Re: [PATCH] selftests/mm: fix uffd move-pmd test crash on 64K base page systems

["Jose A. Perez de Azpillaga" <azpijr@gmail.com>]

On Mon, Oct 12, 2026 at 12:35:27AM +0800, Zenghui Yu wrote:
> From: "Zenghui Yu (Huawei)" <zenghui.yu@linux.dev>
> 
> uffd_move_pmd_test()/uffd_move_test_common() splits the test area into
> chunks of chunk_size and moves them one by one.  On arm64 with 64K base
> pages the chunk_size (read_pmd_pagesize()) is 512MB, which is larger than
> the whole 32MB (UFFD_TEST_MEM_SIZE) test area:
> 
>   step_count = nr_pages / (chunk_size / page_size) == 0
> 
> Because chunk_size > page_size, the test then aligns both areas up to
> 512MB, which points far outside of the 32MB mappings, and decrements
> step_count, which underflows to ULONG_MAX.  The verification loop then
> runs away and the first dereference of area_src faults with a SIGSEGV.
> 
> Skip the test when a single chunk cannot fit into the test area.
> 
> Fixes: a2bf6a9ca805 ("selftests/mm: add UFFDIO_MOVE ioctl test")
> Signed-off-by: Zenghui Yu (Huawei) <zenghui.yu@linux.dev>
> ---
>  tools/testing/selftests/mm/uffd-unit-tests.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/tools/testing/selftests/mm/uffd-unit-tests.c b/tools/testing/selftests/mm/uffd-unit-tests.c
> index ef9b3956bdcf..2a97ba1d6feb 100644
> --- a/tools/testing/selftests/mm/uffd-unit-tests.c
> +++ b/tools/testing/selftests/mm/uffd-unit-tests.c
> @@ -1937,6 +1937,11 @@ uffd_move_test_common(uffd_global_test_opts_t *gopts,
>  
>  	args.gopts = gopts;
>  
> +	if (chunk_size > gopts->nr_pages * gopts->page_size) {
> +		uffd_test_skip("chunk size does not fit in the test area");
> +		return;
> +	}
> +
>  	/* Prevent source pages from being mapped more than once */
>  	if (madvise(gopts->area_src, gopts->nr_pages * gopts->page_size, MADV_DONTFORK))
>  		err("madvise(MADV_DONTFORK) failure");
> -- 
> 2.53.0
> 
> 

LGTM.

Reviewed-by: Jose A. Perez de Azpillaga <azpijr@gmail.com>

-- 
cheers,
jose a. p-a