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

[Mike Rapoport <rppt@kernel.org>]

On Tue, Oct 13, 2026 at 09:39:54AM +0200, David Hildenbrand (Arm) wrote:
> On 10/11/26 18:35, 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.
> 
> That leaves 64k untested, though? Why not adjust the chunk size accordingly?

To test anything PMD on arm64 with 64k pages we need to adjust
UFFD_TEST_MEM_SIZE to have at least 4 pages, i.e 2G :)

Maybe we want UFFD_TEST_MEM_SIZE to scale with the page size?
 
> -- 
> Cheers,
> 
> David

-- 
Sincerely yours,
Mike.