Re: [PATCH] dm: Implement set_read_only

[Mikulas Patocka <mpatocka@redhat.com>]

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On Wed, 21 Aug 2024, Łukasz Patron wrote:

> This lets us change the read-only flag for device mapper block devices
> via the BLKROSET ioctl.
> 
> Signed-off-by: Łukasz Patron <priv.luk@gmail.com>
> ---
>  drivers/md/dm.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> index 87bb90303435..538a93e596d7 100644
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -410,6 +410,12 @@ static int dm_blk_getgeo(struct block_device *bdev, struct hd_geometry *geo)
>  	return dm_get_geometry(md, geo);
>  }
>  
> +static int dm_blk_set_read_only(struct block_device *bdev, bool ro)
> +{
> +	set_disk_ro(bdev->bd_disk, ro);
> +	return 0;
> +}
> +
>  static int dm_prepare_ioctl(struct mapped_device *md, int *srcu_idx,
>  			    struct block_device **bdev)
>  {
> @@ -3666,6 +3672,7 @@ static const struct block_device_operations dm_blk_dops = {
>  	.release = dm_blk_close,
>  	.ioctl = dm_blk_ioctl,
>  	.getgeo = dm_blk_getgeo,
> +	.set_read_only = dm_blk_set_read_only,
>  	.report_zones = dm_blk_report_zones,
>  	.pr_ops = &dm_pr_ops,
>  	.owner = THIS_MODULE
> -- 
> 2.46.0

Hi

Device mapper already calls set_disk_ro in the do_resume function. So, the 
problem here is that the value set using set_read_only will be overwritten 
as soon as a new table will be loaded.

I'd like to ask why is this patch needed? Why do you want to set read-only 
status using this ioctl instead of using the existing table flag?

If this is needed, we need to add another flag that is being set by 
dm_blk_set_read_only, so that dm_blk_set_read_only and dm_resume won't 
step over each other's changes.

Mikulas