[PATCH 0/5] userfaultfd shmem updates

[Andrea Arcangeli]

Hello,

Jann found two bugs in the userfaultfd shmem MAP_SHARED backend: the
lack of the VM_MAYWRITE check and the lack of i_size checks.

Then looking into the above we also fixed the MAP_PRIVATE case.

Hugh by source review also found a data loss source if UFFDIO_COPY is
used on shmem MAP_SHARED PROT_READ mappings (the production usages
incidentally run with PROT_READ|PROT_WRITE, so the data loss couldn't
happen in those production usages like with QEMU).

The whole patchset is marked for stable.

We verified QEMU postcopy live migration with guest running on shmem
MAP_PRIVATE run as well as before after the fix of shmem
MAP_PRIVATE. Regardless if it's shmem or hugetlbfs or MAP_PRIVATE or
MAP_SHARED, QEMU unconditionally invokes a punch hole if the guest
mapping is filebacked and a MADV_DONTNEED too (needed to get rid of
the MAP_PRIVATE COWs and for the anon backend).

Thank you,
Andrea

Andrea Arcangeli (5):
  userfaultfd: use ENOENT instead of EFAULT if the atomic copy user
    fails
  userfaultfd: shmem: allocate anonymous memory for MAP_PRIVATE shmem
  userfaultfd: shmem/hugetlbfs: only allow to register VM_MAYWRITE vmas
  userfaultfd: shmem: add i_size checks
  userfaultfd: shmem: UFFDIO_COPY: set the page dirty if VM_WRITE is not
    set

 fs/userfaultfd.c | 15 ++++++++++++
 mm/hugetlb.c     |  2 +-
 mm/shmem.c       | 31 +++++++++++++++++++++---
 mm/userfaultfd.c | 62 +++++++++++++++++++++++++++++++++++-------------
 4 files changed, 90 insertions(+), 20 deletions(-)