linux-kernel - Re: [patch 2/5] mm: memcontrol: take a css reference for each charged page

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Message-ID: <20141015184410.GB6442@phnom.home.cmpxchg.org>
Date:	Wed, 15 Oct 2014 14:44:10 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Michal Hocko <mhocko@...e.cz>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Vladimir Davydov <vdavydov@...allels.com>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [patch 2/5] mm: memcontrol: take a css reference for each
 charged page

On Wed, Oct 15, 2014 at 05:18:36PM +0200, Michal Hocko wrote:
> On Tue 14-10-14 12:20:34, Johannes Weiner wrote:
> > Charges currently pin the css indirectly by playing tricks during
> > css_offline(): user pages stall the offlining process until all of
> > them have been reparented, whereas kmemcg acquires a keep-alive
> > reference if outstanding kernel pages are detected at that point.
> > 
> > In preparation for removing all this complexity, make the pinning
> > explicit and acquire a css references for every charged page.
> > 
> > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> > Reviewed-by: Vladimir Davydov <vdavydov@...allels.com>
> 
> Acked-by: Michal Hocko <mhocko@...e.cz>
> 
> > ---
> >  include/linux/cgroup.h          | 26 +++++++++++++++++++++++
> >  include/linux/percpu-refcount.h | 47 +++++++++++++++++++++++++++++++++--------
> >  mm/memcontrol.c                 | 21 ++++++++++++++----
> >  3 files changed, 81 insertions(+), 13 deletions(-)
> > 
> [...]
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 67dabe8b0aa6..a3feead6be15 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -2256,6 +2256,7 @@ static void drain_stock(struct memcg_stock_pcp *stock)
> >  		page_counter_uncharge(&old->memory, stock->nr_pages);
> >  		if (do_swap_account)
> >  			page_counter_uncharge(&old->memsw, stock->nr_pages);
> > +		css_put_many(&old->css, stock->nr_pages);
> 
> I have suggested to add a comment about pairing css_get here because the
> corresponding refill_stock doesn't take any reference which might be
> little bit confusing. Nothing earth shattering of course...

Ah, but this isn't the counter-part to refill_stock(), consume_stock()
is.  The references are taken for the charges, and those two functions
do not change the account.  css get/put pair exactly like page counter
charge/uncharge.
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