linux-kernel - Re: [PATCH v2 18/33] mnt: support ns lookup

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Message-ID: <20250916035633.GM39973@ZenIV>
Date: Tue, 16 Sep 2025 04:56:33 +0100
From: Al Viro <viro@...iv.linux.org.uk>
To: Christian Brauner <brauner@...nel.org>
Cc: Jan Kara <jack@...e.cz>, Amir Goldstein <amir73il@...il.com>,
	linux-fsdevel@...r.kernel.org, Josef Bacik <josef@...icpanda.com>,
	Jeff Layton <jlayton@...nel.org>, Mike Yuan <me@...dnzj.com>,
	Zbigniew Jędrzejewski-Szmek <zbyszek@...waw.pl>,
	Lennart Poettering <mzxreary@...inter.de>,
	Daan De Meyer <daan.j.demeyer@...il.com>,
	Aleksa Sarai <cyphar@...har.com>, Jens Axboe <axboe@...nel.dk>,
	Tejun Heo <tj@...nel.org>, Johannes Weiner <hannes@...xchg.org>,
	Michal Koutný <mkoutny@...e.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
	Simon Horman <horms@...nel.org>,
	Chuck Lever <chuck.lever@...cle.com>, linux-nfs@...r.kernel.org,
	linux-kselftest@...r.kernel.org, linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH v2 18/33] mnt: support ns lookup

On Fri, Sep 12, 2025 at 01:52:41PM +0200, Christian Brauner wrote:
> Move the mount namespace to the generic ns lookup infrastructure.
> This allows us to drop a bunch of members from struct mnt_namespace.

>  static void mnt_ns_tree_remove(struct mnt_namespace *ns)
>  {
>  	/* remove from global mount namespace list */
> -	if (!is_anon_ns(ns)) {
> -		mnt_ns_tree_write_lock();
> -		rb_erase(&ns->mnt_ns_tree_node, &mnt_ns_tree);
> -		list_bidir_del_rcu(&ns->mnt_ns_list);
> -		mnt_ns_tree_write_unlock();
> -	}
> -
> -	call_rcu(&ns->mnt_ns_rcu, mnt_ns_release_rcu);
> -}
> -
> -static int mnt_ns_find(const void *key, const struct rb_node *node)
> -{
> -	const u64 mnt_ns_id = *(u64 *)key;
> -	const struct mnt_namespace *ns = node_to_mnt_ns(node);
> +	if (!is_anon_ns(ns))
> +		ns_tree_remove(ns);

Conflicts with "mnt_ns_tree_remove(): DTRT if mnt_ns had never been
added to mnt_ns_list".

FWIW, the right thing to do here is

	if (!RB_EMPTY_NODE(to_ns_common(ns)->ns_tree_node))
		ns_tree_remove(ns);