linux-kernel - Re: [mm] 4e2c82a409: ltp.overcommit

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Date:   Mon, 6 Jul 2020 16:42:20 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Andi Kleen <andi.kleen@...el.com>
Cc:     Feng Tang <feng.tang@...el.com>, Qian Cai <cai@....pw>,
        Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        kernel test robot <rong.a.chen@...el.com>,
        Michal Hocko <mhocko@...e.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Matthew Wilcox <willy@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Kees Cook <keescook@...omium.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Iurii Zaikin <yzaikin@...gle.com>, tim.c.chen@...el.com,
        dave.hansen@...el.com, ying.huang@...el.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, lkp@...ts.01.org
Subject: Re: [mm] 4e2c82a409: ltp.overcommit_memory01.fail

On Mon, 6 Jul 2020 06:34:34 -0700 Andi Kleen <andi.kleen@...el.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