linux-kernel - Re: [PATCH] mm/backing-dev: Check return value of the debugfs_create

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Message-Id: <20150825140858.8185db77fed42cf5df5faeb5@linux-foundation.org>
Date:	Tue, 25 Aug 2015 14:08:58 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Alexander Kuleshov <kuleshovmail@...il.com>
Cc:	Tejun Heo <tj@...nel.org>, Jan Kara <jack@...e.cz>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/backing-dev: Check return value of the
 debugfs_create_dir()

On Tue, 25 Aug 2015 13:54:23 +0600 Alexander Kuleshov <kuleshovmail@...il.com> wrote:

> The debugfs_create_dir() function may fail and return error. If the
> root directory not created, we can't create anything inside it. This
> patch adds check for this case.
> 
> ...
>
> --- a/mm/backing-dev.c
> +++ b/mm/backing-dev.c
> @@ -117,15 +117,21 @@ static const struct file_operations bdi_debug_stats_fops = {
>  
>  static void bdi_debug_register(struct backing_dev_info *bdi, const char *name)
>  {
> -	bdi->debug_dir = debugfs_create_dir(name, bdi_debug_root);
> -	bdi->debug_stats = debugfs_create_file("stats", 0444, bdi->debug_dir,
> -					       bdi, &bdi_debug_stats_fops);
> +	if (bdi_debug_root) {
> +		bdi->debug_dir = debugfs_create_dir(name, bdi_debug_root);
> +		if (bdi->debug_dir)
> +			bdi->debug_stats = debugfs_create_file("stats", 0444,
> +							bdi->debug_dir, bdi,
> +							&bdi_debug_stats_fops);
> +	}

If debugfs_create_dir() fails, debugfs_create_file() will go ahead and
attempt to create the debugfs file in the debugfs root directory:

: static struct dentry *start_creating(const char *name, struct dentry *parent)
: {
: ...
: 	/* If the parent is not specified, we create it in the root.
: 	 * We need the root dentry to do this, which is in the super
: 	 * block. A pointer to that is in the struct vfsmount that we
: 	 * have around.
: 	 */
: 	if (!parent)
: 		parent = debugfs_mount->mnt_root;

I'm not sure that this is very useful behaviour, and putting the files
in the wrong place is a very obscure way of informing the user that
debugfs_create_dir() failed :(


I don't think it's worth making little changes such as this - handling
debugfs failures needs a deeper rethink.
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