selftests/mm: introduce helper to read every page

FORCE_READ(*addr) ensures that the compiler will emit a load from addr. 
Several tests need to trigger such a load for a range of pages, ensuring
that every page is faulted in, if it wasn't already.

Introduce a new helper force_read_pages() that does exactly that and
replace existing loops with a call to it.

The step size (regular/huge page size) is preserved for all loops, except
in split_huge_page_test.  Reading every byte is unnecessary; we now read
every huge page, matching the following call to check_huge_file().

Link: https://lkml.kernel.org/r/20260122170224.4056513-7-kevin.brodsky@arm.com
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@arm.com>
Acked-by: David Hildenbrand (Red Hat) <david@kernel.org>
Cc: Jason Gunthorpe <jgg@nvidia.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: wang lian <lianux.mm@gmail.com>
Cc: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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

@@ -47,14 +47,7 @@ void write_fault_pages(void *addr, unsigned long nr_pages)
 
 void read_fault_pages(void *addr, unsigned long nr_pages)
 {
-	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(*addr2);
-	}
+	force_read_pages(addr, nr_pages, huge_page_size);
 }
 
 int main(int argc, char **argv)

tools/testing/selftests/mm/pfnmap.c

@@ -35,18 +35,15 @@ static void signal_handler(int sig)
 
 static int test_read_access(char *addr, size_t size, size_t pagesize)
 {
-	size_t offs;
 	int ret;
 
 	if (signal(SIGSEGV, signal_handler) == SIG_ERR)
 		return -EINVAL;
 
 	ret = sigsetjmp(sigjmp_buf_env, 1);
-	if (!ret) {
-		for (offs = 0; offs < size; offs += pagesize)
-			/* Force a read that the compiler cannot optimize out. */
-			*((volatile char *)(addr + offs));
-	}
+	if (!ret)
+		force_read_pages(addr, size/pagesize, pagesize);
+
 	if (signal(SIGSEGV, SIG_DFL) == SIG_ERR)
 		return -EINVAL;
 

tools/testing/selftests/mm/split_huge_page_test.c

@@ -652,11 +652,7 @@ static int create_pagecache_thp_and_fd(const char *testfile, size_t fd_size,
 	}
 	madvise(*addr, fd_size, MADV_HUGEPAGE);
 
-	for (size_t i = 0; i < fd_size; i++) {
-		char *addr2 = *addr + i;
-
-		FORCE_READ(*addr2);
-	}
+	force_read_pages(*addr, fd_size / pmd_pagesize, pmd_pagesize);
 
 	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

@@ -54,6 +54,13 @@ static inline unsigned int pshift(void)
 	return __page_shift;
 }
 
+static inline void force_read_pages(char *addr, unsigned int nr_pages,
+				    size_t pagesize)
+{
+	for (unsigned int i = 0; i < nr_pages; i++)
+		FORCE_READ(addr[i * pagesize]);
+}
+
 bool detect_huge_zeropage(void);
 
 /*