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 <[email protected]> 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;
