diff options
| author | Peter Xu <peterx@redhat.com> | 2022-10-04 22:34:00 +0300 |
|---|---|---|
| committer | Andrew Morton <akpm@linux-foundation.org> | 2022-10-13 04:51:50 +0300 |
| commit | 26c92d37d3dc484157bdb4eb7d29991c017b168b (patch) | |
| tree | 8315ac0af29354c34602ad71f21948f1b53b50da /tools/testing/selftests/vm | |
| parent | f9bf6c03eca1077cae8de0e6d86427656fa42a9b (diff) | |
| download | linux-26c92d37d3dc484157bdb4eb7d29991c017b168b.tar.xz | |
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>
Diffstat (limited to 'tools/testing/selftests/vm')
| -rw-r--r-- | tools/testing/selftests/vm/userfaultfd.c | 22 |
1 files changed, 21 insertions, 1 deletions
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"); |