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

[Mikulas Patocka <mpatocka@redhat.com>]

On Thu, 5 Jun 2025, 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;
> +

len is already in sectors, so why do we shift it right?

Could this logic be moved to the function stripe_io_hints, so that we 
don't have to add a new flag for that and that we don't have to modify the 
generic dm code?

Mikulas

>  	mutex_lock(&q->limits_lock);
>  	/*
>  	 * BLK_FEAT_ATOMIC_WRITES is not inherited from the bottom device in
> diff --git a/include/linux/device-mapper.h b/include/linux/device-mapper.h
> index cb95951547ab..a863523b69ee 100644
> --- a/include/linux/device-mapper.h
> +++ b/include/linux/device-mapper.h
> @@ -309,6 +309,9 @@ struct target_type {
>  #define DM_TARGET_ATOMIC_WRITES		0x00000400
>  #define dm_target_supports_atomic_writes(type) ((type)->features & DM_TARGET_ATOMIC_WRITES)
>  
> +#define DM_TARGET_STRIPED		0x00000800
> +#define dm_target_supports_striped(type) ((type)->features & DM_TARGET_STRIPED)
> +
>  struct dm_target {
>  	struct dm_table *table;
>  	struct target_type *type;
> -- 
> 2.31.1
>