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

[Shakeel Butt <shakeelb@google.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.