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

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Message-ID: <20141015151836.GG23547@dhcp22.suse.cz>
Date:	Wed, 15 Oct 2014 17:18:36 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Johannes Weiner <hannes@...xchg.org>
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 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...

>  		stock->nr_pages = 0;
>  	}
>  	stock->cached = NULL;
> @@ -2513,6 +2514,7 @@ bypass:
>  	return -EINTR;
>  
>  done_restock:
> +	css_get_many(&memcg->css, batch);
>  	if (batch > nr_pages)
>  		refill_stock(memcg, batch - nr_pages);
>  done:
[...]
-- 
Michal Hocko
SUSE Labs
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