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From: Michal Hocko <mhocko@kernel.org>
To: Minchan Kim <minchan@kernel.org>
Cc: Jann Horn <jannh@google.com>, Linux-MM <linux-mm@kvack.org>,
	kernel list <linux-kernel@vger.kernel.org>,
	Daniel Colascione <dancol@google.com>,
	Dave Hansen <dave.hansen@intel.com>,
	"Joel Fernandes (Google)" <joel@joelfernandes.org>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: interaction of MADV_PAGEOUT with CoW anonymous mappings?
Date: Fri, 13 Mar 2020 09:05:46 +0100
Message-ID: <20200313080546.GA21007@dhcp22.suse.cz> (raw)
In-Reply-To: <20200313020851.GD68817@google.com>

On Thu 12-03-20 19:08:51, Minchan Kim wrote:
> On Thu, Mar 12, 2020 at 09:41:55PM +0100, Michal Hocko wrote:
> > On Thu 12-03-20 13:16:02, Minchan Kim wrote:
> > > On Thu, Mar 12, 2020 at 09:22:48AM +0100, Michal Hocko wrote:
> > [...]
> > > > From eca97990372679c097a88164ff4b3d7879b0e127 Mon Sep 17 00:00:00 2001
> > > > From: Michal Hocko <mhocko@suse.com>
> > > > Date: Thu, 12 Mar 2020 09:04:35 +0100
> > > > Subject: [PATCH] mm: do not allow MADV_PAGEOUT for CoW pages
> > > > 
> > > > Jann has brought up a very interesting point [1]. While shared pages are
> > > > excluded from MADV_PAGEOUT normally, CoW pages can be easily reclaimed
> > > > that way. This can lead to all sorts of hard to debug problems. E.g.
> > > > performance problems outlined by Daniel [2]. There are runtime
> > > > environments where there is a substantial memory shared among security
> > > > domains via CoW memory and a easy to reclaim way of that memory, which
> > > > MADV_{COLD,PAGEOUT} offers, can lead to either performance degradation
> > > > in for the parent process which might be more privileged or even open
> > > > side channel attacks. The feasibility of the later is not really clear
> > > 
> > > I am not sure it's a good idea to mention performance stuff because
> > > it's rather arguble. You and Johannes already pointed it out when I sbumit
> > > early draft which had shared page filtering out logic due to performance
> > > reason. You guys suggested the shared pages has higher chance to be touched
> > > so that if it's really hot pages, that whould keep in the memory. I agree.
> > 
> > Yes, the hot memory is likely to be referenced but the point was an
> > unexpected latency because of the major fault. I have to say that I have
> 
> I don't understand your point here. If it's likely to be referenced
> among several processes, it doesn't have the major fault latency.
> What's your point here?

a) the particular CoW page might be cold enough to be reclaimed and b)
nothing really prevents the MADV_PAGEOUT to be called faster than the
reference bit being readded.

> > underestimated the issue because I was not aware of runtimes mentioned
> > in the referenced links. Essentially a lot of CoW memory shared over
> > security domains.
> 
> I tend to agree about security part in the description, but still don't
> agree with performance concern in the description so I'd like to remove
> it in the description. Current situation is caused by security concern
> unfortunately, not performance reason.

Well, I have to admit that I haven't seen any actual numbers here but
considering zygote like workload I would rather not even risk it. Even
if the risk is theoretical I would rather put the restriction and
mention it in the changelog. If somebody would like to drop this
restriction it is at least more clear what to test for.

-- 
Michal Hocko
SUSE Labs


  reply index

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-10 18:08 Jann Horn
2020-03-10 18:48 ` Michal Hocko
2020-03-10 19:11   ` Jann Horn
2020-03-10 21:09     ` Michal Hocko
2020-03-10 22:48       ` Dave Hansen
2020-03-11  8:45         ` Michal Hocko
2020-03-11 22:02           ` Minchan Kim
2020-03-11 23:53           ` Shakeel Butt
2020-03-12  0:18             ` Minchan Kim
2020-03-12  2:03               ` Daniel Colascione
2020-03-12 15:15                 ` Shakeel Butt
2020-03-10 20:19   ` Daniel Colascione
2020-03-10 21:40     ` Jann Horn
2020-03-10 21:52       ` Daniel Colascione
2020-03-10 22:14 ` Minchan Kim
2020-03-12  8:22 ` Michal Hocko
2020-03-12 15:40   ` Vlastimil Babka
2020-03-12 20:16   ` Minchan Kim
2020-03-12 20:26     ` Dave Hansen
2020-03-12 20:41     ` Michal Hocko
2020-03-13  2:08       ` Minchan Kim
2020-03-13  8:05         ` Michal Hocko [this message]
2020-03-13 20:59           ` Minchan Kim
2020-03-16  9:20             ` Michal Hocko
2020-03-17  1:43               ` Minchan Kim
2020-03-17  7:12                 ` Michal Hocko
2020-03-17 15:00                   ` Minchan Kim
2020-03-17 15:58                     ` Michal Hocko
2020-03-17 17:20                       ` Minchan Kim
2020-03-12 21:41     ` Dave Hansen
2020-03-13  2:00       ` Minchan Kim
2020-03-13 16:59         ` Dave Hansen
2020-03-13 21:13           ` Minchan Kim
2020-03-12 23:29     ` Jann Horn

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