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

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Message-ID: <CAJuCfpGtfxmyGDeCH9+YMMd-aX2z9pgdBch+DD7vGZzcfaC+kw@mail.gmail.com>
Date:   Fri, 1 Dec 2023 13:59:14 -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 Fri, Dec 1, 2023 at 12:47 PM David Hildenbrand <david@...hat.com> 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?
>
> I can spot that uffd_move_pmd_test()/uffd_move_pmd_handle_fault() uses
> default_huge_page_size(), which reads the default hugetlb size.
>
> That, however, does not necessarily correspond to the THP size. That one
> can be obtained using read_pmd_pagesize() in vm_util.c

Oh, I didn't realize that default_huge_page_size() is not always the
actual THP size. Will fix.

>
> I quickly scanned the code (still want to take a deeper look), but all
> PAE checks looked sane to me.
>
> I think the issue is folio split handling. I replied to the patch.

Thanks for your hint! That's very possibly the reason for the test
failure. I'm in the process of trying to reproduce this issue on
ARM64. If I'm unable to do so then I'll create a patch to split
PTE-mapped THP when encountered and will ask Ryan to try that.
Thanks,
Suren.

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