Re: "creative" bio usage in the RAID code

[Christoph Hellwig <hch@infradead.org>]

On Mon, Nov 14, 2016 at 10:03:20AM +1100, NeilBrown wrote:
> I would suggest adding a "bi_dev_private" field to the bio which is for
> use by the lowest-level driver (much as bi_private is for use by the
> top-level initiator).
> That could be in a union with any or all of:
> 	unsigned int		bi_phys_segments;
> 	unsigned int		bi_seg_front_size;
> 	unsigned int		bi_seg_back_size;
> 
> (any driver that needs those, would see a 'request' rather than a 'bio'
> and so could use rq->special)
> 
> raid5.c could then use bi_dev_private (or bi_special, or whatever it is call).

All the three above fields are those that could go away with a full
implementation of the multipage bvec scheme.  So any field for driver
use would still be be overhead.  If it's just for raid5 it could
be a smaller 16 bit (or maybe even just 8 bit) one.