Re: [PATCH 2/3] block: cap atomic write size by PI buffer size constraints

[Christoph Hellwig <hch@lst.de>]

On Wed, Sep 09, 2026 at 10:16:17AM +0100, John Garry wrote:
> Furthermore, I don't think that this is the change above is correct. We 
> have 4x atomic limits and I think that 3x would need updating:
> - atomic_write_max_sectors
> - atomic_write_unit_min
> - atomic_write_unit_max
>
> The expectation is that the driver sets atomic_write_hw_max, 
> atomic_write_hw_unit_min, and atomic_write_hw_unit_max individually. We 
> don't assume that atomic_write_hw_unit_max == 
> rounddown_pow_of_two(atomic_write_hw_max). The is because of SCSI and it's 
> granularity and alignment atomic limits :(

>
> Anyway, maybe this is a better change:
>
> diff --git a/block/blk-settings.c b/block/blk-settings.c
> index 8274631290db..b88744d42719 100644
> --- a/block/blk-settings.c
> +++ b/block/blk-settings.c
> @@ -233,11 +233,15 @@ static void blk_atomic_writes_update_limits(struct 
> queue_limits *lim)
>  	unsigned int unit_limit = min(lim->max_hw_sectors << SECTOR_SHIFT, 
> blk_queue_max_guaranteed_bio(lim));
>
> +	unit_limit = min_not_zero(unit_limit, max_integrity_io_size(lim));
>  	unit_limit = rounddown_pow_of_two(unit_limit);
>
>  	lim->atomic_write_max_sectors =
>  		min(lim->atomic_write_hw_max >> SECTOR_SHIFT,
>  			lim->max_hw_sectors);
> +	lim->atomic_write_max_sectors =
> +		min_not_zero(lim->atomic_write_max_sectors,
> +			max_integrity_io_size(lim));

This needs to be conditional on a non-zero lim->integrity.metadata_size.

Otherwise this looks sane.

> I would (naively) assume that if the disk has integrity, then it's atomic 
> limits reported would update accordingly and lim->atomic_write_hw_max et al 
> would already have any integrity limits built in (so that we don't need to 
> do any capping of atomic limits by integrity limits).

The integrity limit is purely a software limit based on the mempool for the
max integrity data allocation.