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

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Date:   Tue, 07 Jul 2020 12:00:09 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Feng Tang <feng.tang@...el.com>
Cc:     Andi Kleen <andi.kleen@...el.com>, Qian Cai <cai@....pw>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "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>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, <lkp@...ts.01.org>
Subject: Re: [mm] 4e2c82a409: ltp.overcommit_memory01.fail

Feng Tang <feng.tang@...el.com> writes:

> 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. 

Can we change the batch firstly, then sync the global counter, finally
change the overcommit policy?

Best Regards,
Huang, Ying