linux-kernel - Re: [PATCH v2 7/7] Documentation: cgroup: add memory.swap.{current,max} description

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Message-ID: <20160113170250.GK17512@dhcp22.suse.cz>
Date:	Wed, 13 Jan 2016 18:02:50 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Vladimir Davydov <vdavydov@...tuozzo.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 7/7] Documentation: cgroup: add
 memory.swap.{current,max} description

On Thu 17-12-15 15:30:00, Vladimir Davydov wrote:
> The rationale of separate swap counter is given by Johannes Weiner.
>
> Signed-off-by: Vladimir Davydov <vdavydov@...tuozzo.com>

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
> Changes in v2:
>  - Add rationale of separate swap counter provided by Johannes.
> 
>  Documentation/cgroup.txt | 33 +++++++++++++++++++++++++++++++++

this went to Documentation/cgroup-v2.txt with the latest Tejun's pull
request.

>  1 file changed, 33 insertions(+)
> 
> diff --git a/Documentation/cgroup.txt b/Documentation/cgroup.txt
> index 31d1f7bf12a1..f441564023e1 100644
> --- a/Documentation/cgroup.txt
> +++ b/Documentation/cgroup.txt
> @@ -819,6 +819,22 @@ PAGE_SIZE multiple when read back.
>  		the cgroup.  This may not exactly match the number of
>  		processes killed but should generally be close.
>  
> +  memory.swap.current
> +
> +	A read-only single value file which exists on non-root
> +	cgroups.
> +
> +	The total amount of swap currently being used by the cgroup
> +	and its descendants.
> +
> +  memory.swap.max
> +
> +	A read-write single value file which exists on non-root
> +	cgroups.  The default is "max".
> +
> +	Swap usage hard limit.  If a cgroup's swap usage reaches this
> +	limit, anonymous meomry of the cgroup will not be swapped out.
> +
>  
>  5-2-2. General Usage
>  
> @@ -1291,3 +1307,20 @@ allocation from the slack available in other groups or the rest of the
>  system than killing the group.  Otherwise, memory.max is there to
>  limit this type of spillover and ultimately contain buggy or even
>  malicious applications.
> +
> +The combined memory+swap accounting and limiting is replaced by real
> +control over swap space.
> +
> +The main argument for a combined memory+swap facility in the original
> +cgroup design was that global or parental pressure would always be
> +able to swap all anonymous memory of a child group, regardless of the
> +child's own (possibly untrusted) configuration.  However, untrusted
> +groups can sabotage swapping by other means - such as referencing its
> +anonymous memory in a tight loop - and an admin can not assume full
> +swappability when overcommitting untrusted jobs.
> +
> +For trusted jobs, on the other hand, a combined counter is not an
> +intuitive userspace interface, and it flies in the face of the idea
> +that cgroup controllers should account and limit specific physical
> +resources.  Swap space is a resource like all others in the system,
> +and that's why unified hierarchy allows distributing it separately.
> -- 
> 2.1.4
> 
> --
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-- 
Michal Hocko
SUSE Labs