Re: [PATCH] loop: properly observe rotational flag of underlying device

[Jens Axboe <axboe@fb.com>]

On 11/11/2015 08:21 AM, Holger Hoffstätte wrote:
>
> The loop driver always declares the rotational flag of its device as
> rotational, even when the device of the mapped file is nonrotational,
> as is the case with SSDs or on tmpfs. This can confuse filesystem tools
> which are SSD-aware; in my case I frequently forget to tell mkfs.btrfs
> that my loop device on tmpfs is nonrotational, and that I really don't
> need any automatic metadata redundancy.
>
> The attached patch fixes this by introspecting the rotational flag of the
> mapped file's underlying block device, if it exists. If the mapped file's
> filesystem has no associated block device - as is the case on e.g. tmpfs -
> we assume nonrotational storage. If there is a better way to identify such
> non-devices I'd love to hear them.
>
> Signed-off-by: Holger Hoffstätte <holger.hoffstaette@googlemail.com>
> ---
>   drivers/block/loop.c | 19 +++++++++++++++++++
>   1 file changed, 19 insertions(+)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 423f4ca..2984aca 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -843,6 +843,24 @@ static void loop_config_discard(struct loop_device *lo)
>   	queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, q);
>   }
>
> +static void loop_update_rotational(struct loop_device *lo)
> +{
> +	struct file *file = lo->lo_backing_file;
> +	struct inode *file_inode = file->f_mapping->host;
> +	struct block_device *file_bdev = file_inode->i_sb->s_bdev;
> +	struct request_queue *q = lo->lo_queue;
> +	bool nonrot = true;
> +
> +	/* not all filesystems (e.g. tmpfs) have a sb->s_bdev */
> +	if (file_bdev)
> +		nonrot = blk_queue_nonrot(bdev_get_queue(file_bdev));
> +
> +	if (nonrot)
> +		queue_flag_set_unlocked(QUEUE_FLAG_NONROT, q);
> +	else
> +		queue_flag_clear_unlocked(QUEUE_FLAG_NONROT, q);
> +}

Are we sure we want to change the default from rot to nonrot?

Apart from that, looks good.

-- 
Jens Axboe