RFC userfaultfd: add write protect support

Hi,

There is plan to support write protect fault into userfaultfd before, but it's not implemented yet. I'm working on a library to support different types of buffer like compressed buffer and file buffer, something like a page cache implementation in userspace. The buffer enables userfaultfd and does something like decompression in userfault handler. When memory size exceeds a threshold, madvise is used to reclaim memory. The problem is data can be corrupted in reclaim without memory protection support.

For example, in the compressed buffer case, reclaim does:
1. compress memory range and store compressed data elsewhere
2. madvise the memory range

But if the memory is changed before 2, new change is lost. memory write protection can solve the issue. With it, the reclaim does:
1. write protect memory range
2. compress memory range and store compressed data elsewhere
3. madvise the memory range
4. undo write protect memory range and wakeup tasks waiting in write protect fault.
If a task changes memory before 3, write protect fault will be triggered. we can put the task into sleep till step 4 runs for example. In this way memory changes will not be lost.

This patch set add write protect support for userfaultfd. One issue is write protect fault can happen even without enabling write protect in userfault. For example, a write to address backed by zero page. There is no way to distinguish if this is a write protect fault expected by userfault. This patch just blindly triggers write protect fault to userfault if corresponding vma enables VM_UFFD_WP. Application should be prepared to handle such write protect fault.

Thanks,
Shaohua

Shaohua Li (8):
userfaultfd: add helper for writeprotect check
userfaultfd: support write protection for userfault vma range
userfaultfd: expose writeprotect API to ioctl
userfaultfd: allow userfaultfd register success with writeprotection
userfaultfd: undo write proctection in unregister
userfaultfd: hook userfault handler to write protection fault
userfaultfd: fault try one more time
userfaultfd: enabled write protection in userfaultfd API

 arch/alpha/mm/fault.c            |  8 ++++-
 arch/arc/mm/fault.c              |  8 ++++-
 arch/arm/mm/fault.c              |  8 ++++-
 arch/arm64/mm/fault.c            |  8 ++++-
 arch/avr32/mm/fault.c            |  8 ++++-
 arch/cris/mm/fault.c             |  8 ++++-
 arch/hexagon/mm/vm_fault.c       |  8 ++++-
 arch/ia64/mm/fault.c             |  8 ++++-
 arch/m68k/mm/fault.c             |  8 ++++-
 arch/metag/mm/fault.c            |  8 ++++-
 arch/microblaze/mm/fault.c       |  8 ++++-
 arch/mips/mm/fault.c             |  8 ++++-
 arch/mn10300/mm/fault.c          |  8 ++++-
 arch/nios2/mm/fault.c            |  8 ++++-
 arch/openrisc/mm/fault.c         |  8 ++++-
 arch/parisc/mm/fault.c           |  8 ++++-
 arch/powerpc/mm/fault.c          |  8 ++++-
 arch/s390/mm/fault.c             |  9 +++++-
 arch/sh/mm/fault.c               |  8 ++++-
 arch/sparc/mm/fault_32.c         |  8 ++++-
 arch/sparc/mm/fault_64.c         |  8 ++++-
 arch/tile/mm/fault.c             |  8 ++++-
 arch/um/kernel/trap.c            |  8 ++++-
 arch/unicore32/mm/fault.c        |  8 ++++-
 arch/x86/mm/fault.c              |  9 +++++-
 arch/xtensa/mm/fault.c           |  8 ++++-
 fs/userfaultfd.c                 | 66 ++++++++++++++++++++++++++++++++++------
 include/linux/mm.h               |  3 +-
 include/linux/userfaultfd_k.h    | 12 ++++++++
 include/uapi/linux/userfaultfd.h | 17 +++++++++--
 mm/memory.c                      | 66 +++++++++++++++++++++++++++++-----------
 mm/userfaultfd.c                 | 52 +++++++++++++++++++++++++++++++
 32 files changed, 369 insertions(+), 57 deletions(-)

2.4.6

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