Re: [RFC PATCH linux-next] ns: do not allocate a new nsproxy at each call

[Eric W. Biederman]

"Guillaume Gaudonville" <gaudonville@xxxxxxxxx> writes:

> Currently, at each call of setns system call a new nsproxy is allocated,
> the old nsproxy namespaces are copied into the new one and the old nsproxy
> is freed if the task was the only one to use it.
>
> It can creates large delays on hardware with large number of cpus since
> to free a nsproxy a synchronize_rcu() call is done.
>
> When a task is the only one to use a nsproxy, only the task can do an action
> that will make this nsproxy to be shared by another task or thread (fork,...).
> So when the refcount of the nsproxy is equal to 1, we can simply update the
> current nsproxy field without allocating a new one and freeing the old one.
>
> The install operations of each kind of namespace cannot fails, so there's no
> need to check for an error and calling ops->install().
>
> Tested on TileGX (36 cores) and Intel (32 cores).

This may be worth doing (I am a little scared of a design that has setns
on a fast path) but right now this isn't safe.

Currently pidns_install ends with:

	put_pid_ns(nsproxy->pid_ns_for_children);
	nsproxy->pid_ns_for_children = get_pid_ns(new);
	return 0;


And netns_install ends with:

	put_net(nsproxy->net_ns);
	nsproxy->net_ns = get_net(net);
	return 0;

The put before the set is not atomic and is not safe unless
the nsproxy is private.  I think this is fixable but it requires a more
indepth look at the code than you have done.

Mind if I ask where this comes up?


> Reported-by: Chris Metcalf <cmetcalf@xxxxxxxxxx>
> Signed-off-by: Guillaume Gaudonville <guillaume.gaudonville@xxxxxxxxx>
> ---
>  kernel/nsproxy.c |   12 ++++++++++++
>  1 files changed, 12 insertions(+), 0 deletions(-)
>
> diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
> index afc0456..afc04ac 100644
> --- a/kernel/nsproxy.c
> +++ b/kernel/nsproxy.c
> @@ -255,6 +255,18 @@ SYSCALL_DEFINE2(setns, int, fd, int, nstype)
>  	if (nstype && (ops->type != nstype))
>  		goto out;
>  
> +	/*
> +	 * If count == 1, only the current task can increment it,
> +	 * by doing a fork for example so we can safely update the
> +	 * current nsproxy pointers without allocate a new one,
> +	 * update it and destroy the old one
> +	 */
> +	if (atomic_read(&tsk->nsproxy->count) == 1) {
> +		err = ops->install(tsk->nsproxy, ei->ns);
> +		fput(file);
> +		return err;
> +	}

As a minor nit, but to match the rest of the code in this function that
should read:

> +	if (atomic_read(&tsk->nsproxy->count) == 1) {
> +		err = ops->install(tsk->nsproxy, ei->ns);
> +		goto out;
> +	}

There is no need to add an additional exit point to reason about.

> +
>  	new_nsproxy = create_new_namespaces(0, tsk, current_user_ns(), tsk->fs);
>  	if (IS_ERR(new_nsproxy)) {
>  		err = PTR_ERR(new_nsproxy);
--
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