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

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Message-ID: <5435B6C0.8020704@kernel.dk>
Date:	Wed, 08 Oct 2014 16:12:16 -0600
From:	Jens Axboe <axboe@...nel.dk>
To:	Mike Snitzer <snitzer@...hat.com>, linux-kernel@...r.kernel.org
CC:	martin.petersen@...cle.com, dm-devel@...hat.com
Subject: Re: [PATCH] block: fix alignment_offset math that assumes io_min
 is a power-of-2

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...

-- 
Jens Axboe

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