Repair SWAP-over_NFS

This 4th version of the series address review comment, particularly
tidying up "NFS: swap IO handling is slightly different for O_DIRECT IO"
and collect reviewed-by etc.

I've also move 3 NFS patches which depend on the MM patches to the end
in case they helps maintainers land the patches in a consistent order.
Those three patches might go through the NFS free after the next merge
window.

Original intro follows.
Thanks,
NeilBrown

swap-over-NFS currently has a variety of problems.

swap writes call generic_write_checks(), which always fails on a swap
file, so it completely fails.
Even without this, various deadlocks are possible - largely due to
improvements in NFS memory allocation (using NOFS instead of ATOMIC)
which weren't tested against swap-out.

NFS is the only filesystem that has supported fs-based swap IO, and it
hasn't worked for several releases, so now is a convenient time to clean
up the swap-via-filesystem interfaces - we cannot break anything !

So the first few patches here clean up and improve various parts of the
swap-via-filesystem code.  ->activate_swap() is given a cleaner
interface, a new ->swap_rw is introduced instead of burdening
->direct_IO, etc.

Current swap-to-filesystem code only ever submits single-page reads and
writes.  These patches change that to allow multi-page IO when adjacent
requests are submitted.  Writes are also changed to be async rather than
sync.  This substantially speeds up write throughput for swap-over-NFS.

---

NeilBrown (21):
 MM - in merge winow
      MM: create new mm/swap.h header file.
      MM: drop swap_set_page_dirty
      MM: move responsibility for setting SWP_FS_OPS to ->swap_activate
      MM: reclaim mustn't enter FS for SWP_FS_OPS swap-space
      MM: introduce ->swap_rw and use it for reads from SWP_FS_OPS swap-space
      MM: perform async writes to SWP_FS_OPS swap-space using ->swap_rw
      DOC: update documentation for swap_activate and swap_rw
      MM: submit multipage reads for SWP_FS_OPS swap-space
      MM: submit multipage write for SWP_FS_OPS swap-space
      VFS: Add FMODE_CAN_ODIRECT file flag

 NFS - in merge window
      NFS: remove IS_SWAPFILE hack
      SUNRPC/call_alloc: async tasks mustn't block waiting for memory
      SUNRPC/auth: async tasks mustn't block waiting for memory
      SUNRPC/xprt: async tasks mustn't block waiting for memory
      SUNRPC: remove scheduling boost for "SWAPPER" tasks.
      NFS: discard NFS_RPC_SWAPFLAGS and RPC_TASK_ROOTCREDS
      SUNRPC: improve 'swap' handling: scheduling and PF_MEMALLOC
      NFSv4: keep state manager thread active if swap is enabled

 NFS - after merge window
      NFS: rename nfs_direct_IO and use as ->swap_rw
      NFS: swap IO handling is slightly different for O_DIRECT IO
      NFS: swap-out must always use STABLE writes.


 Documentation/filesystems/locking.rst |  18 +-
 Documentation/filesystems/vfs.rst     |  17 +-
 drivers/block/loop.c                  |   4 +-
 fs/cifs/file.c                        |   7 +-
 fs/fcntl.c                            |   9 +-
 fs/nfs/direct.c                       |  67 +++++---
 fs/nfs/file.c                         |  39 +++--
 fs/nfs/nfs4_fs.h                      |   1 +
 fs/nfs/nfs4proc.c                     |  20 +++
 fs/nfs/nfs4state.c                    |  39 ++++-
 fs/nfs/read.c                         |   4 -
 fs/nfs/write.c                        |   2 +
 fs/open.c                             |   9 +-
 fs/overlayfs/file.c                   |  13 +-
 include/linux/fs.h                    |   4 +
 include/linux/nfs_fs.h                |  15 +-
 include/linux/nfs_xdr.h               |   2 +
 include/linux/sunrpc/auth.h           |   1 +
 include/linux/sunrpc/sched.h          |   1 -
 include/linux/swap.h                  |   7 +-
 include/linux/writeback.h             |   7 +
 include/trace/events/sunrpc.h         |   1 -
 mm/madvise.c                          |   8 +-
 mm/memory.c                           |   2 +-
 mm/page_io.c                          | 237 +++++++++++++++++++-------
 mm/swap.h                             |  30 +++-
 mm/swap_state.c                       |  22 ++-
 mm/swapfile.c                         |  13 +-
 mm/vmscan.c                           |  38 +++--
 net/sunrpc/auth.c                     |   8 +-
 net/sunrpc/auth_gss/auth_gss.c        |   6 +-
 net/sunrpc/auth_unix.c                |  10 +-
 net/sunrpc/clnt.c                     |   7 +-
 net/sunrpc/sched.c                    |  29 ++--
 net/sunrpc/xprt.c                     |  19 +--
 net/sunrpc/xprtrdma/transport.c       |  10 +-
 net/sunrpc/xprtsock.c                 |   8 +
 37 files changed, 519 insertions(+), 215 deletions(-)

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