ksm_tests: skip hugepage test when Transparent Hugepages are disabled

ksm_tests: skip hugepage test when Transparent Hugepages are disabled

Some systems (e.g.  minimal or real-time kernels) may not enable
Transparent Hugepages (THP), causing MADV_HUGEPAGE to return EINVAL.  This
patch introduces a runtime check using the existing THP sysfs interface
and skips the hugepage merging test (`-H`) when THP is not available.

To avoid those failures:

  # -----------------------------
  # running ./ksm_tests -H -s 100
  # -----------------------------
  # ksm_tests: MADV_HUGEPAGE: Invalid argument
  # [FAIL]
  not ok 1 ksm_tests -H -s 100 # exit=2

  # --------------------
  # running ./khugepaged
  # --------------------
  # Reading PMD pagesize failed# [FAIL]
  not ok 1 khugepaged # exit=1

  # --------------------
  # running ./soft-dirty
  # --------------------
  # TAP version 13
  # 1..15
  # ok 1 Test test_simple
  # ok 2 Test test_vma_reuse dirty bit of allocated page
  # ok 3 Test test_vma_reuse dirty bit of reused address page
  # Bail out! Reading PMD pagesize failed# Planned tests != run tests (15 != 3)
  # # Totals: pass:3 fail:0 xfail:0 xpass:0 skip:0 error:0
  # [FAIL]
  not ok 1 soft-dirty # exit=1
  # SUMMARY: PASS=0 SKIP=0 FAIL=1

  # -------------------
  # running ./migration
  # -------------------
  # TAP version 13
  # 1..3
  # # Starting 3 tests from 1 test cases.
  # #  RUN           migration.private_anon ...
  # #            OK  migration.private_anon
  # ok 1 migration.private_anon
  # #  RUN           migration.shared_anon ...
  # #            OK  migration.shared_anon
  # ok 2 migration.shared_anon
  # #  RUN           migration.private_anon_thp ...
  # # migration.c:196:private_anon_thp:Expected madvise(ptr, TWOMEG, MADV_HUGEPAGE) (-1) == 0 (0)
  # # private_anon_thp: Test terminated by assertion
  # #          FAIL  migration.private_anon_thp
  # not ok 3 migration.private_anon_thp
  # # FAILED: 2 / 3 tests passed.
  # # Totals: pass:2 fail:1 xfail:0 xpass:0 skip:0 error:0
  # [FAIL]
  not ok 1 migration # exit=1

It's true that CONFIG_TRANSPARENT_HUGEPAGE=y is explicitly enabled in
tools/testing/selftests/mm/config, so ideally the runtime environment
should also support THP.

However, in practice, we've found that on some systems:

- THP is disabled at boot time (transparent_hugepage=never)
- Or manually disabled via sysfs
- Or unavailable in RT kernels, containers, or minimal CI environments

In these cases, the test will fail with EINVAL on madvise(MADV_HUGEPAGE),
even though the kernel config is correct.

To make the test suite more robust and avoid false negatives, this patch
adds a runtime check for /sys/kernel/mm/transparent_hugepage/enabled.

If THP is not available, the hugepage test (-H) is skipped with a clear
message.

Link: https://lkml.kernel.org/r/20250624032748.393836-1-liwang@redhat.com
Signed-off-by: Li Wang <liwang@redhat.com>
Cc: Aruna Ramakrishna <aruna.ramakrishna@oracle.com>
Cc: Bagas Sanjaya <bagasdotme@gmail.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Joey Gouly <joey.gouly@arm.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Keith Lucas <keith.lucas@oracle.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>