Re: [PATCH RFC] audit: move the tree pruning to a dedicated thread

[Paul Moore]

On Thursday, December 04, 2014 12:39:21 PM Imre Palik wrote:
> From: "Palik, Imre" <imrep@xxxxxxxxx>
> 
> When file auditing is enabled, during a low memory situation, a memory
> allocation with __GFP_FS can lead to pruning the inode cache.  Which can,
> in turn lead to audit_tree_freeing_mark() being called.  This can call
> audit_schedule_prune(), that tries to fork a pruning thread, and
> waits until the thread is created.  But forking needs memory, and the
> memory allocations there are done with __GFP_FS.
> 
> So we are waiting merrily for some __GFP_FS memory allocations to complete,
> while holding some filesystem locks.  This can take a while ...
> 
> This patch creates a single thread for pruning the tree from
> audit_tree_init(), and thus avoids the deadlock that the on-demand thread
> creation can cause.
> 
> An alternative approach would be to move the thread creation outside of the
> lock.  This would assume that other layers of the filesystem code don't
> hold any locks, and it would need some rewrite of the code to limit the
> amount of threads possibly spawned.
> 
> Reported-by: Matt Wilson <msw@xxxxxxxxxx>
> Cc: Matt Wilson <msw@xxxxxxxxxx>
> Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Imre Palik <imrep@xxxxxxxxx>
> ---
>  kernel/audit_tree.c |   53 ++++++++++++++++++++++++++++++++---------------
>  1 file changed, 35 insertions(+), 18 deletions(-)

Sorry for the delay, we've changed maintainers recently and some patches/issue 
were lost in the handoff.  Some comments below ...

> diff --git a/kernel/audit_tree.c b/kernel/audit_tree.c
> index 0caf1f8..cf6db88 100644
> --- a/kernel/audit_tree.c
> +++ b/kernel/audit_tree.c
> @@ -37,6 +37,7 @@ struct audit_chunk {
> 
>  static LIST_HEAD(tree_list);
>  static LIST_HEAD(prune_list);
> +static struct task_struct *prune_thread;
> 
>  /*
>   * One struct chunk is attached to each inode of interest.
> @@ -806,30 +807,39 @@ int audit_tag_tree(char *old, char *new)
>   */
>  static int prune_tree_thread(void *unused)
>  {
> -	mutex_lock(&audit_cmd_mutex);
> -	mutex_lock(&audit_filter_mutex);
> +	for (;;) {
> +		set_current_state(TASK_INTERRUPTIBLE);
> +		if (list_empty(&prune_list))
> +			schedule();
> +		__set_current_state(TASK_RUNNING);
> 
> -	while (!list_empty(&prune_list)) {
> -		struct audit_tree *victim;
> +		mutex_lock(&audit_cmd_mutex);
> +		mutex_lock(&audit_filter_mutex);
> 
> -		victim = list_entry(prune_list.next, struct audit_tree, list);
> -		list_del_init(&victim->list);
> +		while (!list_empty(&prune_list)) {
> +			struct audit_tree *victim;
> 
> -		mutex_unlock(&audit_filter_mutex);
> +			victim = list_entry(prune_list.next,
> +					struct audit_tree, list);
> +			list_del_init(&victim->list);
> 
> -		prune_one(victim);
> +			mutex_unlock(&audit_filter_mutex);
> 
> -		mutex_lock(&audit_filter_mutex);
> -	}
> +			prune_one(victim);
> 
> -	mutex_unlock(&audit_filter_mutex);
> -	mutex_unlock(&audit_cmd_mutex);
> +			mutex_lock(&audit_filter_mutex);
> +		}
> +
> +		mutex_unlock(&audit_filter_mutex);
> +		mutex_unlock(&audit_cmd_mutex);
> +	}
>  	return 0;
>  }
> 
>  static void audit_schedule_prune(void)
>  {
> -	kthread_run(prune_tree_thread, NULL, "audit_prune_tree");
> +	BUG_ON(!prune_thread);

I don't really like the BUG_ON() here.  If we can't guarantee that the thread 
is still alive, we should look into some fallback approach so that we can 
still prune the tree.  I imagine something could be done with the parameter to 
prune_tree_thread() to indicate if it is running in a dedicated thread or not.

> +	wake_up_process(prune_thread);
>  }
> 
>  /*
> @@ -896,9 +906,10 @@ static void evict_chunk(struct audit_chunk *chunk)
>  	for (n = 0; n < chunk->count; n++)
>  		list_del_init(&chunk->owners[n].list);
>  	spin_unlock(&hash_lock);
> +	mutex_unlock(&audit_filter_mutex);
>  	if (need_prune)
>  		audit_schedule_prune();
> -	mutex_unlock(&audit_filter_mutex);
> +
>  }
> 
>  static int audit_tree_handle_event(struct fsnotify_group *group,
> @@ -938,10 +949,16 @@ static int __init audit_tree_init(void)
>  {
>  	int i;
> 
> -	audit_tree_group = fsnotify_alloc_group(&audit_tree_ops);
> -	if (IS_ERR(audit_tree_group))
> -		audit_panic("cannot initialize fsnotify group for rectree watches");
> -
> +	prune_thread = kthread_create(prune_tree_thread, NULL,
> +				"audit_prune_tree");
> +	if (IS_ERR(prune_thread)) {
> +		audit_panic("cannot start thread audit_prune_tree");

Only in the most extreme configurations is audit_panic() an actual panic().  
This goes hand in hand with the comment above regarding the case where the 
pruning thread may not exist.

> +	} else {
> +		wake_up_process(prune_thread);
> +		audit_tree_group = fsnotify_alloc_group(&audit_tree_ops);
> +		if (IS_ERR(audit_tree_group))
> +			audit_panic("cannot initialize fsnotify group for rectree 
watches");
> +	}

The above doesn't really need to be in an else block does it?

>  	for (i = 0; i < HASH_SIZE; i++)
>  		INIT_LIST_HEAD(&chunk_hash_heads[i]);

-- 
paul moore
www.paul-moore.com

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