Re: [PATCH -v4 2/2] printk: Add kernel parameter to control writes to /dev/kmsg

From: Dave Young
Date: Sun Jul 17 2016 - 22:18:22 EST

On 07/17/16 at 07:40am, Borislav Petkov wrote:
> On Sat, Jul 16, 2016 at 06:44:25PM +0800, Dave Young wrote:
> ...
> > Mis-ratelimit cause critical userspace messages being lost, that is worse
> The current setting is quite generous so that all critical messages
> should land in dmesg. Besides, we don't ratelimit during boot. The idea
> is that userspace should switch to a different logging facility once the
> system is up... which userspace does reportedly.

I would say avoiding ratelimit during boot make no much sense. Userspace can not
write to /dev/kmsg when system_state == SYSTEM_BOOTING because init process
has not run yet.

> IOW, /dev/kmsg should handle a relatively big logging amount without
> ratelimiting.
> > than use off as default. Suppose we turn off devkmsg by default distributions
> > can still turn on it with sysctl and for us who do not want the flooding we can
> > use printk.devkmsg=off in kernel cmdline to override it.
> That part I cannot parse.

I do not understand, care to elaborate a bit?

Let me explain my comments, I means to set printk.devkmsg=off by default,
userspace can set it to on by sysctl. User can provide kernel cmdline
printk.devkmsg=off if he/she want.

Or set printk.devkmsg=on by default to avoid break userspace, it is also fine.

> > Of course if we turn off it by default we can print a warning to alert user.
> >
> > BTW, for userspace messages maybe they should not go to same log buffer, maybe
> > a separate log buffer for /dev/msg will be better.
> See above.