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

Mike Snitzer
2014-10-08T22:47:13Z

On Wed, Oct 08 2014 at 6:38pm -0400,
Jens Axboe <axboe@kernel.dk> wrote:

> On 10/08/2014 04:28 PM, Mike Snitzer wrote:
> > 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...
>
> I had it mixed up with the recent blk_max_size_offset() - you are right,
> this is not in a hot path. For that case, I don't really care, it's fine.
>
> Is v2 runtime tested?

Yes.

Here is the DM stack for an lvm created dm-thin-pool (dm-5).

# lsblk /dev/skd0
NAME MAJ:MIN RM SIZE RO TYPE MOUNTPOINT
skd0 252:0 0 745.3G 0 disk
├─bricks-mypool_tmeta 253:2 0 15.8G 0 lvm
│ └─bricks-mypool-tpool 253:4 0 512G 0 lvm
│ └─bricks-mypool 253:5 0 512G 0 lvm
└─bricks-mypool_tdata 253:3 0 512G 0 lvm
└─bricks-mypool-tpool 253:4 0 512G 0 lvm
└─bricks-mypool 253:5 0 512G 0 lvm

Before patch:
# cat /sys/block/dm-5/alignment_offset
1048576

After patch:
# cat /sys/block/dm-5/alignment_offset
0