linux-kernel - Re: [PATCH v2 2/2] modules: add CONFIG_MODPROBE

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Message-ID: <YEiVkZAmWm5D68A2@kroah.com>
Date:   Wed, 10 Mar 2021 10:46:57 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     Luis Chamberlain <mcgrof@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jessica Yu <jeyu@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v2 2/2] modules: add CONFIG_MODPROBE_PATH

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@...musvillemoes.dk>
> ---
>  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@...uxfoundation.org>