linux-kernel - Re: [PATCH] xen: Fix Xen domU boot with batched mprotect

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Date:	Mon, 27 Oct 2008 14:11:43 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Chris Lalancette <clalance@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] xen: Fix Xen domU boot with batched mprotect


* Jeremy Fitzhardinge <jeremy@...p.org> wrote:

> From: Chris Lalancette <clalance@...hat.com>
>
> Recent i686 2.6.27 kernels with a certain amount of memory (between 
> 736 and 855MB) have a problem booting under a hypervisor that 
> supports batched mprotect (this includes the RHEL-5 Xen hypervisor 
> as well as any 3.3 or later Xen hypervisor).  The problem ends up 
> being that xen_ptep_modify_prot_commit() is using virt_to_machine to 
> calculate which pfn to update.  However, this only works for pages 
> that are in the p2m list, and the pages coming from 
> change_pte_range() in mm/mprotect.c are kmap_atomic pages.  Because 
> of this, we can run into the situation where the lookup in the p2m 
> table returns an INVALID_MFN, which we then try to pass to the 
> hypervisor, which then (correctly) denies the request to a totally 
> bogus pfn.
>
> The right thing to do is to use arbitrary_virt_to_machine, so that 
> we can be sure we are modifying the right pfn.  This unfortunately 
> introduces a performance penalty because of a full page-table-walk, 
> but we can avoid that penalty for pages in the p2m list by checking 
> if virt_addr_valid is true, and if so, just doing the lookup in the 
> p2m table.
>
> The attached patch implements this, and allows my 2.6.27 i686 based 
> guest with 768MB of memory to boot on a RHEL-5 hypervisor again.  
> Thanks to Jeremy for the suggestions about how to fix this 
> particular issue.
>
> Signed-off-by: Chris Lalancette <clalance@...hat.com>
> Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>
> ---
> arch/x86/xen/mmu.c |   18 ++++++++++++++----
> 1 file changed, 14 insertions(+), 4 deletions(-)

applied to tip/x86/urgent, thanks!

	Ingo
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