linux-kernel - Re: [PATCH] mm: memcontrol: reclaim when shrinking memory.high below usage

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: <20160316054157.GB11006@cmpxchg.org>
Date:	Tue, 15 Mar 2016 22:41:57 -0700
From:	Johannes Weiner <hannes@...xchg.org>
To:	Vladimir Davydov <vdavydov@...tuozzo.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Michal Hocko <mhocko@...e.cz>, linux-mm@...ck.org,
	cgroups@...r.kernel.org, linux-kernel@...r.kernel.org,
	kernel-team@...com
Subject: Re: [PATCH] mm: memcontrol: reclaim when shrinking memory.high below
 usage

On Fri, Mar 11, 2016 at 11:34:40AM +0300, Vladimir Davydov wrote:
> On Thu, Mar 10, 2016 at 03:50:13PM -0500, Johannes Weiner wrote:
> > When setting memory.high below usage, nothing happens until the next
> > charge comes along, and then it will only reclaim its own charge and
> > not the now potentially huge excess of the new memory.high. This can
> > cause groups to stay in excess of their memory.high indefinitely.
> > 
> > To fix that, when shrinking memory.high, kick off a reclaim cycle that
> > goes after the delta.
> 
> I agree that we should reclaim the high excess, but I don't think it's a
> good idea to do it synchronously. Currently, memory.low and memory.high
> knobs can be easily used by a single-threaded load manager implemented
> in userspace, because it doesn't need to care about potential stalls
> caused by writes to these files. After this change it might happen that
> a write to memory.high would take long, seconds perhaps, so in order to
> react quickly to changes in other cgroups, a load manager would have to
> spawn a thread per each write to memory.high, which would complicate its
> implementation significantly.

While I do expect memory.high to be adjusted every once in a while, I
can't see anybody doing it by a significant fraction of the cgroup
every couple of seconds - or tighter than the workingset; and dropping
use-once cache is cheap. What kind of usecase would that be?

But even if we're wrong about it and this becomes a scalability issue,
the knob - even when reclaiming synchroneously - makes no guarantees
about the target being met once the write finishes. It's a best effort
mechanism. What would break if we made it async later on?