Commit 26c92d37 authored by Peter Xu's avatar Peter Xu Committed by Andrew Morton
Browse files

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: default avatarPeter Xu <peterx@redhat.com>
Reviewed-by: default avatarMike Kravetz <mike.kravetz@oracle.com>
Reviewed-by: default avatarDavid 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: default avatarAndrew Morton <akpm@linux-foundation.org>
parent f9bf6c03

tools/testing/selftests/vm/userfaultfd.c

+21 −1
Original line number Diff line number Diff line
@@ -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");