linux-kernel - Re: [Xen-devel] [PATCH] x86/mm/pat: Fix BUG_ON in mmap

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Date:	Fri, 08 Apr 2016 10:56:53 -0600
From:	Toshi Kani <toshi.kani@....com>
To:	David Vrabel <david.vrabel@...rix.com>,
	Borislav Petkov <bp@...e.de>
Cc:	ying.huang@...ux.intel.com, x86@...nel.org,
	linux-kernel@...r.kernel.org, hpa@...or.com,
	xen-devel@...ts.xenproject.org, tglx@...utronix.de,
	mingo@...nel.org
Subject: Re: [Xen-devel] [PATCH] x86/mm/pat: Fix BUG_ON in mmap_mem on
 QEMU/i386

On Fri, 2016-04-08 at 18:00 +0100, David Vrabel wrote:
> On 08/04/16 17:34, Toshi Kani wrote:
> > 
> > On Tue, 2016-04-05 at 09:24 -0600, Toshi Kani wrote:
> > > 
> > > +xen-devl
> > > 
> > > On Tue, 2016-04-05 at 13:09 +0200, Borislav Petkov wrote:
> > > > 
> > > > On Fri, Apr 01, 2016 at 04:19:45PM -0600, Toshi Kani wrote:
> > > > > 
> >  :
> > > > > 
> > > > > When the system does not have much memory, 'high_memory' points
> > > > > to What does "much memory" mean, exactly?
> > >
> > > I meant to say when a 32-bit system does not have ZONE_HIGHMEM,
> > > __pa(high_memory) points to the maximum memory address + 1.
> > > 
> > > I will remove this sentence since it is irrelevant to this
> > > BUG_ON.  Even if a 32-bit system does have ZONE_HIGHMEM,
> > > slow_virt_to_phys() still returns 0 for high_memory because it is set
> > > to the maximum direct mapped address + 1 in this case.  This address
> > > is not covered by page table, either.
> > > 
> > > But this made me realized that this high_memory check can be harmful
> > > in such case, ie. __pa(high_memory) is not the maximum memory address
> > > when ZONE_HIGHMEM is present.
> > > 
> > > I assume when this code block was originally added, legacy systems
> > > without MTRRs did not have ZONE_HIGHMEM.  However, MTRRs are also
> > > disabled on Xen. Reactivating this code may cause an issue on Xen 32-
> > > bit guests with ZONE_HIGHMEM.
> > > 
> > > Question to Xen folks: Does Xen support 32-bit guests with
> > > ZONE_HIGHMEM?
> > > 
> > > If yes, a safer fix may be to remove this code block since it was
> > > deadcode anyway...
> >
> > I have not heard a confirmation from Xen folks, but I believe
> > ZONE_HIGHMEM is supported on 32-bit Xen guests.  So, unless someone has
> > an objection, I am going to remove this code block in the next version
> > of this patch.
>
> 32-bit Xen guests have highmem, yes.

Thanks David!
-Toshi