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

[Suren Baghdasaryan]

On Wed, Dec 6, 2023 at 2:30 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
>
> On Wed, Dec 6, 2023 at 1:21 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
> >
> > On 05.12.23 05:46, Suren Baghdasaryan wrote:
> > > On Mon, Dec 4, 2023 at 10:44 AM Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> > >>
> > >> On Mon, Dec 4, 2023 at 10:27 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > >>>
> > >>> On 04.12.23 17:35, Suren Baghdasaryan wrote:
> > >>>> On Mon, Dec 4, 2023 at 1:27 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
> > >>>>>
> > >>>>> On 04/12/2023 04:09, Suren Baghdasaryan wrote:
> > >>>>>> On Sat, Dec 2, 2023 at 2:11 AM David Hildenbrand <david@xxxxxxxxxx> 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@xxxxxxxxxx>
> > >>>>>>>>>>> ---
> > >>>>>>>>>>>      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.

Posted v6 at https://lore.kernel.org/all/20231206103702.3873743-1-surenb@xxxxxxxxxx/.
Changes are listed in the cover letter.

Andrew, could you please replace the current v5 version in mm-unstable
with this new one?
Thanks,
Suren.

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