Re: [PATCH 3/8] block: grab a device model reference in blkdev_get_no_open

[Ming Lei <ming.lei@redhat.com>]

On Wed, Jul 21, 2021 at 05:35:18PM +0200, Christoph Hellwig wrote:
> Opening a block device needs to ensure it is fully present instead of
> just the allocated memory.  Switch from an inode reference as obtained
> by bdget to a full device model reference.
> 
> In fact we should not use inode references for anything in the block
> layer.  There are three users left, two can be trivially removed
> and the third (xen-blkfront) is a complete mess that needs more
> attention.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/block_dev.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/block_dev.c b/fs/block_dev.c
> index 9ef4f1fc2cb0..24a6970f3623 100644
> --- a/fs/block_dev.c
> +++ b/fs/block_dev.c
> @@ -1342,9 +1342,16 @@ struct block_device *blkdev_get_no_open(dev_t dev)
>  		goto bdput;
>  	if ((disk->flags & (GENHD_FL_UP | GENHD_FL_HIDDEN)) != GENHD_FL_UP)
>  		goto put_disk;
> -	if (!try_module_get(bdev->bd_disk->fops->owner))
> +	if (!try_module_get(disk->fops->owner))
>  		goto put_disk;
> +
> +	/* switch to a device model reference instead of the inode one: */
> +	if (!kobject_get_unless_zero(&bdev->bd_device.kobj))

If bdev->bd_device.kobj is grabbed in every open, getting disk reference might
be moved into add_partition, and drop it in part_release(). Then we can avoid
extra disk reference in open/close().

Not mention two disk references are actually grabbed in case of opening disk.

Thanks,
Ming