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

[Suren Baghdasaryan <surenb@google.com>]

On Wed, Dec 6, 2023 at 1:21 AM David Hildenbrand <david@redhat.com> wrote:
>
> 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).

Agree. Let me post the new version so that mm-unstable does not
produce these failures and will start working on covering additional
cases in the tests. The new patchset is almost ready, just finishing
final tests.

>
> --
> Cheers,
>
> David / dhildenb
>