[PATCH 0/3] NFS/localio: issue IO inline when not reclaiming memory

[Mike Snitzer]

Hi,

Every NFS LOCALIO read, write and commit is currently issued indirectly: it
is queued onto the dedicated !WQ_MEM_RECLAIM nfslocaliod_workqueue rather
than being submitted to the underlying filesystem in the calling context.
That intermediate hop was introduced by commit b9f5dd57f4a5 ("nfs/localio:
use dedicated workqueues for filesystem read and write") because:
LOCALIO submits IO directly into a stacked local filesystem (e.g. XFS)
which may in turn flush its own !WQ_MEM_RECLAIM workqueue. Doing so from a
WQ_MEM_RECLAIM worker -- most importantly writeback's wb_workfn running on
bdi_wq -- or from an explicit PF_MEMALLOC reclaim task trips
check_flush_dependency() and risks a forward-progress deadlock.

However, that hazard only exists when the submitting context is itself a
memory-reclaim context.  For ordinary application/task submission -- e.g.
O_DIRECT, or an fsync-driven commit -- the workqueue hop buys nothing: it
just adds a context switch and scheduling latency per IO and throws away the
NFS client's inherent application-context parallelism.

This series makes the hop conditional.  It adds a small workqueue-core
helper, current_is_workqueue_mem_reclaim(), that reports whether %current is
a WQ_MEM_RECLAIM worker using exactly the predicate check_flush_dependency()
warns on.  LOCALIO uses it (together with the existing PF_MEMALLOC test) in a
new nfs_local_defer_io() helper to decide per-IO whether it must defer to
nfslocaliod_workqueue or may issue the IO inline.  Reclaim contexts still
defer and are unaffected; everything else runs inline.

  Patch 1 adds current_is_workqueue_mem_reclaim() and applies the gating to
          the read and write paths.
  Patch 2 removes never-taken FLUSH_SYNC handling from nfs_local_commit()
          (every caller supplies a FLUSH_SYNC-stripped "how"), which also
          drops the sole user of the ctx->done completion plumbing and the
          now-unused "how" argument.  No functional change.
  Patch 3 extends the same gating to the commit (fsync) path.  Note the
          writeback-triggered commit does reach here in reclaim context --
          nfs_write_inode() (the ->write_inode super_op) runs under wb_workfn
          on the WQ_MEM_RECLAIM bdi_wq -- so that case correctly keeps
          deferring; only app-context commits run inline.

Patch 1 touches the workqueue core (kernel/workqueue.c, include/linux/
workqueue.h), hence the Cc to Tejun Heo, Lai Jiangshan and LKML.  The new
export mirrors the existing current_is_workqueue_rescuer()/current_work()
context-introspection helpers.

Additional note for reviewers:
 - The inline path now stacks the NFS pgio path plus the underlying
   filesystem's ->write_iter/->read_iter (and vfs_fsync_range) in task
   context.  The reclaim-context stack-depth concern of b9f5dd57f4a5 (2) is
   avoided for the deferred paths; feedback on inline stack headroom under
   deeply-stacking filesystems (XFS especially) is welcome.

All review appreciated, thanks.
Mike

Mike Snitzer (3):
  NFS/localio: issue IO inline when not in a memory-reclaim context
  NFS/localio: remove dead FLUSH_SYNC handling from nfs_local_commit
  NFS/localio: issue commit inline when not in a memory-reclaim context

 fs/nfs/internal.h         |  4 +--
 fs/nfs/localio.c          | 56 ++++++++++++++++++++++++++++++---------
 fs/nfs/write.c            |  2 +-
 include/linux/workqueue.h |  1 +
 kernel/workqueue.c        | 24 +++++++++++++++++
 5 files changed, 71 insertions(+), 16 deletions(-)

-- 
2.44.0