Re: [PATCH v7 2/5] block: Introduce provisioning primitives

[Mike Snitzer <snitzer@kernel.org>]

On Thu, May 18 2023 at  6:33P -0400,
Sarthak Kukreti <sarthakkukreti@chromium.org> wrote:

> Introduce block request REQ_OP_PROVISION. The intent of this request
> is to request underlying storage to preallocate disk space for the given
> block range. Block devices that support this capability will export
> a provision limit within their request queues.
> 
> This patch also adds the capability to call fallocate() in mode 0
> on block devices, which will send REQ_OP_PROVISION to the block
> device for the specified range,
> 
> Signed-off-by: Sarthak Kukreti <sarthakkukreti@chromium.org>
> ---
...
> diff --git a/block/blk-settings.c b/block/blk-settings.c
> index 896b4654ab00..d303e6614c36 100644
> --- a/block/blk-settings.c
> +++ b/block/blk-settings.c
> @@ -59,6 +59,7 @@ void blk_set_default_limits(struct queue_limits *lim)
>  	lim->zoned = BLK_ZONED_NONE;
>  	lim->zone_write_granularity = 0;
>  	lim->dma_alignment = 511;
> +	lim->max_provision_sectors = 0;
>  }
>  
>  /**
> @@ -82,6 +83,7 @@ void blk_set_stacking_limits(struct queue_limits *lim)
>  	lim->max_dev_sectors = UINT_MAX;
>  	lim->max_write_zeroes_sectors = UINT_MAX;
>  	lim->max_zone_append_sectors = UINT_MAX;
> +	lim->max_provision_sectors = UINT_MAX;
>  }
>  EXPORT_SYMBOL(blk_set_stacking_limits);
>  
> @@ -578,6 +594,9 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
>  	t->max_segment_size = min_not_zero(t->max_segment_size,
>  					   b->max_segment_size);
>  
> +	t->max_provision_sectors = min_not_zero(t->max_provision_sectors,
> +						b->max_provision_sectors);
> +

This needs to use min() since max_provision_sectors also serves to
indicate if the device supports REQ_OP_PROVISION.  Otherwise, if I set
max_provision_sectors to 0 on a dm thin-pool the blk_stack_limits()
will ignore my having set it to 0 (to disable) and it'll remain as
UINT_MAX (thanks to blk_set_default_limits).

Mike