Re: [PATCH v1] selftests/mm: Set allocated memory to non-zero content in cow test

[David Hildenbrand <david@redhat.com>]

On 07.01.25 15:25, Ryan Roberts wrote:
> After commit b1f202060afe ("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.
> 
> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
> ---
> 
> Applies on top of mm-unstable (f349e79bfbf3)
> 
> Thanks,
> Ryan
> 
>   tools/testing/selftests/mm/cow.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/tools/testing/selftests/mm/cow.c b/tools/testing/selftests/mm/cow.c
> index 32c6ccc2a6be..1238e1c5aae1 100644
> --- a/tools/testing/selftests/mm/cow.c
> +++ b/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);

Interesting, thanks!

"1" should work, because we use "0xff" to detect modifications.

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb