selftests/mm: set allocated memory to non-zero content in cow test

After commit b1f2020 ("mm: remap unused subpages to shared zeropage
when splitting isolated thp"), cow test cases involving swapping out THPs
via madvise(MADV_PAGEOUT) started to be skipped due to the subsequent
check via pagemap determining that the memory was not actually swapped
out.  Logs similar to this were emitted:

   ...

   # [RUN] Basic COW after fork() ... with swapped-out, PTE-mapped THP (16 kB)
   ok 2 # SKIP MADV_PAGEOUT did not work, is swap enabled?
   # [RUN] Basic COW after fork() ... with single PTE of swapped-out THP (16 kB)
   ok 3 # SKIP MADV_PAGEOUT did not work, is swap enabled?
   # [RUN] Basic COW after fork() ... with swapped-out, PTE-mapped THP (32 kB)
   ok 4 # SKIP MADV_PAGEOUT did not work, is swap enabled?

   ...

The commit in question introduces the behaviour of scanning THPs and if
their content is predominantly zero, it splits them and replaces the pages
which are wholly zero with the zero page.  These cow test cases were
getting caught up in this.

So let's avoid that by filling the contents of all allocated memory with
a non-zero value. With this in place, the tests are passing again.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: b1f2020 ("mm: remap unused subpages to shared zeropage when splitting isolated thp")
Signed-off-by: Ryan Roberts <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Cc: Usama Arif <[email protected]>
Cc: Yu Zhao <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Ryan Roberts

tools/testing/selftests/mm/cow.c

@@ -758,7 +758,7 @@ static void do_run_with_base_page(test_fn fn, bool swapout)
 	}
 
 	/* Populate a base page. */
-	memset(mem, 0, pagesize);
+	memset(mem, 1, pagesize);
 
 	if (swapout) {
 		madvise(mem, pagesize, MADV_PAGEOUT);
@@ -824,12 +824,12 @@ static void do_run_with_thp(test_fn fn, enum thp_run thp_run, size_t thpsize)
 	 * Try to populate a THP. Touch the first sub-page and test if
 	 * we get the last sub-page populated automatically.
 	 */
-	mem[0] = 0;
+	mem[0] = 1;
 	if (!pagemap_is_populated(pagemap_fd, mem + thpsize - pagesize)) {
 		ksft_test_result_skip("Did not get a THP populated\n");
 		goto munmap;
 	}
-	memset(mem, 0, thpsize);
+	memset(mem, 1, thpsize);
 
 	size = thpsize;
 	switch (thp_run) {
@@ -1012,7 +1012,7 @@ static void run_with_hugetlb(test_fn fn, const char *desc, size_t hugetlbsize)
 	}
 
 	/* Populate an huge page. */
-	memset(mem, 0, hugetlbsize);
+	memset(mem, 1, hugetlbsize);
 
 	/*
 	 * We need a total of two hugetlb pages to handle COW/unsharing