Re: [RFC PATCH 1/4] vfs: Create fs_context-aware mount_bdev() replacement

[Andrew Price]

On 27/03/2019 11:23, David Howells wrote:
> Andrew Price <anprice@xxxxxxxxxx> wrote:
>
> > > +		up_write(&s->s_umount);
> > > +		blkdev_put(bdev, fc->bdev_mode);
> > > +		down_write(&s->s_umount);
> >
> > fc->bdev should be NULLed here (or, on the way out of sget_fc() might be more
> > appropriate) otherwise we get a double-blkdev_put() leading to NULL pointer
> > derefs later. This happens when I mount a device twice and then unmount them,
> > or mount it 3 times.
>
> How about the attached changes?  Note that they conflict a bit with the mtd
> changes from where I took the destructor piece.
>
> David

It fixes my bug and I see nothing unexpected from an xfstests -g quick 
run with my gfs2 patch on top. Looks good.

Thanks,
Andy

> ---
> commit 161602f963ae5a05bdb834461f7a4896286fbde0
> Author: David Howells <dhowells@xxxxxxxxxx>
> Date:   Wed Mar 27 11:12:57 2019 +0000
>
>      bdev handling fix
>
> diff --git a/fs/fs_context.c b/fs/fs_context.c
> index 9e22fe6aafe7..fc8fda4b9fe9 100644
> --- a/fs/fs_context.c
> +++ b/fs/fs_context.c
> @@ -425,10 +425,8 @@ void put_fs_context(struct fs_context *fc)
>   
>   	if (fc->need_free && fc->ops && fc->ops->free)
>   		fc->ops->free(fc);
> -#ifdef CONFIG_BLOCK
> -	if (fc->bdev)
> -		blkdev_put(fc->bdev, fc->bdev_mode);
> -#endif
> +	if (fc->dev_destructor)
> +		fc->dev_destructor(fc);
>   
>   	security_free_mnt_opts(&fc->security);
>   	put_net(fc->net_ns);
> diff --git a/fs/super.c b/fs/super.c
> index 85851adb0f19..e9e678d2c37c 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -1211,8 +1211,17 @@ int vfs_get_super(struct fs_context *fc,
>   EXPORT_SYMBOL(vfs_get_super);
>   
>   #ifdef CONFIG_BLOCK
> +static void fc_bdev_destructor(struct fs_context *fc)
> +{
> +	if (fc->bdev) {
> +		blkdev_put(fc->bdev, fc->bdev_mode);
> +		fc->bdev = NULL;
> +	}
> +}
> +
>   static int set_bdev_super_fc(struct super_block *s, struct fs_context *fc)
>   {
> +	s->s_mode = fc->bdev_mode;
>   	s->s_bdev = fc->bdev;
>   	s->s_dev = s->s_bdev->bd_dev;
>   	s->s_bdi = bdi_get(s->s_bdev->bd_bdi);
> @@ -1252,6 +1261,9 @@ int vfs_get_block_super(struct fs_context *fc,
>   		return PTR_ERR(bdev);
>   	}
>   
> +	fc->dev_destructor = fc_bdev_destructor;
> +	fc->bdev = bdev;
> +
>   	/* Once the superblock is inserted into the list by sget_fc(), s_umount
>   	 * will protect the lockfs code from trying to start a snapshot while
>   	 * we are mounting
> @@ -1260,18 +1272,14 @@ int vfs_get_block_super(struct fs_context *fc,
>   	if (bdev->bd_fsfreeze_count > 0) {
>   		mutex_unlock(&bdev->bd_fsfreeze_mutex);
>   		warnf(fc, "%pg: Can't mount, blockdev is frozen", bdev);
> -		error = -EBUSY;
> -		goto error_bdev;
> +		return -EBUSY;
>   	}
>   
> -	fc->bdev = bdev;
>   	fc->sb_flags |= SB_NOSEC;
>   	s = sget_fc(fc, test_bdev_super_fc, set_bdev_super_fc);
>   	mutex_unlock(&bdev->bd_fsfreeze_mutex);
> -	if (IS_ERR(s)) {
> -		error = PTR_ERR(s);
> -		goto error_bdev;
> -	}
> +	if (IS_ERR(s))
> +		return PTR_ERR(s);
>   
>   	if (s->s_root) {
>   		/* Don't summarily change the RO/RW state. */
> @@ -1281,16 +1289,10 @@ int vfs_get_block_super(struct fs_context *fc,
>   			goto error_sb;
>   		}
>   
> -		/* s_umount nests inside bd_mutex during __invalidate_device().
> -		 * blkdev_put() acquires bd_mutex and can't be called under
> -		 * s_umount.  Drop s_umount temporarily.  This is safe as we're
> -		 * holding an active reference.
> +		/* Leave fc->bdev to fc_bdev_destructor() to clean up to avoid
> +		 * locking conflicts.
>   		 */
> -		up_write(&s->s_umount);
> -		blkdev_put(bdev, fc->bdev_mode);
> -		down_write(&s->s_umount);
>   	} else {
> -		s->s_mode = fc->bdev_mode;
>   		snprintf(s->s_id, sizeof(s->s_id), "%pg", bdev);
>   		sb_set_blocksize(s, block_size(bdev));
>   		error = fill_super(s, fc);
> @@ -1307,11 +1309,7 @@ int vfs_get_block_super(struct fs_context *fc,
>   
>   error_sb:
>   	deactivate_locked_super(s);
> -error_bdev:
> -	if (fc->bdev) {
> -		blkdev_put(fc->bdev, fc->bdev_mode);
> -		fc->bdev = NULL;
> -	}
> +	/* Leave fc->bdev to fc_bdev_destructor() to clean up */
>   	return error;
>   }
>   EXPORT_SYMBOL(vfs_get_block_super);
> @@ -1521,8 +1519,13 @@ int vfs_get_tree(struct fs_context *fc)
>   	 * on the superblock.
>   	 */
>   	error = fc->ops->get_tree(fc);
> -	if (error < 0)
> +	if (error < 0) {
> +		if (fc->dev_destructor) {
> +			fc->dev_destructor(fc);
> +			fc->dev_destructor = NULL;
> +		}
>   		return error;
> +	}
>   
>   	if (!fc->root) {
>   		pr_err("Filesystem %s get_tree() didn't set fc->root\n",
> diff --git a/include/linux/fs_context.h b/include/linux/fs_context.h
> index cb49b92f02af..9af788a3ef8e 100644
> --- a/include/linux/fs_context.h
> +++ b/include/linux/fs_context.h
> @@ -76,7 +76,9 @@ struct fs_context {
>   	const struct fs_context_operations *ops;
>   	struct file_system_type	*fs_type;
>   	void			*fs_private;	/* The filesystem's context */
> -	struct block_device	*bdev;		/* The backing blockdev (if applicable) */
> +	union {
> +		struct block_device *bdev;	/* The backing blockdev (if applicable) */
> +	};
>   	struct dentry		*root;		/* The root and superblock */
>   	struct user_namespace	*user_ns;	/* The user namespace for this mount */
>   	struct net		*net_ns;	/* The network namespace for this mount */
> @@ -93,6 +95,7 @@ struct fs_context {
>   	enum fs_context_purpose	purpose:8;
>   	bool			need_free:1;	/* Need to call ops->free() */
>   	bool			global:1;	/* Goes into &init_user_ns */
> +	void (*dev_destructor)(struct fs_context *fc); /* For block or mtd */
>   };
>   
>   struct fs_context_operations {