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

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Date:   Tue, 7 Jul 2020 13:41:20 +0800
From:   Feng Tang <feng.tang@...el.com>
To:     "Huang, Ying" <ying.huang@...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

On Tue, Jul 07, 2020 at 12:00:09PM +0800, Huang, Ying wrote:
> 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?

These reorderings are really head scratching :)

I've thought about this before when Qian Cai first reported the warning
message, as kernel had a check: 

	VM_WARN_ONCE(percpu_counter_read(&vm_committed_as) <
			-(s64)vm_committed_as_batch * num_online_cpus(),
			"memory commitment underflow");

If the batch is decreased first, the warning will be easier/earlier to be
triggered, so I didn't brought this up when handling the warning message.

But it might work now, as the warning has been removed.

Thanks,
Feng