kbuild: handle module compression while running 'make modules_install'.

Since module-init-tools (gzip) and kmod (gzip and xz) support compressed
modules, it could be useful to include a support for compressing modules
right after having them installed. Doing this in kbuild instead of per
distro can permit to make this kind of usage more generic.

This patch add a Kconfig entry to "Enable loadable module support" menu
and let you choose to compress using gzip (default) or xz.

Both gzip and xz does not used any extra -[1-9] option since Andi Kleen
and Rusty Russell prove no gain is made using them. gzip is called with -n
argument to avoid storing original filename inside compressed file, that
way we can save some more bytes.

On a v3.16 kernel, 'make allmodconfig' generated 4680 modules for a
total of 378MB (no strip, no sign, no compress), the following table
shows observed disk space gain based on the allmodconfig .config :

       |           time                |
       +-------------+-----------------+
       | manual .ko  |       make      | size | percent
       | compression | modules_install |      | gain
       +-------------+-----------------+------+--------
  -    |             |     18.61s      | 378M |
  GZIP |   3m16s     |     3m37s       | 102M | 73.41%
  XZ   |   5m22s     |     5m39s       |  77M | 79.83%

The gain for restricted environnement seems to be interesting while
uncompress can be time consuming but happens only while loading a module,
that is generally done only once.

This is fully compatible with signed modules while the signed module is
compressed. module-init-tools or kmod handles decompression
and provide to other layer the uncompressed but signed payload.

Reviewed-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Bertrand Jacquin <beber@meleeweb.net>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 43 insertions, 0 deletions

diff --git a/init/Kconfig b/init/Kconfig
index e84c642..4980925 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -1906,6 +1906,49 @@ config MODULE_SIG_HASH
 	default "sha384" if MODULE_SIG_SHA384
 	default "sha512" if MODULE_SIG_SHA512
 
+config MODULE_COMPRESS
+	bool "Compress modules on installation"
+	depends on MODULES
+	help
+	  This option compresses the kernel modules when 'make
+	  modules_install' is run.
+
+	  The modules will be compressed either using gzip or xz depend on the
+	  choice made in "Compression algorithm".
+
+	  module-init-tools has support for gzip format while kmod handle gzip
+	  and xz compressed modules.
+
+	  When a kernel module is installed from outside of the main kernel
+	  source and uses the Kbuild system for installing modules then that
+	  kernel module will also be compressed when it is installed.
+
+	  This option provides little benefit when the modules are to be used inside
+	  an initrd or initramfs, it generally is more efficient to compress the whole
+	  initrd or initramfs instead.
+
+	  This is fully compatible with signed modules while the signed module is
+	  compressed. module-init-tools or kmod handles decompression and provide to
+	  other layer the uncompressed but signed payload.
+
+choice
+	prompt "Compression algorithm"
+	depends on MODULE_COMPRESS
+	default MODULE_COMPRESS_GZIP
+	help
+	  This determines which sort of compression will be used during
+	  'make modules_install'.
+
+	  GZIP (default) and XZ are supported.
+
+config MODULE_COMPRESS_GZIP
+	bool "GZIP"
+
+config MODULE_COMPRESS_XZ
+	bool "XZ"
+
+endchoice
+
 endif # MODULES
 
 config INIT_ALL_POSSIBLE