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[37.188.247.35]) by smtp.gmail.com with ESMTPSA id 133sm16541367wmd.5.2020.03.13.01.05.47 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 13 Mar 2020 01:05:49 -0700 (PDT) Date: Fri, 13 Mar 2020 09:05:46 +0100 From: Michal Hocko To: Minchan Kim Cc: Jann Horn , Linux-MM , kernel list , Daniel Colascione , Dave Hansen , "Joel Fernandes (Google)" , Andrew Morton Subject: Re: interaction of MADV_PAGEOUT with CoW anonymous mappings? Message-ID: <20200313080546.GA21007@dhcp22.suse.cz> References: <20200312082248.GS23944@dhcp22.suse.cz> <20200312201602.GA68817@google.com> <20200312204155.GE23944@dhcp22.suse.cz> <20200313020851.GD68817@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200313020851.GD68817@google.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > > > > 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