V2: Reduce mmap_sem hold times during file backed page faults

This is the second iteration of our change dropping mmap_sem when a disk
access occurs during a page fault to a file backed VMA.

Changes since V1:
- Cleaned up 'Retry page fault when blocking on disk transfer' applying
  linus's suggestions
- Added 'access_error API cleanup'

Tests:

- microbenchmark: thread A mmaps a large file and does random read accesses
  to the mmaped area - achieves about 55 iterations/s. Thread B does
  mmap/munmap in a loop at a separate location - achieves 55 iterations/s
  before, 15000 iterations/s after.
- We are seeing related effects in some applications in house, which show
  significant performance regressions when running without this change.
- I am looking for a microbenchmark to expose the worst case overhead of
  the page fault retry. Would FIO be a good match for that use ?

Michel Lespinasse (3):
  filemap_fault: unique path for locking page
  Retry page fault when blocking on disk transfer.
  access_error API cleanup

 arch/x86/mm/fault.c |   44 +++++++++++++++++++++++++++++---------------
 include/linux/mm.h  |    2 ++
 mm/filemap.c        |   41 ++++++++++++++++++++++++++++++++---------
 mm/memory.c         |    3 ++-
 4 files changed, 65 insertions(+), 25 deletions(-)
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