userfaultfd: support access/write hints

From: Nadav Amit <[email protected]>

Setting the access-bit and dirty-bit introduces a tradeoff. When the bit is set access/write is faster, but memory reclamation might be slower. Currently, in the common userfaultfd cases the access-bit is not set on and the dirty-bit is set. This is a questionable behavior.

Allow userspace to control this behavior through hints access- and write-likely hints. These hints are used to control access- and dirty-bits. For zero-pages that with write-likely hint, allocate a clear page instead of mapping the zero-page.

RFCv2 -> v1:
* Adding hints to zeropage and continue
* Fixing other issues pointed by David H. & Peter Xu
* Adding tests to ./run_vmtests.sh

Nadav Amit (5):
userfaultfd: introduce uffd_flags
userfaultfd: introduce access-likely mode for common operations
userfaultfd: introduce write-likely mode for uffd operations
userfaultfd: zero access/write hints selftest/userfaultfd: test read/write hints

 fs/userfaultfd.c                          |  56 +++++++++--
 include/linux/hugetlb.h                   |   4 +-
 include/linux/shmem_fs.h                  |   8 +-
 include/linux/userfaultfd_k.h             |  26 +++--
 include/uapi/linux/userfaultfd.h          |  31 +++++-
 mm/hugetlb.c                              |   6 +-
 mm/shmem.c                                |   9 +-
 mm/userfaultfd.c                          | 117 ++++++++++++++++------
 tools/testing/selftests/vm/run_vmtests.sh |  23 ++---
 tools/testing/selftests/vm/userfaultfd.c  |  32 ++++++
 10 files changed, 239 insertions(+), 73 deletions(-)

2.25.1