Re: [PATCH v2] block: Increase BLK_DEF_MAX_SECTORS_CAP

[Damien Le Moal <dlemoal@kernel.org>]

On 6/18/25 18:06, John Garry wrote:
> On 18/06/2025 07:00, Damien Le Moal wrote:
>> Back in 2015, commit d2be537c3ba3 ("block: bump BLK_DEF_MAX_SECTORS to
>> 2560") increased the default maximum size of a block device I/O to 2560
>> sectors (1280 KiB) to "accommodate a 10-data-disk stripe write with
>> chunk size 128k". This choice is rather arbitrary and since then,
>> improvements to the block layer have software RAID drivers correctly
>> advertize their stripe width through chunk_sectors and abuses of
>> BLK_DEF_MAX_SECTORS_CAP by drivers (to set the HW limit rather than the
>> default user controlled maximum I/O size) have been fixed.
>>
>> Since many block devices can benefit from a larger value of
>> BLK_DEF_MAX_SECTORS_CAP, and in particular HDDs, increase this value to
>> be 4MiB, or 8192 sectors.
>>
>> And given that BLK_DEF_MAX_SECTORS_CAP is only used in the block layer
>> and should not be used by drivers directly, move this macro definition
>> to the block layer internal header file block/blk.h.
>>
>> Suggested-by: Martin K . Petersen <martin.petersen@oracle.com>
>> Signed-off-by: Damien Le Moal <dlemoal@kernel.org>
>> Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
> 
> Regardless of comment below:
> Reviewed-by: John Garry <john.g.garry@oracle.com>
> 
>> ---
>> Changes from v1:
>>   - Move BLK_DEF_MAX_SECTORS_CAP definition to block/blk.h
> 
> it's only referenced in blk-settings.c, so I don't know why it doesn't 
> live there.
> 
> However it is co-located with enum blk_default_limits and the same 
> comment goes for members of enum blk_default_limits. I think all those 
> in enum blk_default_limits could potentially be moved to blk-settings.c 
> after Christoph's work for atomic queue limit updates.

I actually checked that and a few drivers are still using 2 of the 4 enum defaults.

Jens,

DO you prefer we move BLK_DEF_MAX_SECTORS_CAP to blk-settings.c ? blk.h has a
couple of settings macro at the top, it is together with that for now.


-- 
Damien Le Moal
Western Digital Research