hugetlb_dio test uses sub-page offsets (pagesize / 2) to verify that
hugepages used as DIO user buffers are correctly unpinned at completion.

However, on filesystems with a logical block size larger than half the
page size (e.g., 4K-sector block devices), these unaligned DIO writes
are rejected with -EINVAL, causing the test to fail unexpectedly.

Add check_dio_alignment() which queries the filesystem's DIO alignment
requirement via statx(STATX_DIOALIGN) and skips the test early if the
sub-page offset used by the test is not compatible with the alignment
constraint.

=== Reproduce Steps ===

  # dd if=/dev/zero of=/tmp/test.img bs=1M count=512
  # losetup --sector-size 4096 /dev/loop0 /tmp/test.img
  # mkfs.xfs /dev/loop0
  # mkdir -p /mnt/dio_test
  # mount /dev/loop0 /mnt/dio_test

  // Modify test to open /mnt/dio_test and rebuild it:
  -       fd = open("/tmp", O_TMPFILE | O_RDWR | O_DIRECT, 0664);
  +       fd = open("/mnt/dio_test", O_TMPFILE | O_RDWR | O_DIRECT, 0664);

  # getconf PAGESIZE
  4096

  # echo 100 >/proc/sys/vm/nr_hugepages

  # ./hugetlb_dio
  TAP version 13
  1..4
  # No. Free pages before allocation : 100
  # No. Free pages after munmap : 100
  ok 1 free huge pages from 0-12288
  Bail out! Error writing to file
  : Invalid argument (22)
  # Planned tests != run tests (4 != 1)
  # Totals: pass:1 fail:0 xfail:0 xpass:0 skip:0 error:0

Signed-off-by: Li Wang <liwang@redhat.com>
---
 tools/testing/selftests/mm/hugetlb_dio.c | 37 +++++++++++++++++-------
 1 file changed, 27 insertions(+), 10 deletions(-)

diff --git a/tools/testing/selftests/mm/hugetlb_dio.c b/tools/testing/selftests/mm/hugetlb_dio.c
index 9ac62eb4c97d..afcca50d190e 100644
--- a/tools/testing/selftests/mm/hugetlb_dio.c
+++ b/tools/testing/selftests/mm/hugetlb_dio.c
@@ -20,6 +20,31 @@
 #include "vm_util.h"
 #include "kselftest.h"
 
+#ifndef STATX_DIOALIGN
+#define STATX_DIOALIGN		0x00002000U
+#endif
+
+void check_dio_alignment(size_t pagesize)
+{
+	int fd;
+	struct statx stx;
+	unsigned int dio_align = 1;
+
+	fd = open("/tmp", O_TMPFILE | O_RDWR, 0664);
+	if (fd < 0)
+		ksft_exit_skip("Unable to allocate file: %s\n", strerror(errno));
+
+	if (statx(fd, "", AT_EMPTY_PATH, STATX_DIOALIGN, &stx) == 0 &&
+				(stx.stx_mask & STATX_DIOALIGN))
+		dio_align = stx.stx_dio_offset_align;
+
+	close(fd);
+
+	if ((pagesize / 2) % dio_align != 0)
+		ksft_exit_skip("DIO alignment (%u) incompatible with sub-page offset %lu\n",
+				dio_align, pagesize / 2);
+}
+
 void run_dio_using_hugetlb(unsigned int start_off, unsigned int end_off)
 {
 	int fd;
@@ -89,16 +114,11 @@ void run_dio_using_hugetlb(unsigned int start_off, unsigned int end_off)
 
 int main(void)
 {
-	size_t pagesize = 0;
-	int fd;
+	size_t pagesize = psize();
 
 	ksft_print_header();
 
-	/* Open the file to DIO */
-	fd = open("/tmp", O_TMPFILE | O_RDWR | O_DIRECT, 0664);
-	if (fd < 0)
-		ksft_exit_skip("Unable to allocate file: %s\n", strerror(errno));
-	close(fd);
+	check_dio_alignment(pagesize);
 
 	/* Check if huge pages are free */
 	if (!get_free_hugepages())
@@ -106,9 +126,6 @@ int main(void)
 
 	ksft_set_plan(4);
 
-	/* Get base page size */
-	pagesize  = psize();
-
 	/* start and end is aligned to pagesize */
 	run_dio_using_hugetlb(0, (pagesize * 3));
 
-- 
2.53.0