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Date:   Mon, 4 Dec 2023 19:27:32 +0100
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Subject: Re: [PATCH v5 5/5] selftests/mm: add UFFDIO_MOVE ioctl test
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To:     Suren Baghdasaryan <surenb@google.com>,
        Ryan Roberts <ryan.roberts@arm.com>
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From:   David Hildenbrand <david@redhat.com>
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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.

-- 
Cheers,

David / dhildenb