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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

> -     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.

Jan


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