Re: [PATCH 0/3] make vm_committed_as_batch aware of vm overcommit policy

[Feng Tang]

On Fri, May 29, 2020 at 08:50:25AM -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 is not atomic.
> There's still a race window here over all the CPUs where the WARN_ON could
> happen because you change the global first.
 
The re-computing of batch number comes after this sync, so at this
point the batch is still the bigger one, and won't trigger the warning.

> Probably you would need another global that says "i'm currently changing
> the mode" and then skip the WARN_ON in that window. Maybe a sequence lock.
> 
> Seems all overkill to me. Better to kill the warning.

Yes, the cost is high, schedule_on_each_cpu is labeled as "very slow"
in the code comments.

Thanks,
Feng

> 
> -Andi