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

[David Hildenbrand (Arm)]

On 10/29/26 14:37, Zenghui Yu wrote:
> Hi David,
> 
> On 10/13/26 3:39 PM, David Hildenbrand (Arm) wrote:
>> On 10/11/26 18:35, Zenghui Yu wrote:
>>> From: "Zenghui Yu (Huawei)" <zenghui.yu@xxxxxxxxx>
>>>
>>> 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?
> 
> Right. Apparently this has never been tested on 64k :)

Yeah. And if we start supporting it we should properly test it :)

> 
> I agree that scaling the chunk size is the better approach -- it gives us
> actual test coverage on such configuration, instead of bailing out early.
> 
> If anyone is interested in it, please feel free to send a patch. Otherwise
> I'll work on it when I get some spare time.
I guess no rush on this one.

-- 
Cheers,

David