[Bug 270810] munmap does not always sync the underlying file

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=270810

--- Comment #3 from Konstantin Belousov <kib@FreeBSD.org> ---
NFS is not POSIX compliant, for many reasons besides mmap.  It is inherent in
the protocol.  This is not going to change.

FWIW, try to think how could reliable write-back from client to server on mmap
could be implemented?  System has the cache on client, and the data on server.
You need to implement coherency for async userspace writes to the cached pages
which are invisible to OS until it actively queries for dirty page status.

The only known reliable way is to make each write fault to detect it.  Then you
will complain about performance being 100x times worse than now.

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