Re: [mm] 4e2c82a409: ltp.overcommit_memory01.fail

[Feng Tang]

On Mon, Jul 06, 2020 at 06:34:34AM -0700, Andi Kleen wrote:
> >  	ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
> > -	if (ret == 0 && write)
> > +	if (ret == 0 && write) {
> > +		if (sysctl_overcommit_memory == OVERCOMMIT_NEVER)
> > +			schedule_on_each_cpu(sync_overcommit_as);
> 
> The schedule_on_each_cpu is not atomic, so the problem could still happen
> in that window.
> 
> I think it may be ok if it eventually resolves, but certainly needs
> a comment explaining it. Can you do some stress testing toggling the
> policy all the time on different CPUs and running the test on
> other CPUs and see if the test fails?

For the raw test case reported by 0day, this patch passed in 200 times
run. And I will read the ltp code and try stress testing it as you
suggested.


> The other alternative would be to define some intermediate state
> for the sysctl variable and only switch to never once the schedule_on_each_cpu
> returned. But that's more complexity.

One thought I had is to put this schedule_on_each_cpu() before
the proc_dointvec_minmax() to do the sync before sysctl_overcommit_memory
is really changed. But the window still exists, as the batch is
still the larger one. 

Thanks,
Feng

> 
> 
> -Andi