mem_cgroup.