selftests/mm: run_vmtests.sh: fix hugetlb mem size calculation

The script calculates a mininum required size of hugetlb memories, but
it'll stop working with <1MB huge page sizes, reporting all zeros even if
huge pages are available.

In reality, the calculation doesn't really need to be as complicated
either.  Make it simpler and work for KB-level hugepages too.

[peterx@redhat.com: run_vmtests.sh: fix hugetlb mem size calculation]
  Link: https://lkml.kernel.org/r/20240403200324.1603493-1-peterx@redhat.com
Link: https://lkml.kernel.org/r/20240321215047.678172-1-peterx@redhat.comSigned-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Tested-by: Ryan Roberts <ryan.roberts@arm.com>
Cc: Nico Pache <npache@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

tools/testing/selftests/mm/run_vmtests.sh

@@ -152,9 +152,13 @@ done < /proc/meminfo
 # both of these requirements into account and attempt to increase
 # number of huge pages available.
 nr_cpus=$(nproc)
-hpgsize_MB=$((hpgsize_KB / 1024))
-half_ufd_size_MB=$((((nr_cpus * hpgsize_MB + 127) / 128) * 128))
-needmem_KB=$((half_ufd_size_MB * 2 * 1024))
+uffd_min_KB=$((hpgsize_KB * nr_cpus * 2))
+hugetlb_min_KB=$((256 * 1024))
+if [[ $uffd_min_KB -gt $hugetlb_min_KB ]]; then
+	needmem_KB=$uffd_min_KB
+else
+	needmem_KB=$hugetlb_min_KB
+fi
 
 # set proper nr_hugepages
 if [ -n "$freepgs" ] && [ -n "$hpgsize_KB" ]; then
@@ -294,7 +298,8 @@ CATEGORY="userfaultfd" run_test ./uffd-unit-tests
 uffd_stress_bin=./uffd-stress
 CATEGORY="userfaultfd" run_test ${uffd_stress_bin} anon 20 16
 # Hugetlb tests require source and destination huge pages. Pass in half
-# the size ($half_ufd_size_MB), which is used for *each*.
+# the size of the free pages we have, which is used for *each*.
+half_ufd_size_MB=$((freepgs / 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