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

[Sarthak Sharma <sarthak.sharma@arm.com>]

Hi Zenghui!

On 10/11/26 10:05 PM, 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>
> ---

I run the test on a 64k kernel. Before the patch, a segmentation fault
occurred, which was fixed by the patch. So:

Tested-by: Sarthak Sharma <sarthak.sharma@arm.com>

Though Sashiko has a few nits [1] you might take a look at.

[1]
https://sashiko.dev/#/patchset/20261011163527.95988-1-zenghui.yu%40linux.dev