Re: [Xen-devel] [PATCH] xen: p2m: correctly initialize partial p2m leave

[Stefan Bader <stefan.bader@canonical.com>]

On 01/21/2011 03:26 PM, Konrad Rzeszutek Wilk wrote:
> On Thu, Jan 20, 2011 at 03:38:23PM +0100, Stefan Bader wrote:
>> There have been changes and code been moved around, so this is just a quick
>> rebase of the change I tested on a 2.6.37 based kernel. The basic problem seem
>> still valid, though.
> 
> Yup.
>>
>> Initially I thought of adding a cc to stable into the s-o-b, but the patch needs
>> to be adapted anyway (I can supply that version if the way I fixed the issue
>> looks ok).
> 
> OK, let me send this upstream to Linus for adaption. I fixed one compile warning
> but otherwsie it is the same. Look below for details.
> 
> And when that is done I would appreciate you sending a copy to stable.
> 
Sure, will do that as soon as the change hits 2.6.38 upstream.

-Stefan

>>
>> Regards,
>> Stefan
> 
>> >From 1e9c9514caf0399c88ae9288e6db8e3d1c4b4be5 Mon Sep 17 00:00:00 2001
>> From: Stefan Bader <stefan.bader@canonical.com>
>> Date: Thu, 20 Jan 2011 11:37:43 +0100
>> Subject: [PATCH] xen: p2m: correctly initialize partial p2m leave
>>
>> After changing the p2m mapping to a tree by
>>
>>   commit 58e05027b530ff081ecea68e38de8d59db8f87e0
>>     xen: convert p2m to a 3 level tree
>>
>> and trying to boot a DomU with 615MB of memory, the following crash was
>> observed in the dump:
>>
>> kernel direct mapping tables up to 26f00000 @ 1ec4000-1fff000
>> BUG: unable to handle kernel NULL pointer dereference at (null)
>> IP: [<c0107397>] xen_set_pte+0x27/0x60
>> *pdpt = 0000000000000000 *pde = 0000000000000000
>>
>> Adding further debug statements showed that when trying to set up
>> pfn=0x26700 the returned mapping was invalid.
>>
>> pfn=0x266ff calling set_pte(0xc1fe77f8, 0x6b3003)
>> pfn=0x26700 calling set_pte(0xc1fe7800, 0x3)
>>
>> Although the last_pfn obtained from the startup info is 0x26700, which
>> should in turn not be hit, the additional 8MB which are added as extra
>> memory normally seem to be ok. This lead to looking into the initial
>> p2m tree construction, which uses the smaller value and assuming that
>> there is other code handling the extra memory.
>>
>> When the p2m tree is set up, the leaves are directly pointed to the
>> array which the domain builder set up. But if the mapping is not on a
>> boundary that fits into one p2m page, this will result in the last leaf
>> being only partially valid. And as the invalid entries are not
>> initialized in that case, things go badly wrong.
>>
>> I am trying to fix that by checking whether the current leaf is a
>> complete map and if not, allocate a completely new page and copy only
>> the valid pointers there. This may not be the most efficient or elegant
>> solution, but at least it seems to allow me booting DomUs with memory
>> assignments all over the range.
>>
>> BugLink: http://bugs.launchpad.net/bugs/686692
>>
>> Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
>> ---
>>  arch/x86/xen/p2m.c |   20 +++++++++++++++++++-
>>  1 files changed, 19 insertions(+), 1 deletions(-)
>>
>> diff --git a/arch/x86/xen/p2m.c b/arch/x86/xen/p2m.c
>> index 8f2251d..c9307ec 100644
>> --- a/arch/x86/xen/p2m.c
>> +++ b/arch/x86/xen/p2m.c
>> @@ -237,7 +237,25 @@ void __init xen_build_dynamic_phys_to_machine(void)
>>  			p2m_top[topidx] = mid;
>>  		}
>>  
>> -		p2m_top[topidx][mididx] = &mfn_list[pfn];
>> +		/*
>> +		 * As long as the mfn_list has enough entries to completely
>> +		 * fill a p2m page, pointing into the array is ok. But if
>> +		 * not the entries beyond the last pfn will be undefined.
>> +		 * And guessing that the 'what-ever-there-is' does not take it
>> +		 * too kindly when changing it to invalid markers, a new page
>> +		 * is allocated, initialized and filled with the valid part.
>> +		 */
>> +		if (unlikely(pfn + P2M_PER_PAGE > max_pfn)) {
>> +			unsigned long p2midx;
>> +			unsigned long **p2m = extend_brk(PAGE_SIZE, PAGE_SIZE);
> 
> 			unsigned long  *p2m.
> 
> 
>> +			p2m_init(p2m);
>> +
>> +			for (p2midx = 0; pfn + p2midx < max_pfn; p2midx++) {
>> +				p2m[p2midx] = mfn_list[pfn + p2midx];
>> +			}
>> +			p2m_top[topidx][mididx] = p2m;
>> +		} else
>> +			p2m_top[topidx][mididx] = &mfn_list[pfn];
>>  	}
>>  
>>  	m2p_override_init();
>> -- 
>> 1.7.0.4
>>
> 
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