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

[David Hildenbrand <david@redhat.com>]

On 05.12.23 05:46, Suren Baghdasaryan wrote:
> On Mon, Dec 4, 2023 at 10:44 AM Suren Baghdasaryan <surenb@google.com> wrote:
>>
>> On Mon, Dec 4, 2023 at 10:27 AM David Hildenbrand <david@redhat.com> wrote:
>>>
>>> On 04.12.23 17:35, Suren Baghdasaryan wrote:
>>>> On Mon, Dec 4, 2023 at 1:27 AM Ryan Roberts <ryan.roberts@arm.com> wrote:
>>>>>
>>>>> On 04/12/2023 04:09, Suren Baghdasaryan wrote:
>>>>>> On Sat, Dec 2, 2023 at 2:11 AM David Hildenbrand <david@redhat.com> wrote:
>>>>>>>
>>>>>>> On 02.12.23 09:04, Ryan Roberts wrote:
>>>>>>>> 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.
>>>>>>>>
>>>>>>>
>>>>>>> Okay, so that likely won't affect it.
>>>>>>>
>>>>>>> I can only guess that it has to do with the alignment of the virtual
>>>>>>> area we are testing with, and that we do seem to get more odd patterns
>>>>>>> on arm64.
>>>>>>>
>>>>>>> uffd_move_test_common() is a bit more elaborate, but if we aligned the
>>>>>>> src+start area up, surely "step_count" cannot be left unmodified?
>>>>>>>
>>>>>>> So assuming we get either an unaligned source or an unaligned dst from
>>>>>>> mmap(), I am not convinced that we won't be moving areas that are not
>>>>>>> necessarily fully backed by PMDs and maybe don't even fall into the VMA
>>>>>>> of interest?
>>>>>>>
>>>>>>> Not sure if that could trigger the THP splitting issue, though.
>>>>>>>
>>>>>>> But I just quickly scanned that test setup, could be I am missing
>>>>>>> something. It might make sense to just print the mmap'ed range and the
>>>>>>> actual ranges we are trying to move. Maybe something "obvious" can be
>>>>>>> observed.
>>>>>>
>>>>>> I was able to reproduce the issue on an Android device and after
>>>>>> implementing David's suggestions to split the large folio and after
>>>>>> replacing default_huge_page_size() with read_pmd_pagesize(), the
>>>>>> move-pmd test started working for me. Ryan, could you please apply
>>>>>> attached patches (over mm-unstable) and try the test again?
>>>>>
>>>>> Yep, all fixed with those patches!
>>>>
>>>> Great! Thanks for testing and confirming. I'll post an updated
>>>> patchset later today and will ask Andrew to replace the current one
>>>> with it.
>>>> I'll also look into the reasons we need to split PMD on ARM64 in this
>>>> test. It's good that this happened and we were able to test the PMD
>>>> split path but I'm curious about the reason. It's possible my address
>>>> alignment calculations are  somehow incorrect.
>>>
>>> I only skimmed the diff briefly, but likely you also want to try
>>> splitting in move_pages_pte(), if you encounter an already-pte-mapped THP.
>>
>> Huh, good point. I might be able to move the folio splitting code into
>> pte-mapped case and do a retry after splitting. That should minimize
>> the additional code required. Will do and post a new set shortly.
>> Thanks!
> 
> Was planning to post an update today but need some more time. Will try
> to send it tomorrow.

It would be great to have tests that cover these cases (having to 
PTE-map a PMD-mapped THP, and stumbling over an already-PTE-mapped one).

-- 
Cheers,

David / dhildenb