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Old 02-25-2010, 06:33 AM
Ian Campbell
 
Default Backporting HIGHPTE option patch from tip.git

Is it acceptable to backport
http://git.kernel.org/?p=linux/kernel/git/tip/linux-2.6-tip.git;a=commitdiff;h=a73a040e3b18521f83e726abacc 2f3992a148388 into the sid kernel?

This patch is a precursor to a Xen fix but since it touches generic code
I thought I would run it by the list first.

The patch adds a command line option to disable the effects of
CONFIG_HIGHPTE but does not change anything by default and is about as
safe as any patch could be.

The followup Xen patch is
http://git.kernel.org/?p=linux/kernel/git/jeremy/xen.git;a=commitdiff;h=9be6abceadd654543648b799ae4 30f30e09959ac
and disables HIGHPTE when running under Xen. It prevents crashes under
load due to HIGHPTE races which cannot be otherwise (easily) avoided
under Xen.

Ian.
--
Ian Campbell

It's NO USE ... I've gone to "CLUB MED"!!
 
Old 02-25-2010, 08:18 AM
Bastian Blank
 
Default Backporting HIGHPTE option patch from tip.git

On Thu, Feb 25, 2010 at 07:33:26AM +0000, Ian Campbell wrote:
> Is it acceptable to backport
> http://git.kernel.org/?p=linux/kernel/git/tip/linux-2.6-tip.git;a=commitdiff;h=a73a040e3b18521f83e726abacc 2f3992a148388 into the sid kernel?

Will it show up in Linus tree any time soon?

> The followup Xen patch is
> http://git.kernel.org/?p=linux/kernel/git/jeremy/xen.git;a=commitdiff;h=9be6abceadd654543648b799ae4 30f30e09959ac
> and disables HIGHPTE when running under Xen. It prevents crashes under
> load due to HIGHPTE races which cannot be otherwise (easily) avoided
> under Xen.

The commit message does not describe why, and it is not obvious to me.

I'm not sure, but I think this should be possible to do via stable after
both patches landed where they belong.

Bastian

--
I'm a soldier, not a diplomat. I can only tell the truth.
-- Kirk, "Errand of Mercy", stardate 3198.9
 
Old 02-25-2010, 01:44 PM
Ian Campbell
 
Default Backporting HIGHPTE option patch from tip.git

On Thu, 2010-02-25 at 10:18 +0100, Bastian Blank wrote:
> On Thu, Feb 25, 2010 at 07:33:26AM +0000, Ian Campbell wrote:
> > Is it acceptable to backport
> > http://git.kernel.org/?p=linux/kernel/git/tip/linux-2.6-tip.git;a=commitdiff;h=a73a040e3b18521f83e726abacc 2f3992a148388 into the sid kernel?
>
> Will it show up in Linus tree any time soon?

Hopefully in the 2.6.33 merge window but I will check with x86
maintainers.

> > The followup Xen patch is
> > http://git.kernel.org/?p=linux/kernel/git/jeremy/xen.git;a=commitdiff;h=9be6abceadd654543648b799ae4 30f30e09959ac
> > and disables HIGHPTE when running under Xen. It prevents crashes under
> > load due to HIGHPTE races which cannot be otherwise (easily) avoided
> > under Xen.
>
> The commit message does not describe why, and it is not obvious to me.

It's a race between pinning a page and kmap/kunmap. There's a path in
the pagefault code where the kernel breaks its own locking rules by
kmapping a high pte page without holding the pagetable lock which breaks
Xen's ability to track the pinned/unpinned state of the page. (I'm going
to ask Jeremy to update the commit to include this paragraph of
explanation).

Jeremy gave some explanation in:
http://lists.xensource.com/archives/html/xen-devel/2010-01/msg00921.html
(which I've blatantly stolen above)

The thread which led to the patch is "[Xen-devel] dom0 pvops crash" and
"[Xen-devel] Xen pvops kernel CONFIG_HIGHPTE race/crash" involving Pasi
Kärkkäinen. It's quite long but the final conclusion starts at:
http://lists.xensource.com/archives/html/xen-devel/2010-02/msg00403.html

That thread starts at:
http://lists.xensource.com/archives/html/xen-devel/2010-01/msg00902.html
and there is an older version of the same issue at:
http://lists.xensource.com/archives/html/xen-devel/2009-06/msg00519.html

> I'm not sure, but I think this should be possible to do via stable after
> both patches landed where they belong.

OK will go that route assuming I get confirmation that the patch is
destined for 2.3.32. If not then would a backport be appropriate?

Ian.
--
Ian Campbell
Current Noise: Autopsy - Pagan Saviour (Live '90)

Q: Why haven't you graduated yet?
A: Well, Dad, I could have finished years ago, but I wanted
my dissertation to rhyme.


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