linux-module-builder leads to huge store items

[Ludovic Courtès]

Hello Danny & Mathieu,

While looking for sources of contention on ci.guix.gnu.org, I found that
“linux-libre-module-builder” leads to huuuuge trees in the store;
performing deduplication on these trees takes ages, and more generally
these big trees look unreasaonble to me.

--8<---------------cut here---------------start------------->8---
$ find 
/gnu/store/s9md49vjambvprpp5ba3aqvhxilw0w7k-linux-libre-module-builder-5.8.8|wc 
-l
85604
$ du -hs 
/gnu/store/s9md49vjambvprpp5ba3aqvhxilw0w7k-linux-libre-module-builder-5.8.8
1.1G    
/gnu/store/s9md49vjambvprpp5ba3aqvhxilw0w7k-linux-libre-module-builder-5.8.8
--8<---------------cut here---------------end--------------->8---

Surely we don’t need that much just to build a kernel module.  :-)

(gnu build-system linux-module) has this:

  ;; TODO: Only preserve the minimum, i.e. [Kbuild], Kconfig,
  ;; scripts, include, ".config".

Which files exactly are needed?  Can we go ahead and implement this TODO?

I understand the need to “cut corners” while trying things out, but I’d
argue that this specific corner should have been restored before getting
into master.

Thanks,
Ludo’.