linux-kernel - Re: [PATCH v2 RESEND 2/2] x86/mm/KASLR: Do not adapt the size of the direct mapping section for SGI UV system

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Date:   Thu, 28 Sep 2017 16:31:12 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Ingo Molnar <mingo@...nel.org>, rja@....com, frank.ramsay@....com
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, mingo@...hat.com,
        tglx@...utronix.de, hpa@...or.com, thgarnie@...gle.com,
        keescook@...omium.org, akpm@...ux-foundation.org,
        yamada.masahiro@...ionext.com
Subject: Re: [PATCH v2 RESEND 2/2] x86/mm/KASLR: Do not adapt the size of the
 direct mapping section for SGI UV system

Hi Ingo,

On 09/28/17 at 09:56am, Ingo Molnar wrote:
> > diff --git a/arch/x86/mm/kaslr.c b/arch/x86/mm/kaslr.c
> > index af599167fe3c..4d68c08df82d 100644
> > --- a/arch/x86/mm/kaslr.c
> > +++ b/arch/x86/mm/kaslr.c
> > @@ -27,6 +27,7 @@
> >  #include <asm/pgtable.h>
> >  #include <asm/setup.h>
> >  #include <asm/kaslr.h>
> > +#include <asm/uv/uv.h>
> >  
> >  #include "mm_internal.h"
> >  
> > @@ -123,7 +124,7 @@ void __init kernel_randomize_memory(void)
> >  		CONFIG_RANDOMIZE_MEMORY_PHYSICAL_PADDING;
> >  
> >  	/* Adapt phyiscal memory region size based on available memory */
> > -	if (memory_tb < kaslr_regions[0].size_tb)
> > +	if (memory_tb < kaslr_regions[0].size_tb && !is_early_uv_system())
> >  		kaslr_regions[0].size_tb = memory_tb;
> This is really an ugly hack. Is kaslr_regions[] incorrect? If so then it should be 
> corrected instead of uglifying the code that uses it...

Thanks for looking into this!

If on SGI UV system, the kaslr_regions[0].size_tb, namely the size of
the direct mapping section, is incorrect. 

Its direct mapping size includes two parts:
#1 RAM size of system
#2 MMIOH region size which only SGI UV system has.

However, the #2 can only be got till uv_system_init() is called in
native_smp_prepare_cpus(). That is too late for mm KASLR calculation.
That's why I made this hack.

I checked uv_system_init() code, seems not easy to know the size of
MMIOH region before or inside kernel_randomize_memory(). I have CCed UV
devel experts, not sure if they have any idea about this. Otherwise,
this patch could be the only way I can think of.

Hi Mike and Russ,

Is there any chance we can get the size of MMIOH region before mm KASLR
code, namely before we call kernel_randomize_memory()?

Thanks
Baoquan