mount api: Q on behavior of mount_single vs. get_tree_single

[Eric Sandeen <sandeen@redhat.com>]

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
the covers by various processes, still not sure.)

In any case, that led me to wonder:

Should get_tree_single() be doing an explicit reconfigure like
mount_single does?

mount_single() {
...
        if (!s->s_root) {
                error = fill_super(s, data, flags & SB_SILENT ? 1 : 0);
                if (!error)
                        s->s_flags |= SB_ACTIVE;
        } else {
                error = reconfigure_single(s, flags, data);
        }
...

My pstore problem abovec reminded me of the recent issue with tracefs
after the mount api conversion, fixed with:

e4d32142d1de tracing: Fix tracefs mount options

and discussed at:

https://lore.kernel.org/lkml/20241030171928.4168869-2-kaleshsingh@google.com/

which in turn reminded me of:

a6097180d884 devtmpfs regression fix: reconfigure on each mount

so we've seen this difference in behavior with get_tree_single twice already,
and then I ran into it again on pstore.

Should get_tree_single() callers be fixing this up themselves ala devtmpfs
and tracefs, or should get_tree_single() be handling this internally?

Right now in my pstore patch I'm doing:

static int pstore_get_tree(struct fs_context *fc)
{
       if (fc->root)
               return pstore_reconfigure(fc);

       return get_tree_single(fc, pstore_fill_super);
}

but it really feels like this should be handled by core code instead.

Thoughts?

Thanks,
-Eric