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Date:   Mon, 19 Mar 2018 15:24:46 +0800
From:   Baoquan He <bhe@redhat.com>
To:     Chao Fan <fanc.fnst@cn.fujitsu.com>
Cc:     Ingo Molnar <mingo@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, x86@kernel.org, hpa@zytor.com,
        tglx@linutronix.de, mingo@redhat.com, keescook@chromium.org,
        yasu.isimatu@gmail.com, indou.takao@jp.fujitsu.com,
        lcapitulino@redhat.com
Subject: Re: [PATCH v9 0/5] x86/KASLR: Add parameter
 kaslr_boot_mem=nn[KMG]@ss[KMG]
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On 03/12/18 at 08:04pm, Chao Fan wrote:
> On Mon, Mar 12, 2018 at 11:57:27AM +0100, Ingo Molnar wrote:
> >
> >* Baoquan He <bhe@redhat.com> wrote:
> >
> >> Hi Ingo,
> >> 
> >> On 03/12/18 at 10:35am, Ingo Molnar wrote:
> >> > 
> >> > * Chao Fan <fanc.fnst@cn.fujitsu.com> wrote:
> >> > 
> >> > > Long time no reply, rebase the patchset, change the parameter name
> >> > > from 'kaslr_mem' to 'kaslr_boot_mem'. There's no more code change.
> >> > > 
> >> > > ***Background:
> >> > > People reported that kaslr may randomly chooses some positions
> >> > > which are located in movable memory regions. This will break memory
> >> > > hotplug feature.
> >> > 
> >> > [...]
> >> > 
> >> > > ***Solutions:
> >> > > Introduce a new kernel parameter 'kaslr_boot_mem=nn@ss' to let users to
> >> > > specify the memory regions where kernel can be allowed to randomize
> >> > > safely.
> >> > 
> >> > Manual solutions like that are pretty suboptimal to users, aren't they?
> >> > 
> >> > In what way does memory hotplug feature 'break'? Does it crash or misbehave? Or 
> >> > simply does it not allow the movement of the affected memory region, while still 
> >> > allowing the rest to be moved?
> >> 
> >> AFAIT, if kernel is randomized into the movable memory region, the
> >> affected memory region can not be hot added/removed since it has kernel
> >> data. Surely, the system can still work, the unaffected part still can
> >> be moved. Still it will cause regression on memory hotplug.
> >> 
> >> Mainly we parse SRAT table to get the ranges of memory provided by
> >> hot-added memory devices in initmem_init(), that's very late. During boot,
> >> we don't know it. Chao ever posted patches to grab SRAT at decompressing
> >> stage, the code is very complicated and not elegant, ACPI maintainer
> >> NACKed that.

Hi Chao,

Seems Ingo prefers the handling in kaslr boot code. Maybe you can try
to optimize and split your below patch and post anouther round?

I will see how to sove the hugepage in boot/compressed/kaslr.c . 

Thanks
Baoquan

> 
> Thanks for Ingo's suggestion and Baoquan's explaination.
> 
> Yes, I did ever try to dig SRAT table in boot period in early RFC PATCH:
> https://lkml.org/lkml/2017/9/3/77
> But the change is too huge so made this patchset to avoid this bug in a
> small change, which will not make the code looks messy.
> 
> Thanks,
> Chao Fan
> 
> >
> >So there's apparently a mis-design here:
> >
> > - KASLR needs to be done very early on during bootup: - it's not realistic to 
> >   expect KASLR to be done with a booted up kernel, because pointers to various 
> >   KASLR-ed objects are already widely spread out in memory.
> >
> > - But for some unfathomable reason the memory hotplug attribute of memory
> >   regions is not part of the regular memory map but part of late-init ACPI data
> >   structures.
> >
> >The right solution would be _not_ to fudge the KASLR location, but to provide the 
> >memory hotplug information to early code, preferably via the primary memory map. 
> >KASLR can then make use of it and avoid those regions, just like it avoids other 
> >memory regions already.
> >
> >In addition to that hardware makers (including virtualized hardware) should also 
> >fix their systems to provide memory hotplug information to early code.
> >
> >Thanks,
> >
> >	Ingo
> >
> >
> 
>