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From: Shakeel Butt <shakeelb@google.com>
To: Rafael Aquini <aquini@redhat.com>
Cc: LKML <linux-kernel@vger.kernel.org>,
	linux-kselftest@vger.kernel.org, shuah@kernel.org,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH] tools/testing/selftests/vm/mlock2-tests: fix mlock2 false-negative errors
Date: Sun, 22 Mar 2020 09:31:04 -0700
Message-ID: <CALvZod4GjRFLRX=S_YFYnJk-kL6tjveYEDOBFS76NqrURERHHQ@mail.gmail.com> (raw)
In-Reply-To: <20200322013525.1095493-1-aquini@redhat.com>

On Sat, Mar 21, 2020 at 6:35 PM Rafael Aquini <aquini@redhat.com> wrote:
>
> Changes for commit 9c4e6b1a7027f ("mm, mlock, vmscan: no more skipping pagevecs")
> break this test expectations on the behavior of mlock syscall family immediately
> inserting the recently faulted pages into the UNEVICTABLE_LRU, when MCL_ONFAULT is
> passed to the syscall as part of its flag-set.

mlock* syscalls do not provide any guarantee that the pages will be in
unevictable LRU, only that the pages will not be paged-out. The test
is checking something very internal to the kernel and this is expected
to break.

>
> There is no functional error introduced by the aforementioned commit,
> but it opens up a time window where the recently faulted and locked pages
> might yet not be put back into the UNEVICTABLE_LRU, thus causing a
> subsequent and immediate PFN flag check for the UNEVICTABLE bit
> to trip on false-negative errors, as it happens with this test.
>
> This patch fix the false negative by forcefully resorting to a code path that
> will call a CPU pagevec drain right after the fault but before the PFN flag
> check takes place, sorting out the race that way.
>
> Fixes: 9c4e6b1a7027f ("mm, mlock, vmscan: no more skipping pagevecs")

This is fixing the actual test and not about fixing the mentioned
patch. So, this Fixes line is not needed.

  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
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 [this message]
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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