Re: [Xen-devel] [PATCH] xen: tmem: selfballooning should be enabled when xen tmem is enabled

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Wednesday, November 21, 2012 1:21 AM
> To: Dan Magenheimer
> Cc: xen-devel@xxxxxxxxxxxxx; Konrad Wilk; linux-kernel@xxxxxxxxxxxxxxx
> 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@xxxxxxxxxx> 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@xxxxxxxxxx>
> >
> > 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.
 
> > -   depends on XEN && XEN_BALLOON && CLEANCACHE && SWAP && XEN_TMEM
> > -   default n
> > +   bool
> > +   depends on XEN_BALLOON && SWAP
> > +   default y if XEN_TMEM
> 
> Changing the default, otoh, is certainly acceptable. However, this
> should imo be (assuming that you dropped the CLEANCACHE
> dependency for an unrelated [and unexplained] reason),
> 
>       depends on XEN_BALLOON && SWAP && XEN_TMEM
>       default XEN_TMEM
> 
> i.e. the default selection can be simplified, but if you indeed
> have a good reason to drop the prompt, the
> dependencies should continue to include the symbol referenced
> by the default directive, as otherwise you may end up with a
> .config pointlessly having
> 
> # CONFIG_XEN_SELFBALLOONING is disabled. This is particularly
> annoying when subsequently this gets a prompt re-added, since
> at that point a "make oldconfig" won't ask for the item to get
> possibly enabled as there is a value known for it already.
> 
> >     help
> >       Self-ballooning dynamically balloons available kernel memory driven
> >       by the current usage of anonymous memory ("committed AS") and
> 
> If you take away the prompt, keeping the help text isn't useful
> either.

Thanks for the feedback.  I'm an idiot with Kconfig and have
to play with it until it works as I expect.  Since Konrad
isn't going to accept it for the upcoming window anyway,
I'll play with it some more, but if the answer is obvious,
please spoon feed it to me. :-)

Dan

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

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