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.

Link: https://lkml.kernel.org/r/20221004193400.110155-4-peterx@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Mike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Nadav Amit <nadav.amit@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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");