linux-kernel - Re: [PATCH] mm: memcontrol: fix cgroup creation failure after many small jobs

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20160617164043.GA10485@cmpxchg.org>
Date:	Fri, 17 Jun 2016 12:40:43 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Vladimir Davydov <vdavydov@...tuozzo.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Michal Hocko <mhocko@...e.cz>,
	Li Zefan <lizefan@...wei.com>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
	kernel-team@...com
Subject: Re: [PATCH] mm: memcontrol: fix cgroup creation failure after many
 small jobs

On Fri, Jun 17, 2016 at 12:06:55PM +0300, Vladimir Davydov wrote:
> On Wed, Jun 15, 2016 at 11:42:44PM -0400, Johannes Weiner wrote:
> > The memory controller has quite a bit of state that usually outlives
> > the cgroup and pins its CSS until said state disappears. At the same
> > time it imposes a 16-bit limit on the CSS ID space to economically
> > store IDs in the wild. Consequently, when we use cgroups to contain
> > frequent but small and short-lived jobs that leave behind some page
> > cache, we quickly run into the 64k limitations of outstanding CSSs.
> > Creating a new cgroup fails with -ENOSPC while there are only a few,
> > or even no user-visible cgroups in existence.
> > 
> > Although pinning CSSs past cgroup removal is common, there are only
> > two instances that actually need a CSS ID after a cgroup is deleted:
> > cache shadow entries and swapout records.
> > 
> > Cache shadow entries reference the ID weakly and can deal with the CSS
> > having disappeared when it's looked up later. They pose no hurdle.
> > 
> > Swap-out records do need to pin the css to hierarchically attribute
> > swapins after the cgroup has been deleted; though the only pages that
> > remain swapped out after a process exits are tmpfs/shmem pages. Those
> > references are under the user's control and thus manageable.
> > 
> > This patch introduces a private 16bit memcg ID and switches swap and
> > cache shadow entries over to using that. It then decouples the CSS
> > lifetime from the CSS ID lifetime, such that a CSS ID can be recycled
> > when the CSS is only pinned by common objects that don't need an ID.
> 
> There's already id which is only used for online memory cgroups - it's
> kmemcg_id. May be, instead of introducing one more idr, we could name it
> generically and reuse it for shadow entries?

Good point. But it seems mem_cgroup_idr is more generic, it makes
sense to switch slab accounting over to that. I'll look into that, but
as a refactoring patch on top of this fix.

> Regarding swap entries, would it really make much difference if we used
> 4 bytes per swap page instead of 2? For a 100 GB swap it'd increase
> overhead from 50 MB up to 100 MB, which still doesn't seem too much IMO,
> so may be just use plain unrestricted css->id for swap entries?

Yes and no. I agree that the increased consumption wouldn't be too
crazy, but if we have to maintain a 16-bit ID anyway, we might as well
use it for swap too to save that space. I don't think tmpfs and shmem
pins past offlining will be common enough to significantly eat into
the ID space of online cgroups.