Hi Qian Cai,
On Thu, Jul 09, 2020 at 09:40:40AM -0400, Qian Cai wrote:
> > > > 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?
Thanks for the cleanup suggestion.
> t = *table;
> t.data = &new_policy;
The input table->data is actually &sysctl_overcommit_memory, so
there is a problem for "read" case, it will return the 'new_policy'
value instead of real sysctl_overcommit_memory.
It should work after adding a check
if (write)
t.data = &new_policy;
> ret = proc_dointvec_minmax(table, write, buffer, lenp, ppos);
--> &t
Thanks,
Feng
> 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;
