linux-kernel - Re: [PATCH v2 1/3] mm: memcontrol: fix swap counter leak on swapout from offline cgroup

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Date:	Tue, 2 Aug 2016 13:33:37 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Michal Hocko <mhocko@...nel.org>
Cc:	Vladimir Davydov <vdavydov@...tuozzo.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	stable@...r.kernel.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] mm: memcontrol: fix swap counter leak on swapout
 from offline cgroup

On Tue, Aug 02, 2016 at 06:00:26PM +0200, Michal Hocko wrote:
> On Tue 02-08-16 18:00:48, Vladimir Davydov wrote:
> > @@ -5767,15 +5785,20 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
> >  	if (!memcg)
> >  		return;
> >  
> > -	mem_cgroup_id_get(memcg);
> > -	oldid = swap_cgroup_record(entry, mem_cgroup_id(memcg));
> > +	swap_memcg = mem_cgroup_id_get_active(memcg);
> > +	oldid = swap_cgroup_record(entry, mem_cgroup_id(swap_memcg));
> >  	VM_BUG_ON_PAGE(oldid, page);
> > -	mem_cgroup_swap_statistics(memcg, true);
> > +	mem_cgroup_swap_statistics(swap_memcg, true);
> >  
> >  	page->mem_cgroup = NULL;
> >  
> >  	if (!mem_cgroup_is_root(memcg))
> >  		page_counter_uncharge(&memcg->memory, 1);
> > +	if (memcg != swap_memcg) {
> > +		if (!mem_cgroup_is_root(swap_memcg))
> > +			page_counter_charge(&swap_memcg->memsw, 1);
> > +		page_counter_uncharge(&memcg->memsw, 1);
> > +	}
> >  
> >  	/*
> >  	 * Interrupts should be disabled here because the caller holds the
> 
> The resulting code is a weird mixture of memcg and swap_memcg usage
> which is really confusing and error prone. Do we really have to do
> uncharge on an already offline memcg?

The charge is recursive and includes swap_memcg, i.e. live groups, so
the uncharge is necessary. I don't think the code is too bad, though?
swap_memcg is the target that is being charged for swap, memcg is the
origin group from which we swap out. Seems pretty straightforward...?

But maybe a comment above the memcg != swap_memcg check would be nice:

/*
 * In case the memcg owning these pages has been offlined and doesn't
 * have an ID allocated to it anymore, charge the closest online
 * ancestor for the swap instead and transfer the memory+swap charge.
 */

Thinking about it, mem_cgroup_id_get_active() is a little strange; the
term we use throughout the cgroup code is "online". It might be good
to rename this mem_cgroup_id_get_online().

Thanks