Re: [PATCH v2 2/2] modules: add CONFIG_MODPROBE_PATH

[Greg Kroah-Hartman]

On Tue, Mar 09, 2021 at 10:17:00PM +0100, Rasmus Villemoes wrote:
> Allow the developer to specifiy the initial value of the
> modprobe_path[] string. This can be used to set it to the empty string
> initially, thus effectively disabling request_module() during early
> boot until userspace writes a new value via the
> /proc/sys/kernel/modprobe interface. [1]
> 
> When building a custom kernel (often for an embedded target), it's
> normal to build everything into the kernel that is needed for booting,
> and indeed the initramfs often contains no modules at all, so every
> such request_module() done before userspace init has mounted the real
> rootfs is a waste of time.
> 
> This is particularly useful when combined with the previous patch,
> which allowed the initramfs to be unpacked asynchronously - for that
> to work, it had to make any usermodehelper call wait for the unpacking
> to finish before attempting to invoke the userspace helper. By
> eliminating all such (known-to-be-futile) calls of usermodehelper, the
> initramfs unpacking and the {device,late}_initcalls can proceed in
> parallel for much longer.
> 
> For a relatively slow ppc board I'm working on, the two patches
> combined lead to 0.2s faster boot - but more importantly, the fact
> that the initramfs unpacking proceeds completely in the background
> while devices get probed means I get to handle the gpio watchdog in
> time without getting reset.
> 
> [1] __request_module() already has an early -ENOENT return when
> modprobe_path is the empty string.
> 
> Signed-off-by: Rasmus Villemoes <linux@xxxxxxxxxxxxxxxxxx>
> ---
>  init/Kconfig  | 12 ++++++++++++
>  kernel/kmod.c |  2 +-
>  2 files changed, 13 insertions(+), 1 deletion(-)

Looks sane to me, odd we didn't think of doing this before.

Reviewed-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>