Re: [PATCH] tools/testing/selftests/vm/mlock2-tests: fix mlock2 false-negative errors

[Shakeel Butt <shakeelb@google.com>]

On Sat, Mar 21, 2020 at 9:31 PM Andrew Morton <akpm@linux-foundation.org> wrote:
>
> On Sat, 21 Mar 2020 22:03:26 -0400 Rafael Aquini <aquini@redhat.com> wrote:
>
> > > > + * In order to sort out that race, and get the after fault checks consistent,
> > > > + * the "quick and dirty" trick below is required in order to force a call to
> > > > + * lru_add_drain_all() to get the recently MLOCK_ONFAULT pages moved to
> > > > + * the unevictable LRU, as expected by the checks in this selftest.
> > > > + */
> > > > +static void force_lru_add_drain_all(void)
> > > > +{
> > > > + sched_yield();
> > > > + system("echo 1 > /proc/sys/vm/compact_memory");
> > > > +}
> > >
> > > What is the sched_yield() for?
> > >
> >
> > Mostly it's there to provide a sleeping gap after the fault, whithout
> > actually adding an arbitrary value with usleep().
> >
> > It's not a hard requirement, but, in some of the tests I performed
> > (whithout that sleeping gap) I would still see around 1% chance
> > of hitting the false-negative. After adding it I could not hit
> > the issue anymore.
>
> It's concerning that such deep machinery as pagevec draining is visible
> to userspace.
>

We already have other examples like memcg stats where the
optimizations like batching per-cpu stats collection exposes
differences to the userspace. I would not be that worried here.

> I suppose that for consistency and correctness we should perform a
> drain prior to each read from /proc/*/pagemap.  Presumably this would
> be far too expensive.
>
> Is there any other way?  One such might be to make the MLOCK_ONFAULT
> pages bypass the lru_add_pvecs?
>

I would rather prefer to have something similar to
/proc/sys/vm/stat_refresh which drains the pagevecs.