linux-kernel - Re: [PATCH v5 5/5] selftests/mm: add UFFDIO

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Date:   Sun, 3 Dec 2023 20:09:09 -0800
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     David Hildenbrand <david@...hat.com>
Cc:     Ryan Roberts <ryan.roberts@....com>, akpm@...ux-foundation.org,
        viro@...iv.linux.org.uk, brauner@...nel.org, shuah@...nel.org,
        aarcange@...hat.com, lokeshgidra@...gle.com, peterx@...hat.com,
        hughd@...gle.com, mhocko@...e.com, axelrasmussen@...gle.com,
        rppt@...nel.org, willy@...radead.org, Liam.Howlett@...cle.com,
        jannh@...gle.com, zhangpeng362@...wei.com, bgeffon@...gle.com,
        kaleshsingh@...gle.com, ngeoffray@...gle.com, jdduke@...gle.com,
        linux-mm@...ck.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        kernel-team@...roid.com
Subject: Re: [PATCH v5 5/5] selftests/mm: add UFFDIO_MOVE ioctl test

On Sat, Dec 2, 2023 at 2:11 AM David Hildenbrand <david@...hat.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@...gle.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?
Thanks,
Suren.

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

View attachment "0001-userfaultfd-split-large-pmd-mapped-folio.patch" of type "text/x-patch" (1287 bytes)

View attachment "0002-selftests-mm-use-correct-function-to-obtain-huge-pag.patch" of type "text/x-patch" (1305 bytes)