Re: block: fix alignment_offset math that assumes io_min is a power-of-2

[Mike Snitzer <snitzer@redhat.com>]

On Wed, Oct 08 2014 at  6:12pm -0400,
Jens Axboe <axboe@kernel.dk> wrote:

> On 10/08/2014 04:05 PM, Mike Snitzer wrote:
> > The math in both blk_stack_limits() and queue_limit_alignment_offset()
> > assume that a block device's io_min (aka minimum_io_size) is always a
> > power-of-2.  Fix the math such that it works for non-power-of-2 io_min.
> > 
> > This issue (of alignment_offset != 0) became apparent when testing
> > dm-thinp with a thinp blocksize that matches a RAID6 stripesize of
> > 1280K.  Commit fdfb4c8c1 ("dm thin: set minimum_io_size to pool's data
> > block size") unlocked the potential for alignment_offset != 0 due to
> > the dm-thin-pool's io_min possibly being a non-power-of-2.
> 
> Well that sucks, AND with a mask is considerably cheaper than a MOD...

Yeah, certainly does suck (please note v2 that I just sent).  The MODs
shouldn't kill us, these functions aren't called in any real hot path.
A storm at boot maybe.. or SCSI rescan but...