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RE: [Xen-devel] tmem and construct_dom0 memory allocation race

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>, Dulloor <dulloor@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: RE: [Xen-devel] tmem and construct_dom0 memory allocation race
From: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Date: Mon, 21 Jun 2010 08:35:58 -0700 (PDT)
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Hi Dulloor --

Intel had previously reported a failure for 2.6.18-xen
dom0+tmem with dom0_mem unspecified.  I'm not sure if
this is the same bug or not.

The latest versions of the Linux-side tmem patch disable
tmem by default (in Linux, not Xen!) and require a kernel
boot option to turn it on.  Since dom0 is special and
I've done very little testing with dom0 using tmem (as
tmem is primarily used with guests), I think the correct
(at least short-term) fix for this will be to not enable
tmem for dom0 when dom0_mem is unspecified.  I haven't
gotten around to updating 2.6.18-xen for awhile, assuming
it is increasingly rarely used (except in products where
dom0_mem is always specified).

I'll try to submit a major update to the Linux-side
tmem patch for the 2.6.18-xen tree soon so at least
it is consistent with other Linux-side Xen patches.


> -----Original Message-----
> From: Keir Fraser [mailto:keir.fraser@xxxxxxxxxxxxx]
> Sent: Saturday, June 19, 2010 1:27 AM
> To: Dulloor; xen-devel@xxxxxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] tmem and construct_dom0 memory allocation race
> On 19/06/2010 00:10, "Dulloor" <dulloor@xxxxxxxxx> wrote:
> > Following is the sequence :
> > - init_tmem allocates a set of pages and sets up dstmem and workmem
> to
> > alloc pages in MP case (with cpu notifiers)
> > - construct_dom0 estimates nr_pages by calling avail_domheap_pages
> > - On other CPUs, tmem cpu_notifier gets called and allocates pages
> > from domheap, making the construct_dom0's estimate stale.
> > - construct_dom0 fails
> >
> > tmem=off or dom0_mem=xxx both solve the problem for now.
> Xen boot is pretty serialised. In particular SMP boot, and all cpu
> notification calls, should be done before dom0 is constructed. So, have
> you
> actually seen this race?
>  -- Keir
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