Presumably if "user space isn't involved at all", it must require that
user space has granted caching rights to the kernel over a FUSE
cache coherency protocol?
Otherwise I don't see how the kernel could coherently cache file pages
for some kinds of FUSE filesystems. (E.g. sshfs, for example: every
operation must surely invoke a user space request or involve granting
a caching right to the kernel, to keep accesses coherent with other
users of the same remote files).
Ergo, either its not coherent, or there is some coherency protocol,
which does require _some_ work in the user space implementation.
-- Jamie
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