Re: RFC: checking file systems support UF_HIDDEN, UF_SYSTEM

On Thu, Jul 3, 2025 at 4:01 AM Olivier Certner <olce@freebsd.org> wrote:
>
> > It is not the right usage of the MNT_ flags. They are for feature
> > controlling, not for the feature reporting.
>
> That is not the case today (MNT_LOCAL, MNT_QUOTA, MNT_ROOTFS, MNT_USER, etc.). I also would like this to become true at some point.
>
> > VOP_PATHCONF() is exactly for reporting some mnt features.
>
> No, it is to report features that depend on a path.  It can be (ab)used to report mount features (calling it on any file inside the mount), but that obfuscates the fact that an information is per-mount, whereas statfs()/statvfs() are exactly designed for per-mount information.
Note that, for a NFSv4 client mount, support for hidden/system could
change somewhere in
the mounted server tree, since for NFSv4 the tree can consist of multiple file
systems of different types. For example:
If an ext2 fs was mounted under a ufs file system, the hidden/system support
would go away when the mount point is crossed into the ext2 subtree.

So, I think pathconf is the best way to go (and avoids using up flag bits).

rick

>
> > Also, this namespace should not be used frivolously, we already had to
> > extend flags to 64bit, and again we are not too far from exhausting it.
>
> I don't disagree.  That said, at some point, we'll have to dodge the bullet, including separating control flags from report ones in the MNT_* namespace (and provide compat' functionality), and probably extending control flags with a second 64-bit field.
>
> I suspected from the start that that was the main reason behind you recommending VOP_PATHCONF().  My point here is that it is a slightly non-optimal design choice (as explained just above) that will stay and was made out of a practical issue that anyway will have to be solved at some point, and I find this a bit unfortunate.  Solving the issue now is certainly much more work, and in any case I didn't mean to object to not doing it now.  But if someone(tm) wants to do it now, that would be great (perhaps me, but not sure yet).
>
> Regards.
>
> --
> Olivier Certner