Re: [PATCH v5 5/5] selftests/mm: add UFFDIO_MOVE ioctl test

[Ryan Roberts <ryan.roberts@arm.com>]

On 01/12/2023 20:47, David Hildenbrand wrote:
> On 01.12.23 10:29, Ryan Roberts wrote:
>> On 21/11/2023 17:16, Suren Baghdasaryan wrote:
>>> Add tests for new UFFDIO_MOVE ioctl which uses uffd to move source
>>> into destination buffer while checking the contents of both after
>>> the move. After the operation the content of the destination buffer
>>> should match the original source buffer's content while the source
>>> buffer should be zeroed. Separate tests are designed for PMD aligned and
>>> unaligned cases because they utilize different code paths in the kernel.
>>>
>>> Signed-off-by: Suren Baghdasaryan <surenb@google.com>
>>> ---
>>>   tools/testing/selftests/mm/uffd-common.c     |  24 +++
>>>   tools/testing/selftests/mm/uffd-common.h     |   1 +
>>>   tools/testing/selftests/mm/uffd-unit-tests.c | 189 +++++++++++++++++++
>>>   3 files changed, 214 insertions(+)
>>>
>>> diff --git a/tools/testing/selftests/mm/uffd-common.c
>>> b/tools/testing/selftests/mm/uffd-common.c
>>> index fb3bbc77fd00..b0ac0ec2356d 100644
>>> --- a/tools/testing/selftests/mm/uffd-common.c
>>> +++ b/tools/testing/selftests/mm/uffd-common.c
>>> @@ -631,6 +631,30 @@ int copy_page(int ufd, unsigned long offset, bool wp)
>>>       return __copy_page(ufd, offset, false, wp);
>>>   }
>>>   +int move_page(int ufd, unsigned long offset, unsigned long len)
>>> +{
>>> +    struct uffdio_move uffdio_move;
>>> +
>>> +    if (offset + len > nr_pages * page_size)
>>> +        err("unexpected offset %lu and length %lu\n", offset, len);
>>> +    uffdio_move.dst = (unsigned long) area_dst + offset;
>>> +    uffdio_move.src = (unsigned long) area_src + offset;
>>> +    uffdio_move.len = len;
>>> +    uffdio_move.mode = UFFDIO_MOVE_MODE_ALLOW_SRC_HOLES;
>>> +    uffdio_move.move = 0;
>>> +    if (ioctl(ufd, UFFDIO_MOVE, &uffdio_move)) {
>>> +        /* real retval in uffdio_move.move */
>>> +        if (uffdio_move.move != -EEXIST)
>>> +            err("UFFDIO_MOVE error: %"PRId64,
>>> +                (int64_t)uffdio_move.move);
>>
>> Hi Suren,
>>
>> FYI this error is triggering in mm-unstable (715b67adf4c8):
>>
>> Testing move-pmd on anon... ERROR: UFFDIO_MOVE error: -16 (errno=16,
>> @uffd-common.c:648)
>>
>> I'm running in a VM on Apple M2 (arm64). I haven't debugged any further, but
>> happy to go deeper if you can direct.
> 
> Does it trigger reliably? Which pagesize is that kernel using?

Yep, although very occasionally it fails with EAGAIN. 4K kernel; see other email
for full config.

> 
> I can spot that uffd_move_pmd_test()/uffd_move_pmd_handle_fault() uses
> default_huge_page_size(), which reads the default hugetlb size.

My kernel command line is explicitly seting the default huge page size to 2M.

> 
> That, however, does not necessarily correspond to the THP size. That one can be
> obtained using read_pmd_pagesize() in vm_util.c
> 
> I quickly scanned the code (still want to take a deeper look), but all PAE
> checks looked sane to me.
> 
> I think the issue is folio split handling. I replied to the patch.
>