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

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Message-ID: <20180716113845.GM17280@dhcp22.suse.cz>
Date:   Mon, 16 Jul 2018 13:38:45 +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 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?

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?
-- 
Michal Hocko
SUSE Labs