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

[Suren Baghdasaryan <surenb@google.com>]

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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?
Thanks,
Suren.

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

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From cbd4348484986193c45235babbae6e30318d9e48 Mon Sep 17 00:00:00 2001
From: Suren Baghdasaryan <surenb@google.com>
Date: Sun, 3 Dec 2023 19:53:45 -0800
Subject: [PATCH 1/2] userfaultfd: split large pmd-mapped folio

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 mm/userfaultfd.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
index 71d0281f1162..6e6f672067fe 100644
--- a/mm/userfaultfd.c
+++ b/mm/userfaultfd.c
@@ -1382,8 +1382,25 @@ ssize_t move_pages(struct userfaultfd_ctx *ctx, struct mm_struct *mm,
 			/* Check if we can move the pmd without splitting it. */
 			if (move_splits_huge_pmd(dst_addr, src_addr, src_start + len) ||
 			    !pmd_none(dst_pmdval)) {
+				struct folio *folio = pfn_folio(pmd_pfn(*src_pmd));
+				if (!folio || !PageAnonExclusive(&folio->page)) {
+					spin_unlock(ptl);
+					err = -EBUSY;
+					break;
+				}
+				folio_get(folio);
 				spin_unlock(ptl);
 				split_huge_pmd(src_vma, src_pmd, src_addr);
+				if (folio_test_large(folio)) {
+					folio_lock(folio);
+					err = split_folio(folio);
+					folio_unlock(folio);
+					if (err) {
+						folio_put(folio);
+						break;
+					}
+				}
+				folio_put(folio);
 				continue;
 			}
 
-- 
2.43.0.rc2.451.g8631bc7472-goog


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From ab3e137ce9ebf3a949e14b9f544a8a2144934f86 Mon Sep 17 00:00:00 2001
From: Suren Baghdasaryan <surenb@google.com>
Date: Sun, 3 Dec 2023 20:04:08 -0800
Subject: [PATCH 2/2] selftests/mm: use correct function to obtain huge page
 size

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
---
 tools/testing/selftests/mm/uffd-unit-tests.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/testing/selftests/mm/uffd-unit-tests.c b/tools/testing/selftests/mm/uffd-unit-tests.c
index e4e271511db9..07c8dc490993 100644
--- a/tools/testing/selftests/mm/uffd-unit-tests.c
+++ b/tools/testing/selftests/mm/uffd-unit-tests.c
@@ -1096,7 +1096,7 @@ static void uffd_move_handle_fault(struct uffd_msg *msg,
 static void uffd_move_pmd_handle_fault(struct uffd_msg *msg,
 				       struct uffd_args *args)
 {
-	uffd_move_handle_fault_common(msg, args, default_huge_page_size());
+	uffd_move_handle_fault_common(msg, args, read_pmd_pagesize());
 }
 
 static void
@@ -1199,7 +1199,7 @@ static void uffd_move_test(uffd_test_args_t *targs)
 
 static void uffd_move_pmd_test(uffd_test_args_t *targs)
 {
-	uffd_move_test_common(targs, default_huge_page_size(),
+	uffd_move_test_common(targs, read_pmd_pagesize(),
 			      uffd_move_pmd_handle_fault);
 }
 
-- 
2.43.0.rc2.451.g8631bc7472-goog