Re: [PATCH RFC 3/4] dm-stripe: limit chunk_sectors to the stripe size

[Nilay Shroff <nilay@linux.ibm.com>]

On 6/5/25 8:38 PM, John Garry wrote:
> Currently we use min io size as the chunk size when deciding on the limit
> of atomic write size.
> 
> Using min io size is not reliable, as this may be mutated when stacking
> the bottom device limits.
> 
> The block stacking limits will rely on chunk_sectors in future, so set
> this value (to the chunk size).
> 
> Introduce a flag - DM_TARGET_STRIPED - and check this in
> dm_set_device_limits() when setting this limit.
> 
> Signed-off-by: John Garry <john.g.garry@oracle.com>
> ---
>  drivers/md/dm-stripe.c        | 3 ++-
>  drivers/md/dm-table.c         | 4 ++++
>  include/linux/device-mapper.h | 3 +++
>  3 files changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/md/dm-stripe.c b/drivers/md/dm-stripe.c
> index a7dc04bd55e5..c30df6715149 100644
> --- a/drivers/md/dm-stripe.c
> +++ b/drivers/md/dm-stripe.c
> @@ -466,7 +466,8 @@ static struct target_type stripe_target = {
>  	.name   = "striped",
>  	.version = {1, 7, 0},
>  	.features = DM_TARGET_PASSES_INTEGRITY | DM_TARGET_NOWAIT |
> -		    DM_TARGET_ATOMIC_WRITES | DM_TARGET_PASSES_CRYPTO,
> +		    DM_TARGET_ATOMIC_WRITES | DM_TARGET_PASSES_CRYPTO |
> +		    DM_TARGET_STRIPED,
>  	.module = THIS_MODULE,
>  	.ctr    = stripe_ctr,
>  	.dtr    = stripe_dtr,
> diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
> index 24a857ff6d0b..4f1f7173740c 100644
> --- a/drivers/md/dm-table.c
> +++ b/drivers/md/dm-table.c
> @@ -430,6 +430,10 @@ static int dm_set_device_limits(struct dm_target *ti, struct dm_dev *dev,
>  		return 0;
>  	}
>  
> +	/* For striped types, limit the chunk_sectors to the chunk size */
> +	if (dm_target_supports_striped(ti->type))
> +		limits->chunk_sectors = len >> SECTOR_SHIFT;
> +
I think here "len" refers to the total size of dm target and not the 
chunk sectors. So we need to modify this and take into account chunk sectors..
We can get chunk sectors, for example, like this:

	struct stripe_c *sc = ti->private;
	limits->chunk_sectors = sc->chunk_size;

But again struct stripe_c is private to dm-stripe.c and so we can't access it
here directly in dm-table.c Better we add a new callback function for dm target
type under struct target_type and then use that callback to get chunk sector. 

struct target_type stripe_target = {
        ...
        .chunk_sectors = stripe_chunk_sectors,
        ...
}

Thanks,
--Nilay