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Message-Id: <20221003155630.469263-4-peterx@redhat.com>
Date: Mon, 3 Oct 2022 11:56:30 -0400
From: Peter Xu <peterx@...hat.com>
To: linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc: peterx@...hat.com, Andrea Arcangeli <aarcange@...hat.com>,
Mike Rapoport <rppt@...ux.vnet.ibm.com>,
David Hildenbrand <david@...hat.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Axel Rasmussen <axelrasmussen@...gle.com>,
Mike Kravetz <mike.kravetz@...cle.com>,
Nadav Amit <nadav.amit@...il.com>
Subject: [PATCH 3/3] mm/selftest: uffd: Explain the write missing fault check
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.
Signed-off-by: Peter Xu <peterx@...hat.com>
---
tools/testing/selftests/vm/userfaultfd.c | 22 +++++++++++++++++++++-
1 file changed, 21 insertions(+), 1 deletion(-)
diff --git a/tools/testing/selftests/vm/userfaultfd.c b/tools/testing/selftests/vm/userfaultfd.c
index 74babdbc02e5..297f250c1d95 100644
--- a/tools/testing/selftests/vm/userfaultfd.c
+++ b/tools/testing/selftests/vm/userfaultfd.c
@@ -774,7 +774,27 @@ static void uffd_handle_page_fault(struct uffd_msg *msg,
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");
--
2.37.3
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