linux-kernel - Re: [PATCH v3 2/2] modules: add CONFIG_MODPROBE

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Message-ID: <20210315200655.GX4332@42.do-not-panic.com>
Date:   Mon, 15 Mar 2021 20:06:55 +0000
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>
Cc:     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>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v3 2/2] modules: add CONFIG_MODPROBE_PATH

On Sat, Mar 13, 2021 at 10:25:28PM +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 made the initramfs unpacking asynchronous - 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.
> 
> Reviewed-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
> Acked-by: Jessica Yu <jeyu@...nel.org>
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>

Acked-by: Luis Chamberlain <mcgrof@...nel.org>

  Luis