Re: [PATCH v3 03/10] selftests/mm: Skip uffd-wp-mremap if userfaultfd not available

[Brendan Jackman <jackmanb@google.com>]

On Fri, Feb 28, 2025 at 10:55:00PM +0530, Dev Jain wrote:
> 
> 
> On 28/02/25 10:24 pm, Brendan Jackman wrote:
> > It's obvious that this should fail in that case, but still, save the
> > reader the effort of figuring out that they've run into this by just
> > SKIPping
> > 
> > Signed-off-by: Brendan Jackman <jackmanb@google.com>
> > ---
> >   tools/testing/selftests/mm/uffd-wp-mremap.c | 5 ++++-
> >   1 file changed, 4 insertions(+), 1 deletion(-)
> > 
> > diff --git a/tools/testing/selftests/mm/uffd-wp-mremap.c b/tools/testing/selftests/mm/uffd-wp-mremap.c
> > index 2c4f984bd73caa17e12b9f4a5bb71e7fdf5d8554..c2ba7d46c7b4581a3c32a6b6acd148e3e89c2172 100644
> > --- a/tools/testing/selftests/mm/uffd-wp-mremap.c
> > +++ b/tools/testing/selftests/mm/uffd-wp-mremap.c
> > @@ -182,7 +182,10 @@ static void test_one_folio(size_t size, bool private, bool swapout, bool hugetlb
> >   	/* Register range for uffd-wp. */
> >   	if (userfaultfd_open(&features)) {
> > -		ksft_test_result_fail("userfaultfd_open() failed\n");
> > +		if (errno == ENOENT)
> > +			ksft_test_result_skip("userfaultfd not available\n");
> > +		else
> > +			ksft_test_result_fail("userfaultfd_open() failed\n");
> >   		goto out;
> >   	}
> >   	if (uffd_register(uffd, mem, size, false, true, false)) {
> > 
> 
> I think you are correct, just want to confirm whether "uffd not available"
> if and only if "errno == ENOENT" is true. That is,
> is it possible that errno can be something else and uffd is still not
> available, 

Yeah, I strongly suspect this can happen. This is an attempt to
improve things but I don't think it's a full solution.

I've been pondering this a bit and I think it's impractical to solve
problems like this in the code of individual testst. I think the right
thing to do is either:

1. Have a centralised facility for detecting conditions like
   "userfaultfd not available" that tests can just query it, so they
   say something like:

   ksft_test_requires("userfaultfd");

   Which would do some sort of actual principled check for presence
   and then skip the test with an informative message when it's not
   there. There would be a list of these "system requirements" in the
   code so you can easily see in one place what things might be needed
   to successfully run all the tests.

or

2. Specify out of band that there's a fixed set of requirements for
   running the tests and document that you shouldn't run them without
   satisfying them. Then just don't bother with SKIPs and call it user
   error.

   This would require some reasonably usable tooling for actually
   getting a system that satisfies the requirements.

But both of them require a deeper investment. I would quite like to
explore option 1 a bit but that's for a future Brendan. 

In the meantime I'm just trying to get these tests running on
virtme-ng. (I'm not even gonna add all of them, because e.g. once I
noticed this one I added a `scripts/config -e USERFAULTFD` to my
script, so I won't notice if anything else is missing the check).

> or errno can be ENOENT even if uffd is available.

I think it's probably posible for this to happen too, e.g. if the
system has a perverted /dev or something. But again I think that can
only be solved with the kinda stuff I mentioned above.

Sorry for the essay :D