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SubjectRe: [PATCH] loop: properly observe rotational flag of underlying device
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Ping! Jens, can we please let this finally land in 5.2?

thanks,
Holger

On 2/12/19 11:54 PM, Benjamin Gordon wrote:
> From: Holger Hoffstätte <holger.hoffstaette@googlemail.com>
>
> 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.
>
> Cc: Jens Axboe <axboe@kernel.dk>
> Cc: linux-block@vger.kernel.org
> Cc: holger@applied-asynchrony.com
> Signed-off-by: Holger Hoffstätte <holger.hoffstaette@googlemail.com>
> Signed-off-by: Gwendal Grignou <gwendal@chromium.org>
> Signed-off-by: Benjamin Gordon <bmgordon@chromium.org>
> Reviewed-by: Guenter Roeck <groeck@chromium.org>
> ---
> This is a resend of Holger's original patch from
> https://lkml.org/lkml/2015/11/11/288 with the _unlocked functions
> updated. We keep running into the same problem on Chrome OS that this
> originally solved; any chance it can go in?
>
> drivers/block/loop.c | 19 +++++++++++++++++++
> 1 file changed, 19 insertions(+)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index cf5538942834..6c0fc0d49dc0 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -900,6 +900,24 @@ static int loop_prepare_queue(struct loop_device *lo)
> return 0;
> }
>
> +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)
> + blk_queue_flag_set(QUEUE_FLAG_NONROT, q);
> + else
> + blk_queue_flag_clear(QUEUE_FLAG_NONROT, q);
> +}
> +
> static int loop_set_fd(struct loop_device *lo, fmode_t mode,
> struct block_device *bdev, unsigned int arg)
> {
> @@ -963,6 +981,7 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
> if (!(lo_flags & LO_FLAGS_READ_ONLY) && file->f_op->fsync)
> blk_queue_write_cache(lo->lo_queue, true, false);
>
> + loop_update_rotational(lo);
> loop_update_dio(lo);
> set_capacity(lo->lo_disk, size);
> bd_set_size(bdev, size << 9);
>

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