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

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Date:   Thu, 9 Jul 2020 09:40:40 -0400
From:   Qian Cai <cai@....pw>
To:     Feng Tang <feng.tang@...el.com>
Cc:     "Huang, Ying" <ying.huang@...el.com>,
        Andi Kleen <andi.kleen@...el.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>,
        Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
        Christoph Lameter <cl@...ux.com>,
        kernel test robot <rong.a.chen@...el.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 Thu, Jul 09, 2020 at 12:55:54PM +0800, Feng Tang wrote:
> On Tue, Jul 07, 2020 at 01:41:20PM +0800, Feng Tang wrote:
> > 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.
> 
> I tested the reorder way, and the test could pass in 100 times run. The
> new order when changing policy to OVERCOMMIT_NEVER:
>   1. re-compute the batch ( to the smaller one)
>   2. do the on_each_cpu sync
>   3. really change the policy to NEVER.
> 
> It solves one of previous concern, that after the sync is done on cpuX,
> but before the whole sync on all cpus are done, there is a window that
> the percpu-counter could be enlarged again.
> 
> IIRC Andi had concern about read side cost when doing the sync, my
> understanding is most of the readers (malloc/free/map/unmap) are using
> percpu_counter_read_positive, which is a fast path without involving lock.
> 
> As for the problem itself, I agree with Michal's point, that usually there
> is no normal case that will change the overcommit_policy too frequently.
> 
> The code logic is mainly in overcommit_policy_handler(), based on the
> previous sync fix. please help to review, thanks!
> 
> int overcommit_policy_handler(struct ctl_table *table, int write, void *buffer,
> 		size_t *lenp, loff_t *ppos)
> {
> 	int ret;
> 
> 	if (write) {
> 		int new_policy;
> 		struct ctl_table t;
> 
> 		t = *table;
> 		t.data = &new_policy;
> 		ret = proc_dointvec_minmax(&t, write, buffer, lenp, ppos);
> 		if (ret)
> 			return ret;
> 
> 		mm_compute_batch(new_policy);
> 		if (new_policy == OVERCOMMIT_NEVER)
> 			schedule_on_each_cpu(sync_overcommit_as);
> 		sysctl_overcommit_memory = new_policy;
> 	} else {
> 		ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
> 	}
> 
> 	return ret;
> }

Rather than having to indent those many lines, how about this?

t = *table;
t.data = &new_policy;
ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
if (ret || !write)
	return ret;

mm_compute_batch(new_policy);
if (new_policy == OVERCOMMIT_NEVER)
	schedule_on_each_cpu(sync_overcommit_as);

sysctl_overcommit_memory = new_policy;
return ret;