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Date:   Mon, 6 Jul 2020 16:42:20 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Andi Kleen <andi.kleen@intel.com>
Cc:     Feng Tang <feng.tang@intel.com>, Qian Cai <cai@lca.pw>,
        Dennis Zhou <dennis@kernel.org>, Tejun Heo <tj@kernel.org>,
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        Mel Gorman <mgorman@suse.de>,
        Kees Cook <keescook@chromium.org>,
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Subject: Re: [mm] 4e2c82a409: ltp.overcommit_memory01.fail
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On Mon, 6 Jul 2020 06:34:34 -0700 Andi Kleen <andi.kleen@intel.com> 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.

It sure does.

The new exported-to-everything percpu_counter_sync() should have full
formal documentation as well, please.

> 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?
> 
> 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.
> 
> 
> -Andi