Re: [PATCH v2] ksm: React on changing "sleep_millisecs" parameter faster

[Cyrill Gorcunov <gorcunov@gmail.com>]

On Tue, Dec 11, 2018 at 03:22:42PM +0300, Kirill Tkhai wrote:
> On 11.12.2018 14:13, Cyrill Gorcunov wrote:
> > On Tue, Dec 11, 2018 at 01:26:59PM +0300, Kirill Tkhai wrote:
> >> ksm thread unconditionally sleeps in ksm_scan_thread()
> >> after each iteration:
> >>
> >> 	schedule_timeout_interruptible(
> >> 		msecs_to_jiffies(ksm_thread_sleep_millisecs))
> >>
> >> The timeout is configured in /sys/kernel/mm/ksm/sleep_millisecs.
> >>
> >> In case of user writes a big value by a mistake, and the thread
> >> enters into schedule_timeout_interruptible(), it's not possible
> >> to cancel the sleep by writing a new smaler value; the thread
> >> is just sleeping till timeout expires.
> >>
> >> The patch fixes the problem by waking the thread each time
> >> after the value is updated.
> >>
> >> This also may be useful for debug purposes; and also for userspace
> >> daemons, which change sleep_millisecs value in dependence of
> >> system load.
> >>
> >> Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
> >>
> >> v2: Use wait_event_interruptible_timeout() instead of unconditional
> >>     schedule_timeout().
> > ...
> >> @@ -2844,7 +2849,10 @@ static ssize_t sleep_millisecs_store(struct kobject *kobj,
> >>  	if (err || msecs > UINT_MAX)
> >>  		return -EINVAL;
> >>  
> >> +	mutex_lock(&ksm_thread_mutex);
> >>  	ksm_thread_sleep_millisecs = msecs;
> >> +	mutex_unlock(&ksm_thread_mutex);
> >> +	wake_up_interruptible(&ksm_iter_wait);
> > 
> > Btw, just thought -- if we start using this mutex here don't we
> > open a window for force attack on the thread self execution,
> > iow if there gonna be a million of writers do we have a guarantee
> > thread ksm_scan_thread will grab the mutex earlier than writers
> > (or somewhere inbetween)?
> 
> This file is permitted for global root only. I don't think there is
> a problem.
> 
> If someone wants to make ksm helpless, a person may just write a big
> "sleep_millisecs" value. KSM thread won't be executed almost all the time
> in this case.

True. Still I think if we can leave without taking a lock it a rule of thumb.
Something like

	if (msecs != ksm_thread_sleep_millisecs)
		wake_up_interruptable(&ksm_iter_wait);

Thoughts?