The patch titled
Subject: mm/selftest: uffd: explain the write missing fault check
has been added to the -mm mm-unstable branch. Its filename is
mm-selftest-uffd-explain-the-write-missing-fault-check.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-selftest-uffd-explain-the-write-missing-fault-check.patch
This patch will later appear in the mm-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Peter Xu <peterx@xxxxxxxxxx>
Subject: mm/selftest: uffd: explain the write missing fault check
Date: Tue, 4 Oct 2022 15:34:00 -0400
It's not obvious why we had a write check for each of the missing
messages, especially when it should be a locking op. Add a rich comment
for that, and also try to explain its good side and limitations, so that
if someone hit it again for either a bug or a different glibc impl
there'll be some clue to start with.
Link: https://lkml.kernel.org/r/20221004193400.110155-4-peterx@xxxxxxxxxx
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
Reviewed-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
Reviewed-by: David Hildenbrand <david@xxxxxxxxxx>
Cc: Andrea Arcangeli <aarcange@xxxxxxxxxx>
Cc: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxxxxxxxxxx>
Cc: Nadav Amit <nadav.amit@xxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
tools/testing/selftests/vm/userfaultfd.c | 22 ++++++++++++++++++++-
1 file changed, 21 insertions(+), 1 deletion(-)
--- a/tools/testing/selftests/vm/userfaultfd.c~mm-selftest-uffd-explain-the-write-missing-fault-check
+++ a/tools/testing/selftests/vm/userfaultfd.c
@@ -774,7 +774,27 @@ static void uffd_handle_page_fault(struc
continue_range(uffd, msg->arg.pagefault.address, page_size);
stats->minor_faults++;
} else {
- /* Missing page faults */
+ /*
+ * Missing page faults.
+ *
+ * Here we force a write check for each of the missing mode
+ * faults. It's guaranteed because the only threads that
+ * will trigger uffd faults are the locking threads, and
+ * their first instruction to touch the missing page will
+ * always be pthread_mutex_lock().
+ *
+ * Note that here we relied on an NPTL glibc impl detail to
+ * always read the lock type at the entry of the lock op
+ * (pthread_mutex_t.__data.__type, offset 0x10) before
+ * doing any locking operations to guarantee that. It's
+ * actually not good to rely on this impl detail because
+ * logically a pthread-compatible lib can implement the
+ * locks without types and we can fail when linking with
+ * them. However since we used to find bugs with this
+ * strict check we still keep it around. Hopefully this
+ * could be a good hint when it fails again. If one day
+ * it'll break on some other impl of glibc we'll revisit.
+ */
if (msg->arg.pagefault.flags & UFFD_PAGEFAULT_FLAG_WRITE)
err("unexpected write fault");
_
Patches currently in -mm which might be from peterx@xxxxxxxxxx are
mm-uffd-fix-warning-without-pte_marker_uffd_wp-compiled-in.patch
mm-hugetlb-fix-race-condition-of-uffd-missing-minor-handling.patch
mm-hugetlb-use-hugetlb_pte_stable-in-migration-race-check.patch
mm-selftest-uffd-explain-the-write-missing-fault-check.patch
