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SubjectRe: [Xen-devel] Re: [PATCH] xen: p2m: correctly initialize partial p2m leave
On 01/20/2011 07:10 AM, 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.
> Nice catch..


>> 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).
>> Regards,
>> Stefan
>> >From 1e9c9514caf0399c88ae9288e6db8e3d1c4b4be5 Mon Sep 17 00:00:00 2001
>> From: Stefan Bader <>
>> 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.

Since the p2m page is just a normal page that happens to have been
initialized by the domain builder, I think we can just fill the tail of
the page with INVALID_P2M_ENTRY in place, rather than having to allocate
a new one.


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