Re: [PATCH] selftests/mm: pagemap_scan ioctl: add PFN ZERO test cases

On 30.06.25 12:24, Muhammad Usama Anjum wrote:

Add test cases to test the correctness of PFN ZERO flag of pagemap_scan ioctl. Test with normal pages backed memory and huge pages backed memory.

Just to verify: would this trigger on kernels before my fix?

Cc: David Hildenbrand david@redhat.com Signed-off-by: Muhammad Usama Anjum usama.anjum@collabora.com

---

The bug has been fixed [1].

[1] https://lore.kernel.org/all/20250617143532.2375383-1-david@redhat.com

tools/testing/selftests/mm/pagemap_ioctl.c | 57 +++++++++++++++++++++- 1 file changed, 56 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/mm/pagemap_ioctl.c b/tools/testing/selftests/mm/pagemap_ioctl.c index 57b4bba2b45f3..6138de0087edf 100644 --- a/tools/testing/selftests/mm/pagemap_ioctl.c +++ b/tools/testing/selftests/mm/pagemap_ioctl.c @@ -1,4 +1,5 @@ // SPDX-License-Identifier: GPL-2.0

• #define _GNU_SOURCE #include <stdio.h> #include <fcntl.h>

@@ -1480,6 +1481,57 @@ static void transact_test(int page_size) extra_thread_faults); } +void zeropfn_tests(void) +{

• unsigned long long mem_size;
• struct page_region vec;
• int i, ret;
• char *mem;
• /* Test with page backed memory */

What is "page backed memory" ? :)

• mem_size = 10 * page_size;
• mem = mmap(NULL, mem_size, PROT_READ, MAP_PRIVATE | MAP_ANON, -1, 0);
• if (mem == MAP_FAILED)

• ksft_exit_fail_msg("error nomem\n");
  

• /* Touch each page to ensure it's mapped */
• for (i = 0; i < mem_size; i += page_size)

• (void)((volatile char *)mem)[i];
  

• ret = pagemap_ioctl(mem, mem_size, &vec, 1, 0,

• 	    (mem_size / page_size), PAGE_IS_PFNZERO, 0, 0, PAGE_IS_PFNZERO);
  

• if (ret < 0)

• ksft_exit_fail_msg("error %d %d %s\n", ret, errno, strerror(errno));
  

• ksft_test_result(ret == 1 && LEN(vec) == (mem_size / page_size),

• 	 "%s all pages must have PFNZERO set\n", __func__);
  

• munmap(mem, mem_size);
• /* Test with huge page */
• mem_size = 10 * hpage_size;
• mem = memalign(hpage_size, mem_size);
• if (!mem)

• ksft_exit_fail_msg("error nomem\n");
  

• ret = madvise(mem, mem_size, MADV_HUGEPAGE);
• if (ret)

• ksft_exit_fail_msg("madvise failed %d %s\n", errno, strerror(errno));
  

Might fail on older kernels, so we usually treat this as a skip.

• for (i = 0; i < mem_size; i += hpage_size)

• (void)((volatile char *)mem)[i];
  

• ret = pagemap_ioctl(mem, mem_size, &vec, 1, 0,

• 	    (mem_size / page_size), PAGE_IS_PFNZERO, 0, 0, PAGE_IS_PFNZERO);
  

• if (ret < 0)

• ksft_exit_fail_msg("error %d %d %s\n", ret, errno, strerror(errno));
  

• ksft_test_result(ret == 1 && LEN(vec) == (mem_size / page_size),

• 	 "%s all huge pages must have PFNZERO set\n", __func__);
  

Wouldn't this be able to fail if /sys/kernel/mm/transparent_hugepage/use_zero_page is set to false, or if mmap() gave us a suboptimally-aligned range?

You'd have to read each and every page to get the ordinary shared zeropage in these configs instead without making the test too complicated.

• free(mem);

Shouldn't this be an munmap() ?

+}

• int main(int __attribute__((unused)) argc, char *argv[]) { int shmid, buf_size, fd, i, ret;

@@ -1494,7 +1546,7 @@ int main(int __attribute__((unused)) argc, char *argv[]) if (init_uffd()) ksft_exit_pass();

• ksft_set_plan(115);

• ksft_set_plan(117);

We should probably look into converting this test to kselftest_harness.