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, 15 insertions, 0 deletions

diff --git a/Makefile b/Makefile
index f64fc7804a03..82eeaa2f9e1c 100644
--- a/Makefile
+++ b/Makefile
@@ -842,6 +842,21 @@ mod_strip_cmd = true
 endif # INSTALL_MOD_STRIP
 export mod_strip_cmd
 
+# CONFIG_MODULE_COMPRESS, if defined, will cause module to be compressed
+# after they are installed in agreement with CONFIG_MODULE_COMPRESS_GZIP
+# or CONFIG_MODULE_COMPRESS_XZ.
+
+mod_compress_cmd = true
+ifdef CONFIG_MODULE_COMPRESS
+  ifdef CONFIG_MODULE_COMPRESS_GZIP
+    mod_compress_cmd = gzip -n
+  endif # CONFIG_MODULE_COMPRESS_GZIP
+  ifdef CONFIG_MODULE_COMPRESS_XZ
+    mod_compress_cmd = xz
+  endif # CONFIG_MODULE_COMPRESS_XZ
+endif # CONFIG_MODULE_COMPRESS
+export mod_compress_cmd
+
 # Select initial ramdisk compression format, default is gzip(1).
 # This shall be used by the dracut(8) tool while creating an initramfs image.
 #