Re: [PATCH V2] block: make segment size limit workable for > 4K PAGE_SIZE

[Ming Lei <ming.lei@redhat.com>]

On Mon, Feb 10, 2025 at 01:14:00PM +0100, Hannes Reinecke wrote:
> On 2/10/25 10:03, Ming Lei wrote:
> > PAGE_SIZE is applied in some block device queue limits, this way is
> > very fragile and is wrong:
> > 
> > - queue limits are read from hardware, which is often one readonly
> > hardware property
> > 
> > - PAGE_SIZE is one config option which can be changed during build time.
> > 
> > In RH lab, it has been found that max segment size of some mmc card is
> > less than 64K, then this kind of card can't work in case of 64K PAGE_SIZE.
> > 
> So why isn't this reflected in the blk_min_segment settings?
> Or, rather, why isn't setting blk_min_segment not enough?

There isn't min_segment_size setting, at block layer takes PAGE_SIZE
as the actual min_segment_size.

> 
> > Fix this issue by using BLK_MIN_SEGMENT_SIZE in related code for dealing
> > with queue limits and checking if bio needn't split. Define BLK_MIN_SEGMENT_SIZE
> > as 4K(minimized PAGE_SIZE).
> > 
> But why 4k then? That is a value like anything else, and what is the
> rationale to use that instead of the more natural sector size?

The comment explains it already: 4K = min(PAGE_SIZE).


Thanks,
Ming