linux-kernel - Re: [PATCH 10/18] mm: memcontrol: switch to native NR_ANON

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Date:   Thu, 23 Apr 2020 14:27:01 +0900
From:   Joonsoo Kim <js1304@...il.com>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Alex Shi <alex.shi@...ux.alibaba.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Hugh Dickins <hughd@...gle.com>,
        Michal Hocko <mhocko@...e.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Roman Gushchin <guro@...com>, linux-mm@...ck.org,
        cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com
Subject: Re: [PATCH 10/18] mm: memcontrol: switch to native NR_ANON_MAPPED
 counter

On Wed, Apr 22, 2020 at 08:28:18AM -0400, Johannes Weiner wrote:
> Hello Joonsoo,
> 
> On Wed, Apr 22, 2020 at 03:51:52PM +0900, Joonsoo Kim wrote:
> > On Mon, Apr 20, 2020 at 06:11:18PM -0400, Johannes Weiner wrote:
> > > @@ -3768,7 +3761,7 @@ static int memcg_numa_stat_show(struct seq_file *m, void *v)
> > >  
> > >  static const unsigned int memcg1_stats[] = {
> > >  	NR_FILE_PAGES,
> > > -	MEMCG_RSS,
> > > +	NR_ANON_MAPPED,
> > >  	MEMCG_RSS_HUGE,
> > >  	NR_SHMEM,
> > >  	NR_FILE_MAPPED,
> > > @@ -5395,7 +5388,12 @@ static int mem_cgroup_move_account(struct page *page,
> > >  
> > >  	lock_page_memcg(page);
> > >  
> > > -	if (!PageAnon(page)) {
> > > +	if (PageAnon(page)) {
> > > +		if (page_mapped(page)) {
> > 
> > This page_mapped() check is newly inserted. Could you elaborate more
> > on why mem_cgroup_charge_statistics() doesn't need this check?
> 
> MEMCG_RSS extended from when the page was charged until it was
> uncharged, but NR_ANON_MAPPED is only counted while the page is really
> mapped into page tables. That starts shortly after we charge and ends
> shortly before we uncharge, so pages could move between cgroups before
> or after they are mapped, while they aren't counted in NR_ANON_MAPPED.
> 
> So to know that the page is counted, charge_statistics() only needed
> to know that the page is charged and Anon; move_account() also needs
> to know that the page is mapped.

Got it!

> 
> > > @@ -1181,7 +1187,7 @@ void page_add_new_anon_rmap(struct page *page,
> > >  		/* increment count (starts at -1) */
> > >  		atomic_set(&page->_mapcount, 0);
> > >  	}
> > > -	__mod_node_page_state(page_pgdat(page), NR_ANON_MAPPED, nr);
> > > +	__mod_lruvec_page_state(page, NR_ANON_MAPPED, nr);
> > >  	__page_set_anon_rmap(page, vma, address, 1);
> > >  }
> > 
> > memcg isn't setup yet and accounting isn't applied to proper memcg.
> > Maybe, it would be applied to root memcg. With this change, we don't
> > need the mapping to commit the charge so switching the order of
> > page_add_new_anon_rmap() and mem_cgroup_commit_charge() will solve the
> > issue.
> 
> Good catch, it's that dreaded circular dependency. It's fixed two
> patches down when I charge anon pages earlier as well. But I'll change
> the rmap<->commit order in this patch to avoid the temporary bug.

Okay.

> Thanks for your thorough review!

Thanks.