From:   Giuseppe Scrivano <gscrivan@redhat.com>
To:     Rik van Riel <riel@surriel.com>
Cc:     viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org,
        kernel-team@meta.com, linux-fsdevel@vger.kernel.org,
        Chris Mason <clm@meta.com>
Subject: Re: [PATCH 2/2] ipc,namespace: batch free ipc_namespace structures
References: <20230127011535.1265297-1-riel@surriel.com>
        <20230127011535.1265297-3-riel@surriel.com>
Date:   Fri, 27 Jan 2023 12:03:10 +0100
In-Reply-To: <20230127011535.1265297-3-riel@surriel.com> (Rik van Riel's
        message of "Thu, 26 Jan 2023 20:15:35 -0500")
Message-ID: <878rhome8h.fsf@redhat.com>
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Rik van Riel <riel@surriel.com> writes:

> Instead of waiting for an RCU grace period between each ipc_namespace
> structure that is being freed, wait an RCU grace period for every batch
> of ipc_namespace structures.
>
> Thanks to Al Viro for the suggestion of the helper function.
>
> This speeds up the run time of the test case that allocates ipc_namespaces
> in a loop from 6 minutes, to a little over 1 second:
>
> real	0m1.192s
> user	0m0.038s
> sys	0m1.152s
>
> Signed-off-by: Rik van Riel <riel@surriel.com>
> Reported-by: Chris Mason <clm@meta.com>
> Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
> ---
>  fs/namespace.c        | 10 ++++++++++
>  include/linux/mount.h |  1 +
>  ipc/namespace.c       | 13 ++++++++++---
>  3 files changed, 21 insertions(+), 3 deletions(-)
>
> diff --git a/fs/namespace.c b/fs/namespace.c
> index ab467ee58341..296432ba3716 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -1397,6 +1397,16 @@ struct vfsmount *mntget(struct vfsmount *mnt)
>  }
>  EXPORT_SYMBOL(mntget);
>  
> +/*
> + * Make a mount point inaccessible to new lookups.
> + * Because there may still be current users, the caller MUST WAIT
> + * for an RCU grace period before destroying the mount point.
> + */
> +void mnt_make_shortterm(struct vfsmount *mnt)
> +{
> +	real_mount(mnt)->mnt_ns = NULL;
> +}
> +
>  /**
>   * path_is_mountpoint() - Check if path is a mount in the current namespace.
>   * @path: path to check
> diff --git a/include/linux/mount.h b/include/linux/mount.h
> index 62475996fac6..ec55a031aa8c 100644
> --- a/include/linux/mount.h
> +++ b/include/linux/mount.h
> @@ -88,6 +88,7 @@ extern void mnt_drop_write(struct vfsmount *mnt);
>  extern void mnt_drop_write_file(struct file *file);
>  extern void mntput(struct vfsmount *mnt);
>  extern struct vfsmount *mntget(struct vfsmount *mnt);
> +extern void mnt_make_shortterm(struct vfsmount *mnt);
>  extern struct vfsmount *mnt_clone_internal(const struct path *path);
>  extern bool __mnt_is_readonly(struct vfsmount *mnt);
>  extern bool mnt_may_suid(struct vfsmount *mnt);
> diff --git a/ipc/namespace.c b/ipc/namespace.c
> index a26860a41dac..6ecc30effd3e 100644
> --- a/ipc/namespace.c
> +++ b/ipc/namespace.c
> @@ -145,10 +145,11 @@ void free_ipcs(struct ipc_namespace *ns, struct ipc_ids *ids,
>  
>  static void free_ipc_ns(struct ipc_namespace *ns)
>  {
> -	/* mq_put_mnt() waits for a grace period as kern_unmount()
> -	 * uses synchronize_rcu().
> +	/*
> +	 * Caller needs to wait for an RCU grace period to have passed
> +	 * after making the mount point inaccessible to new accesses.
>  	 */
> -	mq_put_mnt(ns);

mq_put_mnt() is not needed anymore, should it be removed?

> +	mntput(ns->mq_mnt);
>  	sem_exit_ns(ns);
>  	msg_exit_ns(ns);
>  	shm_exit_ns(ns);
> @@ -168,6 +169,12 @@ 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)
> +		mnt_make_shortterm(n->mq_mnt);
> +
> +	/* Wait for any last users to have gone away. */
> +	synchronize_rcu();
> +
>  	llist_for_each_entry_safe(n, t, node, mnt_llist)
>  		free_ipc_ns(n);
>  }