PUFFS_KFLAG_WTCACHE. Second, create separate fids for reading and
writing. If opening for read, open a read-only fid and for write
a write-only fid; use these for reading and writing. When the
open-count for a node drops to zero, clunk both. This avoids hitting
the fid limit when accessing large directory hierarchies.
Two problems remain:
* does not take credentials into account, although we can only mount
the remote 9P file server with one set of credentials, so not a
huge worry
* doesn't work for the open/mmap/close/access_memory_window case, but
that will require some further kernel changes
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