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/[email protected]
Signed-off-by: Pankaj Raghav <[email protected]>
Reviewed-by: Zi Yan <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Cc: Luis Chamberlain <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Pankaj Raghav <[email protected]>
Cc: Shuah Khan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Panky-codes

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");