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

From: Jens Axboe
Date: Wed Oct 08 2014 - 18:38:20 EST

On 10/08/2014 04:28 PM, Mike Snitzer wrote:
> On Wed, Oct 08 2014 at 6:12pm -0400,
> Jens Axboe <axboe@xxxxxxxxx> 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?

Jens Axboe

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