Hi,

On Tue, Oct 23, 2012 at 08:45:25PM -0700, Jonathan Nieder wrote:
> Dave Jones wrote:
> > Can the wheezy s390x kernel be saved as an NSS? If so, how?
> Based on [1], it looks like the answer is currently no, based on the
> following line in the kernel config file:
> 
>  # CONFIG_SHARED_KERNEL is not set
> 
> The description of that option says
> 
>       Select this option, if you want to share the text segment of the
>       Linux kernel between different VM guests. This reduces memory
>       usage with lots of guests but greatly increases kernel size.
> 
> You can build a custom kernel using that option if you wish.  It works
> like this[2]:
> 
>       $ apt-get source linux
>       # apt-get install build-essential fakeroot
>       # apt-get build-dep linux
>       $ cd linux-<version>
>       $ fakeroot debian/rules source
>       $ fakeroot make -f debian/rules.gen setup_s390_none_s390x

as we're talking about s390x here that would be setup_s390x_none_s390x.

>       $ cd debian/build/build_s390_none_s390x

Same here.

>       $ scripts/config --disable DEBUG_INFO
>       $ scripts/config --enable SHARED_KERNEL

That's «../source_none/scripts/config».

>       $ cd ../../..
>       $ fakeroot make -f debian/rules.gen binary-arch_s390_none_s390x

And again s390x_none_s390x instead of s390_none_s390x.

I just did the recompilation. AFAICS the kernel size is indeed increased quite
a bit. We currently have:

-rw-r--r-- root/root   6303232 2012-10-22 15:36 ./boot/vmlinuz-3.2.0-4-s390x

With SHARED_KERNEL on and DEBUG_INFO off as above:

-rw-r--r-- root/root   7945728 2012-10-25 21:53 ./boot/vmlinuz-3.2.0-4-s390x

So a NSS shareable kernel is 1.26 times larger than a plain one. And I fear
that the additional bits cannot be discarded at runtime neither, but I cannot
test this right now.

Interestingly enough the kernel is already in its plain form 2.23 times bigger
than an amd64 build of vmlinuz-3.2.0-3-amd64.

Kind regards
Philipp Kern

> [1] http://www.vm.ibm.com/linux/linuxnss.html
> [2] http://kernel-handbook.alioth.debian.org/ch-common-tasks.html


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