[PATCH v3] selftests/mm: use asm volatile to not optimize mmap read variable

[Pankaj Raghav (Samsung)]

From: Pankaj Raghav <p.raghav@xxxxxxxxxxx>

create_pagecache_thp_and_fd() in split_huge_page_test.c used the
variable dummy to perform mmap read.

However, this test was skipped even on XFS which has large folio
support. The issue was compiler (gcc 13.2.0) was optimizing out the
dummy variable, therefore, not creating huge page in the page cache.

Use asm volatile() trick to force the compiler not to optimize out
the loop where we read from the mmaped addr. This is similar to what is
being done in other tests (cow.c, etc)

As the variable is now used in the asm statement, remove the unused
attribute.

Signed-off-by: Pankaj Raghav <p.raghav@xxxxxxxxxxx>
---
Changes since v2:
- Use the asm volatile trick to force the compiler to not optimize the
  read into dummy variable. (David)

 tools/testing/selftests/mm/split_huge_page_test.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/mm/split_huge_page_test.c b/tools/testing/selftests/mm/split_huge_page_test.c
index d3c7f5fb3e7b..e5e8dafc9d94 100644
--- a/tools/testing/selftests/mm/split_huge_page_test.c
+++ b/tools/testing/selftests/mm/split_huge_page_test.c
@@ -300,7 +300,7 @@ int create_pagecache_thp_and_fd(const char *testfile, size_t fd_size, int *fd,
 		char **addr)
 {
 	size_t i;
-	int __attribute__((unused)) dummy = 0;
+	int dummy = 0;
 
 	srand(time(NULL));
 
@@ -341,6 +341,7 @@ int create_pagecache_thp_and_fd(const char *testfile, size_t fd_size, int *fd,
 
 	for (size_t i = 0; i < fd_size; i++)
 		dummy += *(*addr + i);
+	asm volatile("" : "+r" (dummy));
 
 	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");

base-commit: d97496ca23a2d4ee80b7302849404859d9058bcd
-- 
2.44.1