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Date:   Sat, 21 Mar 2020 21:31:42 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Rafael Aquini <aquini@redhat.com>
Cc:     linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org,
        shuah@kernel.org, shakeelb@google.com
Subject: Re: [PATCH] tools/testing/selftests/vm/mlock2-tests: fix mlock2
 false-negative errors
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        <20200321184352.826d3dba38aecc4ff7b32e72@linux-foundation.org>
        <20200322020326.GB1068248@t490s>
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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.

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?