Re: [PATCH][RFC] ipc,fs: use rcu_work to free struct ipc_namespace

["Paul E. McKenney" <paulmck@kernel.org>]

On Thu, Feb 17, 2022 at 03:36:20PM -0500, Rik van Riel wrote:
> The patch works, but a cleanup question for Al Viro:
> 
> How do we get rid of #include "../fs/mount.h" and the raw ->mnt_ns = NULL thing
> in the cleanest way?
> 
> ---8<---
> Currently freeing ipc_namespace structures is done through a
> workqueue, with every single item on the queue waiting in
> synchronize_rcu before it is freed, limiting the rate at which
> ipc_namespace structures can be freed to something on the order
> of 100 a second.
> 
> Getting rid of that workqueue and just using rcu_work instead
> allows a whole batch of ipc_namespace frees to wait one single
> RCU grace period, after which they can all get freed quickly.
> 
> Without this patch, a test program that simply calls
> unshare(CLONE_NEWIPC) a million times in a loop eventually
> gets -ENOSPC as the total number of ipc_namespace structures
> exceeds the limit, due to slow freeing.
> 
> With this patch, the test program runs successfully every time.
> 
> Reported-by: Chris Mason <clm@fb.com>
> Signed-off-by: Rik van Riel <riel@surriel.com>

From an RCU perspective, with the ever-dangerous presumption that the
prior code was functionally correct:

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>

> ---
>  include/linux/ipc_namespace.h |  2 +-
>  ipc/namespace.c               | 30 ++++++++----------------------
>  2 files changed, 9 insertions(+), 23 deletions(-)
> 
> diff --git a/include/linux/ipc_namespace.h b/include/linux/ipc_namespace.h
> index b75395ec8d52..ee26fdbb2ce4 100644
> --- a/include/linux/ipc_namespace.h
> +++ b/include/linux/ipc_namespace.h
> @@ -67,7 +67,7 @@ struct ipc_namespace {
>  	struct user_namespace *user_ns;
>  	struct ucounts *ucounts;
>  
> -	struct llist_node mnt_llist;
> +	struct rcu_work free_rwork;
>  
>  	struct ns_common ns;
>  } __randomize_layout;
> diff --git a/ipc/namespace.c b/ipc/namespace.c
> index ae83f0f2651b..3d151bc5f723 100644
> --- a/ipc/namespace.c
> +++ b/ipc/namespace.c
> @@ -17,6 +17,7 @@
>  #include <linux/proc_ns.h>
>  #include <linux/sched/task.h>
>  
> +#include "../fs/mount.h"
>  #include "util.h"
>  
>  static struct ucounts *inc_ipc_namespaces(struct user_namespace *ns)
> @@ -115,12 +116,11 @@ void free_ipcs(struct ipc_namespace *ns, struct ipc_ids *ids,
>  	up_write(&ids->rwsem);
>  }
>  
> -static void free_ipc_ns(struct ipc_namespace *ns)
> +static void free_ipc_ns(struct work_struct *work)
>  {
> -	/* mq_put_mnt() waits for a grace period as kern_unmount()
> -	 * uses synchronize_rcu().
> -	 */
> -	mq_put_mnt(ns);
> +	struct ipc_namespace *ns = container_of(to_rcu_work(work),
> +				   struct ipc_namespace, free_rwork);
> +	mntput(ns->mq_mnt);
>  	sem_exit_ns(ns);
>  	msg_exit_ns(ns);
>  	shm_exit_ns(ns);
> @@ -131,21 +131,6 @@ static void free_ipc_ns(struct ipc_namespace *ns)
>  	kfree(ns);
>  }
>  
> -static LLIST_HEAD(free_ipc_list);
> -static void free_ipc(struct work_struct *unused)
> -{
> -	struct llist_node *node = llist_del_all(&free_ipc_list);
> -	struct ipc_namespace *n, *t;
> -
> -	llist_for_each_entry_safe(n, t, node, mnt_llist)
> -		free_ipc_ns(n);
> -}
> -
> -/*
> - * The work queue is used to avoid the cost of synchronize_rcu in kern_unmount.
> - */
> -static DECLARE_WORK(free_ipc_work, free_ipc);
> -
>  /*
>   * put_ipc_ns - drop a reference to an ipc namespace.
>   * @ns: the namespace to put
> @@ -166,10 +151,11 @@ void put_ipc_ns(struct ipc_namespace *ns)
>  {
>  	if (refcount_dec_and_lock(&ns->ns.count, &mq_lock)) {
>  		mq_clear_sbinfo(ns);
> +		real_mount(ns->mq_mnt)->mnt_ns = NULL;
>  		spin_unlock(&mq_lock);
>  
> -		if (llist_add(&ns->mnt_llist, &free_ipc_list))
> -			schedule_work(&free_ipc_work);
> +		INIT_RCU_WORK(&ns->free_rwork, free_ipc_ns);
> +		queue_rcu_work(system_wq, &ns->free_rwork);
>  	}
>  }
>  
> -- 
> 2.34.1
> 
>