From mboxrd@z Thu Jan  1 00:00:00 1970
From: NeilBrown <neilb@suse.de>
Subject: Problem with DISCARD and RAID5
Date: Thu, 1 Nov 2012 17:38:54 +1100
Message-ID: <20121101173854.62061307@notabene.brown>
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Return-path: <linux-kernel-owner@vger.kernel.org>
Sender: linux-kernel-owner@vger.kernel.org
To: Shaohua Li <shli@kernel.org>
Cc: linux RAID <linux-raid@vger.kernel.org>, Jens Axboe <axboe@kernel.dk>, lkml <linux-kernel@vger.kernel.org>
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Hi Shaohua,
 I've been doing some testing and discovered a problem with your discard
 support for RAID5.

 The code in blkdev_issue_discard assumes that the 'granularity' is a power
 of 2, and for example subtracts 1 to get a mask.

 However RAID5 sets the granularity to be the stripe size which often is not
 a power of two.  When this happens you can easily get into an infinite loo=
p.

 I suspect that to make this work properly, blkdev_issue_discard will need =
to
 be changed to allow 'granularity' to be an arbitrary value.
 When it is a power of two, the current masking can be used.
 When it is anything else, it will need to use sector_div().

 Could you look into this please?

Thanks,
NeilBrown

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