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

From: <bugzilla-noreply_at_freebsd.org>
Date: Tue, 25 Apr 2023 01:08:18 UTC

--- Comment #10 from geoffrey@dommett.com ---
what you describe is not what happens running nfsv3

running nfsv3 the syncer seems to do this as long as the process does not exit
before it has finished, if it does the syncer stops, and all writes not yet
synced are permanently lost

you can't call fsync on a mapped file, mmap uses its own file handle. For
completeness, I have verified that calling fsync on the file handle makes no
difference. The writes are still lost.

surely msync with the flag MS_SYNC set should perform synchronous writes as per
the man page. It seems to for local file systems and nfsv2 mounts at least.
nfsv3 it does not.

I know linux had issues like this about 15 years ago

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