Re: [PATCH 2/3] namespace: rework connected mounts

[Al Viro <viro@zeniv.linux.org.uk>]

On Fri, Oct 02, 2026 at 04:14:28PM +0200, Christian Brauner wrote:

> +/*
> + * What an unmounted mount leaves behind at its unmounted parent instead of
> + * staying attached to it. A lookup on the parent at the mountpoint finds
> + * a stand-in for as long as the cover is there. The parent frees it.
> + */
> +struct mnt_cover {
> +	struct hlist_node node;		/* parent->mnt_covers, RCU */
> +	struct hlist_node pin;		/* mp->m_covers, keeps the mountpoint */

Is there any point for that to be an hlist?  I mean, you only ever add to
the head, walk it only forward and remove them one by one as you walk...
What's wrong with plain struct mnt_cover *next; instead?

> +	struct dentry *dentry;
> +	struct mountpoint *mp;
> +	struct rcu_head rcu;
> +};

... and seeing that the only place where you have a lockless traversal of
->node is only looking at ->dentry, it might make sense to make that
	struct hlist_node node;		/* parent->mnt_covers, RCU */
	struct dentry *dentry;
	union {
		struct {	// never used by lockless readers
			struct mnt_cover *next;
			struct mountpoint *mp;
		}
		struct rcu_head rcu;
	}
perhaps?