selftests/mm: report errno when things fail in gup_longterm

selftests/mm: report errno when things fail in gup_longterm

Patch series "selftests/mm: Some cleanups from trying to run them", v4.

I never had much luck running mm selftests so I spent a few hours digging
into why.

Looks like most of the reason is missing SKIP checks, so this series is
just adding a bunch of those that I found.  I did not do anything like all
of them, just the ones I spotted in gup_longterm, gup_test, mmap,
userfaultfd and memfd_secret.

It's a bit unfortunate to have to skip those tests when ftruncate() fails,
but I don't have time to dig deep enough into it to actually make them
pass.  I have observed the issue on 9pfs and heard rumours that NFS has a
similar problem.

I'm now able to run these test groups successfully:

- mmap
- gup_test
- compaction
- migration
- page_frag
- userfaultfd
- mlock

I've never gone past "Waiting for hugetlb memory to get depleted", in the
hugetlb tests.  I don't know if they are stuck or if they would eventually
work if I was patient enough (testing on a 1G machine).  I have not
investigated further.

I had some issues with mlock tests failing due to -ENOSRCH from mlock2(),
I can no longer reproduce that though, things work OK now.

Of the remaining tests there may be others that work fine, but there's no
convenient way to survey the whole output of run_vmtests.sh so I'm just
going test by test here.

In my spare moments I am slowly chipping away at a setup to run these
tests continuously in a reasonably hermetic QEMU environment via
virtme-ng:

https://github.com/bjackman/linux/blob/5fad4b9c592290f38e0f8bc73c9abb9c99d8787c/README.md

Hopefully that will eventually offer a way to provide a "canned"
environment where the tests are known to work, which can be fairly easily
reproduced by any developer.

This patch (of 12):

Just reporting failure doesn't tell you what went wrong.  This can fail in
different ways so report errno to help the reader get started debugging.

Link: https://lkml.kernel.org/r/20250311-mm-selftests-v4-0-dec210a658f5@google.com
Link: https://lkml.kernel.org/r/20250311-mm-selftests-v4-1-dec210a658f5@google.com
Signed-off-by: Brendan Jackman <jackmanb@google.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>