linux-kernel - Re: Bug report about KASLR and ZONE

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Message-ID: <20180716152410.GU17280@dhcp22.suse.cz>
Date:   Mon, 16 Jul 2018 17:24:10 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Baoquan He <bhe@...hat.com>
Cc:     Chao Fan <fanc.fnst@...fujitsu.com>,
        Dou Liyang <douly.fnst@...fujitsu.com>,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        yasu.isimatu@...il.com, keescook@...omium.org,
        indou.takao@...fujitsu.com, caoj.fnst@...fujitsu.com,
        vbabka@...e.cz, mgorman@...hsingularity.net
Subject: Re: Bug report about KASLR and ZONE_MOVABLE

On Mon 16-07-18 21:02:02, Baoquan He wrote:
> On 07/16/18 at 01:38pm, Michal Hocko wrote:
> > On Fri 13-07-18 07:52:40, Baoquan He wrote:
> > > Hi Michal,
> > > 
> > > On 07/12/18 at 02:32pm, Michal Hocko wrote:
> > [...]
> > > > I am not able to find the beginning of the email thread right now. Could
> > > > you summarize what is the actual problem please?
> > > 
> > > The bug is found on x86 now. 
> > > 
> > > When added "kernelcore=" or "movablecore=" into kernel command line,
> > > kernel memory is spread evenly among nodes. However, this is right when
> > > KASLR is not enabled, then kernel will be at 16M of place in x86 arch.
> > > If KASLR enabled, it could be put any place from 16M to 64T randomly.
> > >  
> > > Consider a scenario, we have 10 nodes, and each node has 20G memory, and
> > > we specify "kernelcore=50%", means each node will take 10G for
> > > kernelcore, 10G for movable area. But this doesn't take kernel position
> > > into consideration. E.g if kernel is put at 15G of 2nd node, namely
> > > node1. Then we think on node1 there's 10G for kernelcore, 10G for
> > > movable, in fact there's only 5G available for movable, just after
> > > kernel.
> > 
> > OK, I guess I see that part. But who is going to use movablecore along
> > with KASLR enabled? I mean do we really have to support those two
> > obscure command line parameters for KASLR?
> 
> Not very sure whether we have to support both of those to work with
> KASLR. Maybe it's time to make clear of it now.

Yes, I would really like to deprecate this. It is an ugly piece of code
and it's far from easily maintainable as well.

> For 'kernelcore=mirror', we have solved the conflict to make it work well
> with KASLR. For 'movable_node' conflict with KASLR, Chao is posting
> patches to fix it. As for 'kernelcore=' and 'movablecore=', 
> 
> 1) solve the conflict between them with KASLR in
>    find_zone_movable_pfns_for_nodes();
> 2) disable KASLR when 'kernelcore=' | 'movablecore=' is set;
> 3) disable 'kernelcore=' | 'movablecore=' when KASLR is enabled;
> 4) add note in doc to notice people to not add them at the same time;

I would simply warn that those kernel parameters are not supported
anymore. If somebody shows up with a valid usecase we can reconsider.

> 2) and 3) may need be fixed in arch/x86 code. As long as come to an
> agreement, any one is fine to me.
> > 
> > In fact I would be much more concerned about memory hotplug and
> > pre-defined movable nodes. Does the current KASLR code work in that
> > case?
> 
> As said above, kernelcore=mirror works well with KASLR now. Making
> 'movable_node' work with KASLR is in progress.

OK, thanks for the info.

-- 
Michal Hocko
SUSE Labs