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

[Andrew Morton]

On Wed, 8 Apr 2026 13:35:42 -0700 anthony.yznaga@xxxxxxxxxx wrote:

> 
> On 4/3/26 12:31 PM, Andrew Morton wrote:
> > On Thu,  2 Apr 2026 16:59:33 -0700 Anthony Yznaga <anthony.yznaga@xxxxxxxxxx> 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@xxxxxxxxxx
> 
> 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.