Re: [PATCH v2 2/2] selftests/mm: verify droppable mappings cannot be locked

[Lorenzo Stoakes <ljs@kernel.org>]

On Wed, Apr 08, 2026 at 03:26:14PM -0700, anthony.yznaga@oracle.com wrote:
>
> On 4/8/26 2:02 PM, Andrew Morton wrote:
> > On Wed, 8 Apr 2026 13:35:42 -0700 anthony.yznaga@oracle.com wrote:
> >
> > > On 4/3/26 12:31 PM, Andrew Morton wrote:
> > > > On Thu,  2 Apr 2026 16:59:33 -0700 Anthony Yznaga <anthony.yznaga@oracle.com> wrote:
> > > >
> > > > > For configs that support MAP_DROPPABLE verify that a mapping created
> > > > > with MAP_DROPPABLE cannot be locked via mlock(), and that it will not
> > > > > be locked if it's created after mlockall(MCL_FUTURE).
> > > > There are a few queries from the AI reviewbot;
> > > > 	https://sashiko.dev/#/patchset/20260402235933.10588-1-anthony.yznaga@oracle.com
> > > Interesting. Of the two issues, one is certainly legit. I need to add an
> > > munlockall() on early return from test_mlockall_future_droppable().
> > Cool.
> >
> > > For the other, the question posed was whether the tests should handle
> > > possibly being run on an older kernel that doesn't implement
> > > MAP_DROPPABLE. It seems to me to that a selftest should not be expected
> > > to work (or even necessarily compile) on kernels older than when the
> > > selftest was introduced, but I don't want to assume.
> > I don't know that there's any policy on that.  My attitude is that
> > selftests are not intended to be forward- or backward-compatible.
> > That's why we ship them with the kernel source!
> >
> > If we get a selftests fixup then I do like to backport that into
> > earlier kernels if appropriate, to keep those in good shape.  And that
> > has the effect of reducing people's motivation to run a later kernel's
> > selftests on their current kernel.
> >
> That makes sense. It's trivial to skip the tests if MAP_DROPPABLE is not
> defined so I'll do that.
>
>

Yeah in general, I think we best-effort try to keep them working for older
kernels (but no guarantees IMO), so checking if something's implemented is
cheap-and-easy.

Cheers, Lorenzo