Re: mount api: Q on behavior of mount_single vs. get_tree_single

[Christian Brauner <brauner@kernel.org>]

On Thu, Jan 16, 2025 at 09:38:07AM -0600, Eric Sandeen wrote:
> On 1/16/25 4:28 AM, Christian Brauner wrote:
> > On Wed, Jan 15, 2025 at 10:50:31AM -0600, Eric Sandeen wrote:
> >> I was finally getting back to some more mount API conversions,
> >> and was looking at pstore, which I thought would be straightforward.
> >>
> >> I noticed that mount options weren't being accepted, and I'm fairly
> >> sure that this is because there's some internal or hidden mount of
> >> pstore, and mount is actually a remount due to it using a single
> >> superblock. (looks like it was some sort of clone mount done under
> > 
> > Yes, some legacy filesystems behave that way unforunately.
> 
> Is it not the case that every current (or past) user of mount_single
> behave[sd] this way due to the internals of mount_single()?
> 
> gadgetfs, configfs, debugfs, tracefs, efivars, openpromfs and more all
> currently call get_tree_single (and used to call mount_single, which did
> the reconfigure for them).
> 
> Or am I missing something?
> 
> ...
> 
> > I would think we should make this the filesystems problem or add a
> > get_tree_single_reconfigure() helper that the few filesystems that need
> > this behavior can use. Thoughts?
> 
> I wasn't seeing this as individual filesystems needing it, it looked to
> me like every filesystem that used to call mount_single() automatically
> got the behavior. So was looking at this from a consistency/regression
> POV.
> 
> While it seems odd to me that mounting any "_single" sb filesystem would
> reconfigure all of its current mounts on a new mount, that's the way it
> used to work for everyone, as far as I can tell, so I was assuming that
> we'd need to keep the behavior. But if it's a question of just fixing it
> up per-filesystem, I don't mind going that route either.

So far it hasn't lead to any regressions and we had the new behavior for
quite a while.