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Re: [PATCH] Re: tmem on 4.1 (was [Xen-devel] Re: Freeze schedule)

To: "Tim Deegan" <Tim.Deegan@xxxxxxxxxx>, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx>, "Keir Fraser" <keir@xxxxxxx>
Subject: Re: [PATCH] Re: tmem on 4.1 (was [Xen-devel] Re: Freeze schedule)
From: "Jan Beulich" <JBeulich@xxxxxxxxxx>
Date: Thu, 20 Jan 2011 10:04:12 +0000
Cc: xen-devel@xxxxxxxxxxxxxxxxxxx, Ian Jackson <Ian.Jackson@xxxxxxxxxxxxx>
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>>> On 20.01.11 at 08:17, Keir Fraser <keir@xxxxxxx> wrote:
> On 19/01/2011 21:38, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:
>> Just to check again, has anyone actually seen a problem with
>> tmem enabled by default recently?  I agree that there is still
>> theoretically a problem, but there is the same problem with
>> normal guests doing lots of ballooning as well.  Also, note
>> that even if tmem defaults to enabled, the problem is impossible
>> unless a guest enables tmem (or, in the case of SuSE, dom0).
>> And even if a guest does enable tmem, the problem manifested
>> largely due to shadow pages using order>0 (now fixed?)...
>> failure on domain creation can happen for many reasons and
>> is much less of an issue, true?
>> Feel free to shoot me down with more evidence, but I have
>> to at least provide token resistance to this patch.  Distros
>> might certainly choose to disable it to avoid any risk at
>> all, but turning it off anymore seems overkill for xen.org
>> open source Xen IMHO.
> Tbh I was wondering whether anyone is really using it in earnest. No
> upstream kernels support it? If noone's using it, who really cares whether
> it's enabled or not, apart from its author.

As Dan wrote, all our kernels 2.6.31 and newer use it if the
hypervisor has it enabled, which was also the reason why
we noticed the problems it being enabled by default caused
during the 4.0 release cycle.

Since the 2.6.32 XCP kernel is derived from ours and
nothing in the patch queue there removes the tmem bits
afaict, it ought to be affected as much.


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