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From: Shakeel Butt <shakeelb@google.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: Rafael Aquini <aquini@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>,
	linux-kselftest@vger.kernel.org, shuah@kernel.org
Subject: Re: [PATCH] tools/testing/selftests/vm/mlock2-tests: fix mlock2 false-negative errors
Date: Sun, 22 Mar 2020 09:36:49 -0700
Message-ID: <CALvZod7LiMiK1JtfdvvU3W36cGSUKhhKf6dMZpsNZv6nMiJ5=g@mail.gmail.com> (raw)
In-Reply-To: <20200321213142.597e23af955de653fc4db7a1@linux-foundation.org>

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.

  parent reply index

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-22  1:35 Rafael Aquini
2020-03-22  1:43 ` Andrew Morton
2020-03-22  2:03   ` Rafael Aquini
2020-03-22  4:31     ` Andrew Morton
2020-03-22  5:41       ` Rafael Aquini
2020-03-22 16:40         ` Shakeel Butt
2020-03-22 16:36       ` Shakeel Butt [this message]
2020-03-23  7:52         ` Michal Hocko
2020-03-23 14:42           ` Rafael Aquini
2020-03-23 14:51             ` Michal Hocko
2020-03-23 15:02               ` Rafael Aquini
2020-03-23 15:12                 ` Michal Hocko
2020-03-23 15:41                   ` Rafael Aquini
2020-03-23 15:51                     ` Michal Hocko
2020-03-23 15:54                       ` Rafael Aquini
2020-03-24 15:42                         ` Michal Hocko
2020-03-24 15:49                           ` Rafael Aquini
2020-03-26  0:49                             ` Andrew Morton
2020-03-26  2:22                               ` Rafael Aquini
2020-03-26  6:49                               ` Michal Hocko
2020-03-26 19:58                                 ` Andrew Morton
2020-03-26 20:16                                   ` Rafael Aquini
2020-03-26 20:19                                   ` Rafael Aquini
2020-03-22 16:31 ` Shakeel Butt
2020-03-23 14:16   ` Rafael Aquini
2020-03-23 14:29     ` Michal Hocko
2020-03-23 14:55       ` Rafael Aquini
2020-03-23 15:01       ` Michal Hocko
2020-03-23 15:07         ` Rafael Aquini

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