selftests/mm: use asm volatile to not optimize mmap read variable

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.

Link: https://lkml.kernel.org/r/20240606203619.677276-1-kernel@pankajraghav.com
Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Pankaj Raghav <p.raghav@samsung.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 2 insertions, 1 deletions

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");