linux-kernel - Re: [PATCH REPOST] pv-grub: Fix for incorrect dom->p2m

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Message-ID: <20110426143436.GB27417@router-fw-old.local.net-space.pl>
Date:	Tue, 26 Apr 2011 16:34:36 +0200
From:	Daniel Kiper <dkiper@...-space.pl>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Cc:	Daniel Kiper <dkiper@...-space.pl>,
	stefano.stabellini@...citrix.com, xen-devel@...ts.xensource.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH REPOST] pv-grub: Fix for incorrect dom->p2m_host[] list initialization

On Tue, Apr 26, 2011 at 09:42:42AM -0400, Konrad Rzeszutek Wilk wrote:
> On Fri, Apr 22, 2011 at 11:25:45PM +0200, Daniel Kiper wrote:
> > Added missed Signed-off-by line.
> >
> > After a lot of debugging and long reading of Linux Kernel and Xen code
> > finally I killed deeply hidden bug in pv-grub. Details below.
> > Additionally, I am CC'ing this e-mail to LKML because this issue
> > looks like Linux Kernel problem, however, it is not.
> >
> > This patch applies to Xen Ver. 4.0, Xen Ver. 4.1 and unstable tree.
> >
> > # HG changeset patch
> > # User dkiper@...-space.pl
> > # Date 1303474763 -7200
> > # Node ID b33bf24be129b7b9cd2248460beb1298088c6af5
> > # Parent  dbf2ddf652dc3dd927447e79ef4bc586de55d708
> > Introduction of Linux Kernel git commit ceefccc93932b920a8ec6f35f596db05202a12fe
> > (x86: default CONFIG_PHYSICAL_START and CONFIG_PHYSICAL_ALIGN to 16 MB) revealed
> > deeply hidden bug in pv-grub. During kernel load stage dom->p2m_host[] list has
> > been incorrectly initialized.
> >
> > At the beginning of kernel load stage dom->p2m_host[] list is populated with
> > current pfn->mfn layout. Later during memory allocation (memory is allocated
> > page by page in kexec_allocate()) page order is changed to establish linear
> > layout in new domain. It is done by exchanging subsequent mfns with newly
> > allocated mfns. dom->p2m_host[] list is indexed by currently requested pfn
> > (it is incremented from 0) and pfn of newly allocated paged. If pfn of newly
> > allocated page is less than currently requested pfn then relevant earlier
> > allocated mfn is overwritten which leads to domain crash later. This patch
> > fix that issue. If pfn of newly allocated page is less then currently
> > requested pfn then relevant pfn/mfn pair is properly calculated and usual
> > exchange occurs later.
>
> Nice! I presume this fixes the issue you had at the Xen Hack-O-Thon with
> your guest right?

Yes, it does. It was very difficult to discover because that
issue overlapped with other memory management issues which
were coming out last time. Currently, I am working on time
optimized version of that patch.

Daniel
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