Re: Inexplicable disk activity trying to load modules on devfs

[Daniel Jacobowitz]

On Mon, Jun 17, 2002 at 10:59:26AM +0400, Borsenkow Andrej wrote:
> >> 
> >> I just booted into 2.4.19-pre10-ac2 for the first time, and noticed 
> >> something very odd: my disk activity light was flashing at about 
> >> half-second intervals, very regularly, and I could hear the disk 
> >> moving. I was only able to track it down to which disk controller, via 
> >> /proc/interrupts (are there any tools for monitoring VFS activity? 
> >> They'd be really useful). Eventually I hunted down the program causing 
> >> it: xmms. 
> >> 
> >> The reason turned out to be that I hadn't remembered to build my sound 
> >> driver for this kernel version. Every half-second xmms tried to open 
> >> /dev/mixer (and failed, ENOENT). Every time it did that there was 
> >> actual disk activity. Easily reproducible without xmms. Reproducible 
> >> on any non-existant device in devfs, but not for nonexisting files on 
> >> other filesystems. Is something bypassing the normal disk cache 
> >> mechanisms here? That doesn't seem right at all. 
> >> 
> >
> >
> >syslog activity from a printk, perhaps? 
> 
> No. It is most probably devfsd trying to load sound modules.
> 
> This is exactly the reason Mandrake does not enable devfs in kernel-secure.
> You can badly hit your system by doing in a loop ls /dev/foo for some device
> foo that is configured for module autoloading.
> 
> It is very fascist decision; the slightly more forgiving way is to disable
> devfsd module autoloading (or disable devfsd entirely, just run it once
> after all drivers are loaded to execute actions) but then you lose support
> for hot plugging and some people do use kernel-secure on desktops. 

For the curious, the reason is that modprobe writes even failed attempts
to a log in /var/log/ksymoops, and calls fdatasync() on that file
afterwards.  There is no way to disable this without removing that
directory, as a design decision.  I don't personally see the point in
logging attempts which fail because there is no driver...

-- 
Daniel Jacobowitz                           Carnegie Mellon University
MontaVista Software                         Debian GNU/Linux Developer