linux-kernel - Re: block: fix alignment_offset math that assumes io

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Message-ID: <20141008222841.GA15345@redhat.com>
Date:	Wed, 8 Oct 2014 18:28:41 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	linux-kernel@...r.kernel.org, dm-devel@...hat.com,
	martin.petersen@...cle.com
Subject: Re: block: fix alignment_offset math that assumes io_min is a
 power-of-2

On Wed, Oct 08 2014 at  6:12pm -0400,
Jens Axboe <axboe@...nel.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...
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