userfaultfd: change mmap_changing to atomic

Patch series "userfaultfd: minor bug fixes".

Three unrelated bug fixes. The first two addresses possible issues (not
too theoretical ones), but I did not encounter them in practice.

The third patch addresses a test bug that causes the test to fail on my
system. It has been sent before as part of a bigger RFC.

This patch (of 3):

mmap_changing is currently a boolean variable, which is set and cleared
without any lock that protects against concurrent modifications.

mmap_changing is supposed to mark whether userfaultfd page-faults handling
should be retried since mappings are undergoing a change.  However,
concurrent calls, for instance to madvise(MADV_DONTNEED), might cause
mmap_changing to be false, although the remove event was still not read
(hence acknowledged) by the user.

Change mmap_changing to atomic_t and increase/decrease appropriately.  Add
a debug assertion to see whether mmap_changing is negative.

Link: https://lkml.kernel.org/r/20210808020724.1022515-1-namit@vmware.com
Link: https://lkml.kernel.org/r/20210808020724.1022515-2-namit@vmware.com
Fixes: df2cc96e

 ("userfaultfd: prevent non-cooperative events vs mcopy_atomic races")
Signed-off-by: Nadav Amit <namit@vmware.com>
Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Cc: Peter Xu <peterx@redhat.com>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>