RFC mm: some enhancements to the page fault mechanism

v3:
- fix up issues that krobot reported, rebase

This is an RFC series as cleanup and enhancements to current page fault logic. The whole idea comes from the discussion between Andrea and Linus on the bug reported by syzbot here:

https://lkml.org/lkml/2017/11/2/833

Basically it does two things:

(a) Allows the page fault logic to be more interactive on not only SIGKILL, but also the rest of userspace signals, and,

(b) Allows the page fault retry (VM_FAULT_RETRY) to happen for more than once.

For (a): with the changes we should be able to react faster when page faults are working in parallel with userspace signals like SIGSTOP and SIGCONT (and more), and with that we can remove the buggy part in userfaultfd and benefit the whole page fault mechanism on faster signal processing to reach the userspace.

For (b), we should be able to allow the page fault handler to loop for even more than twice. Some context: for now since we have FAULT_FLAG_ALLOW_RETRY we can allow to retry the page fault once with the same interrupt context, however never more than twice. This can be not only a potential cleanup to remove this assumption since AFAIU the code itself doesn't really have this twice-only limitation (though that should be a protective approach in the past), at the same time it'll greatly simplify future works like userfaultfd write-protect where it's possible to retry for more than twice (please have a look at [1] below for a possible user that might require the page fault to be handled for a third time; if we can remove the retry limitation we can simply drop that patch and those complexity).

Some more details on each of the patch (even more in commit messages):

Patch 1: A cleanup of existing GUP code to rename the confusing
"nonblocking" parameter to "locked" which seems suite more.

Patch 2: Complete the page fault faster for non-sigkill signals

Patch 3: Remove the limitation to only allow to retry page fault for
twice (page fault part)

Patch 4: Similar work of patch 3, but for GUP.

The series is only lightly tested. Before running more tests, I'd be really glad to see whether there's any feedback first.

Looking forward to your comments. Thanks,

[1] https://git.kernel.org/pub/scm/linux/kernel/git/andrea/aa.git/commit/?h=userfault&id=b245ecf6cf59156966f3da6e6b674f6695a5ffa5

Peter Xu (4):
mm: gup: rename "nonblocking" to "locked" where proper
mm: userfault: return VM_FAULT_RETRY on signals
mm: allow VM_FAULT_RETRY for multiple times
mm: gup: allow VM_FAULT_RETRY for multiple times

 arch/alpha/mm/fault.c      |  4 +--
 arch/arc/mm/fault.c        | 12 ++++----
 arch/arm/mm/fault.c        | 17 ++++++-----
 arch/arm64/mm/fault.c      | 11 ++-----
 arch/hexagon/mm/vm_fault.c |  3 +-
 arch/ia64/mm/fault.c       |  3 +-
 arch/m68k/mm/fault.c       |  5 +---
 arch/microblaze/mm/fault.c |  3 +-
 arch/mips/mm/fault.c       |  3 +-
 arch/nds32/mm/fault.c      |  7 ++---
 arch/nios2/mm/fault.c      |  5 +---
 arch/openrisc/mm/fault.c   |  3 +-
 arch/parisc/mm/fault.c     |  4 +--
 arch/powerpc/mm/fault.c    |  9 ++----
 arch/riscv/mm/fault.c      |  9 ++----
 arch/s390/mm/fault.c       | 14 ++++-----
 arch/sh/mm/fault.c         |  5 +++-
 arch/sparc/mm/fault_32.c   |  4 ++-
 arch/sparc/mm/fault_64.c   |  4 ++-
 arch/um/kernel/trap.c      |  6 ++--
 arch/unicore32/mm/fault.c  | 10 ++-----
 arch/x86/mm/fault.c        | 13 ++++++--
 arch/xtensa/mm/fault.c     |  4 ++-
 fs/userfaultfd.c           | 24 ---------------
 mm/gup.c                   | 61 +++++++++++++++++++++-----------------
 mm/hugetlb.c               |  8 ++---
 26 files changed, 114 insertions(+), 137 deletions(-)

2.17.1