selftests/mm: fix FORCE_READ to read input value correctly

FORCE_READ() converts input value x to its pointer type then reads from
address x. This is wrong. If x is a non-pointer, it would be caught it
easily. But all FORCE_READ() callers are trying to read from a pointer
and FORCE_READ() basically reads a pointer to a pointer instead of the
original typed pointer. Almost no access violation was found, except the
one from split_huge_page_test.

Fix it by implementing a simplified READ_ONCE() instead.

Link: https://lkml.kernel.org/r/[e-mail]
Fixes: 3f6bfd4789a0 ("selftests/mm: reuse FORCE_READ to replace "asm volatile("" : "+r" (XXX));"")
Signed-off-by: Zi Yan <[e-mail]>
Reviewed-by: Lorenzo Stoakes <[e-mail]>
Acked-by: David Hildenbrand <[e-mail]>
Reviewed-by: wang lian <[e-mail]>
Reviewed-by: Wei Yang <[e-mail]>
Cc: Christian Brauner <[e-mail]>
Cc: Jann Horn <[e-mail]>
Cc: Kairui Song <[e-mail]>
Cc: Liam Howlett <[e-mail]>
Cc: Mark Brown <[e-mail]>
Cc: SeongJae Park <[e-mail]>
Cc: Shuah Khan <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Cc: Zi Yan <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

tools/testing/selftests/mm/cow.c

···
 	}
 
 	/* Read from the page to populate the shared zeropage. */
-	FORCE_READ(mem);
-	FORCE_READ(smem);
+	FORCE_READ(*mem);
+	FORCE_READ(*smem);
 
 	fn(mem, smem, pagesize);
 munmap:

tools/testing/selftests/mm/guard-regions.c

···
 		if (write)
 			*ptr = 'x';
 		else
-			FORCE_READ(ptr);
+			FORCE_READ(*ptr);
 	}
 
 	signal_jump_set = false;

tools/testing/selftests/mm/hugetlb-madvise.c

···
 	unsigned long i;
 
 	for (i = 0; i < nr_pages; i++) {
+		unsigned long *addr2 =
+			((unsigned long *)(addr + (i * huge_page_size)));
 		/* Prevent the compiler from optimizing out the entire loop: */
-		FORCE_READ(((unsigned long *)(addr + (i * huge_page_size))));
+		FORCE_READ(*addr2);
 	}
 }
 

tools/testing/selftests/mm/migration.c

···
 		 * the memory access actually happens and prevents the compiler
 		 * from optimizing away this entire loop.
 		 */
-		FORCE_READ((uint64_t *)ptr);
+		FORCE_READ(*(uint64_t *)ptr);
 	}
 
 	return NULL;

tools/testing/selftests/mm/pagemap_ioctl.c

···
 
 	ret = madvise(mem, hpage_size, MADV_HUGEPAGE);
 	if (!ret) {
-		FORCE_READ(mem);
+		FORCE_READ(*mem);
 
 		ret = pagemap_ioctl(mem, hpage_size, &vec, 1, 0,
 				    0, PAGE_IS_PFNZERO, 0, 0, PAGE_IS_PFNZERO);

tools/testing/selftests/mm/split_huge_page_test.c

···
 	}
 	madvise(*addr, fd_size, MADV_HUGEPAGE);
 
-	for (size_t i = 0; i < fd_size; i++)
-		FORCE_READ((*addr + i));
+	for (size_t i = 0; i < fd_size; i++) {
+		char *addr2 = *addr + i;
+
+		FORCE_READ(*addr2);
+	}
 
 	if (!check_huge_file(*addr, fd_size / pmd_pagesize, pmd_pagesize)) {
 		ksft_print_msg("No large pagecache folio generated, please provide a filesystem supporting large folio\n");

tools/testing/selftests/mm/vm_util.h

···
  * anything with it in order to trigger a read page fault. We therefore must use
  * volatile to stop the compiler from optimising this away.
  */
-#define FORCE_READ(x) (*(volatile typeof(x) *)x)
+#define FORCE_READ(x) (*(const volatile typeof(x) *)&(x))
 
 extern unsigned int __page_size;
 extern unsigned int __page_shift;