Re: [PATCH] xen: p2m: correctly initialize partial p2m leave

Konrad Rzeszutek Wilk
2011-01-20T15:11:05Z

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 <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);
> + 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
>