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

[John Garry <john.garry@linux.dev>]

On 9/10/26 06:36, Christoph Hellwig wrote:
> 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.

ok, I can add that.

I'd like to test this, I suppose scsi_debug is all I have to sanity test 
this...