linux-kernel - RE: [Xen-devel] [PATCH] xen: tmem: selfballooning should be enabled when xen tmem is enabled

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Date:	Wed, 21 Nov 2012 15:54:34 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Dan Magenheimer" <dan.magenheimer@...cle.com>
Cc:	<xen-devel@...ts.xen.org>, "Konrad Wilk" <konrad.wilk@...cle.com>,
	<linux-kernel@...r.kernel.org>
Subject: RE: [Xen-devel] [PATCH] xen: tmem: selfballooning should be
 enabled when xen tmem is enabled

>>> On 21.11.12 at 16:42, Dan Magenheimer <dan.magenheimer@...cle.com> wrote:
>>  From: Jan Beulich [mailto:JBeulich@...e.com]
>> Sent: Wednesday, November 21, 2012 1:21 AM
>> To: Dan Magenheimer
>> Cc: xen-devel@...ts.xen.org; Konrad Wilk; linux-kernel@...r.kernel.org 
>> Subject: Re: [Xen-devel] [PATCH] xen: tmem: selfballooning should be enabled 
> when xen tmem is enabled
>> 
>> >>> On 20.11.12 at 23:42, Dan Magenheimer <dan.magenheimer@...cle.com> wrote:
>> > Konrad: Any chance this can get in for the upcoming window?
>> > (Or is it enough of a bug fix that it can go in at an -rcN?)
>> >
>> > It was just pointed out to me that some kernels have
>> > cleancache and frontswap and xen_tmem enabled but NOT
>> > xen_selfballooning!  While this configuration should be
>> > possible, nearly all kernels that have CONFIG_XEN_TMEM=y should
>> > also have CONFIG_XEN_SELFBALLOONING=y, since Transcendent
>> > Memory (tmem) for Xen has very limited value without
>> > selfballooning.
>> >
>> > This is probably a result of a Kconfig mistake fixed I think
>> > by the patch below.  Note that the year-old Oracle UEK2 kernel
>> > distro has both CONFIG_XEN_TMEM and CONFIG_XEN_SELFBALLOONING
>> > enabled, as does a Fedora 17 kernel update (3.6.6-1.fc17), so
>> > the combination should be well tested.  Also, Xen tmem (and thus
>> > selfballooning) are currently only enabled when a kernel boot
>> > parameter is supplied so there is no runtime impact without
>> > that boot parameter.
>> >
>> > Signed-off-by: Dan Magenheimer <dan.magenheimer@...cle.com>
>> >
>> > diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
>> > index d4dffcd..b5f02f3 100644
>> > --- a/drivers/xen/Kconfig
>> > +++ b/drivers/xen/Kconfig
>> > @@ -10,9 +10,9 @@ config XEN_BALLOON
>> >  	  return unneeded memory to the system.
>> >
>> >  config XEN_SELFBALLOONING
>> > -	bool "Dynamically self-balloon kernel memory to target"
>> 
>> Why would you want to take away the configurability of this?
>> You wanting it always on in your use case doesn't mean everyone
>> agrees. This would be the right way only when the option being
>> off despite all its dependencies being enabled is actively wrong.
> 
> I'd like it to be configurable, but my config steps
> (e.g. yes "" | make oldconfig, after enabling CLEANCACHE and
> FRONTSWAP and XEN_TMEM) failed to enable it.  Removing the
> prompt is the only thing that worked.

If all of the dependencies are enabled, then I can't see why the
prompt wouldn't show up.

> P.S. I removed the CLEANCACHE dependency because that dependency
> is present for XEN_TMEM and I assumed dependencies are transitive.

That's correct.

Jan

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