selftests/mm: run_vmtests.sh: fix half_ufd_size_MB calculation

We noticed that uffd-stress test was always failing to run when invoked
for the hugetlb profiles on x86_64 systems with a processor count of 64 or
bigger:

  ...
  # ------------------------------------
  # running ./uffd-stress hugetlb 128 32
  # ------------------------------------
  # ERROR: invalid MiB (errno=9, @uffd-stress.c:459)
  ...
  # [FAIL]
  not ok 3 uffd-stress hugetlb 128 32 # exit=1
  ...

The problem boils down to how run_vmtests.sh (mis)calculates the size of
the region it feeds to uffd-stress.  The latter expects to see an amount
of MiB while the former is just giving out the number of free hugepages
halved down.  This measurement discrepancy ends up violating uffd-stress'
assertion on number of hugetlb pages allocated per CPU, causing it to bail
out with the error above.

This commit fixes that issue by adjusting run_vmtests.sh's
half_ufd_size_MB calculation so it properly renders the region size in
MiB, as expected, while maintaining all of its original constraints in
place.

Link: https://lkml.kernel.org/r/20250218192251.53243-1-aquini@redhat.com
Fixes: 2e47a44 ("selftests/mm: run_vmtests.sh: fix hugetlb mem size calculation")
Signed-off-by: Rafael Aquini <raquini@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Rafael Aquini

tools/testing/selftests/mm/run_vmtests.sh

@@ -304,7 +304,9 @@ uffd_stress_bin=./uffd-stress
 CATEGORY="userfaultfd" run_test ${uffd_stress_bin} anon 20 16
 # Hugetlb tests require source and destination huge pages. Pass in half
 # the size of the free pages we have, which is used for *each*.
-half_ufd_size_MB=$((freepgs / 2))
+# uffd-stress expects a region expressed in MiB, so we adjust
+# half_ufd_size_MB accordingly.
+half_ufd_size_MB=$(((freepgs * hpgsize_KB) / 1024 / 2))
 CATEGORY="userfaultfd" run_test ${uffd_stress_bin} hugetlb "$half_ufd_size_MB" 32
 CATEGORY="userfaultfd" run_test ${uffd_stress_bin} hugetlb-private "$half_ufd_size_MB" 32
 CATEGORY="userfaultfd" run_test ${uffd_stress_bin} shmem 20 16