[Bug 273942] fusefs(5): read operation changes ctime on FUSE filesystems

From: <bugzilla-noreply_at_freebsd.org>
Date: Tue, 19 Sep 2023 20:54:44 UTC
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=273942

--- Comment #7 from Alan Somers <asomers@FreeBSD.org> ---
When a FUSE file system uses the data cache, then the FUSE daemon cannot handle
atime updates itself, because a read may be serviced by cache.  In such a case,
the OS must issue the atime updates.

If the data cache is not enabled, but atime and the attribute cache are, then
the OS has a choice.  Either it can track atime updates itself, or it can ask
the server, which causes a new FUSE_GETATTR request after every read.  That
kind of defeats the purpose of the attribute cache.

It seems to me that if the OS sends a FUSE_SETATTR with the valid field set to
FATTR_ATIME, then the server should not update the CTIME.  Do you disagree? 
Fir if the atime update actually came from userland, for example from
utimensat(), then the FATTR_CTIME bit will be set, too.

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