Re: [PATCH v3] selftests/mm: Add UFFDIO_MOVE huge zeropage PMD regression test

["David Hildenbrand (Arm)" <david@kernel.org>]

On 3/18/26 05:18, Chris Down wrote:
> The existing uffd-unit-tests move-pmd coverage exercises PMD-sized
> UFFDIO_MOVE on anonymous THPs, but it does not force the huge zeropage
> PMD path in move_pages_huge_pmd().
> 
> Add a dedicated anonymous UFFDIO_MOVE PMD test that exercises this
> relatively comprehensively.
> 
> Signed-off-by: Chris Down <chris@chrisdown.name>
> ---
>  tools/testing/selftests/mm/uffd-unit-tests.c | 100 +++++++++++++++++++
>  tools/testing/selftests/mm/vm_util.c         |  12 +++
>  tools/testing/selftests/mm/vm_util.h         |   1 +
>  3 files changed, 113 insertions(+)
> 
> diff --git a/tools/testing/selftests/mm/uffd-unit-tests.c b/tools/testing/selftests/mm/uffd-unit-tests.c
> index 6f5e404a446c..9cc4cbab9a23 100644
> --- a/tools/testing/selftests/mm/uffd-unit-tests.c
> +++ b/tools/testing/selftests/mm/uffd-unit-tests.c
> @@ -1227,6 +1227,99 @@ static void uffd_move_pmd_test(uffd_global_test_opts_t *gopts, uffd_test_args_t
>  			      uffd_move_pmd_handle_fault);
>  }
>  
> +static void uffd_move_pmd_huge_zeropage_test(uffd_global_test_opts_t *gopts,
> +					     uffd_test_args_t *targs)
> +{
> +	unsigned long pmd_size = read_pmd_pagesize();
> +	unsigned long pmd_pages;
> +	unsigned long bytes = gopts->nr_pages * gopts->page_size;
> +	char *orig_area_src = gopts->area_src, *orig_area_dst = gopts->area_dst;
> +	char *aligned_src, *aligned_dst;
> +	unsigned long src_offs, dst_offs, max_offs;
> +	pthread_t uffd_mon;
> +	struct uffd_args args = { 0 };
> +	char c = '\0';
> +	int pagemap_fd;
> +
> +	if (pmd_size <= gopts->page_size) {
> +		uffd_test_skip("huge page size is 0, feature missing?");

Wouldn't a check for 0 be

	"!pmd_size" ? :)

But yes, this is unexpected. Maybe simply

	"Could not detect PMD size"


> +		return;
> +	}
> +	if (!detect_huge_zeropage()) {
> +		uffd_test_skip("transparent huge zeropage disabled");
> +		return;
> +	}
> +
> +	pmd_pages = pmd_size / gopts->page_size;

Might consider s/pmd_pages/nr_pages_per_pmd/

[...]

>  
> +bool pagemap_is_huge_zero(int fd, char *start)
> +{
> +	uint64_t categories;
> +
> +	if (!pagemap_scan_supported(fd, start))
> +		return false;
> +
> +	categories = pagemap_scan_get_categories(fd, start);


A slightly nicer way to write it might be

	const uint64_t expected = PAGE_IS_PRESENT | PAGE_IS_PFNZERO | PAGE_IS_HUGE;

	...
	return (categories & expected) == huge_zero;


In general, lgtm, thanks!

-- 
Cheers,

David