linux-kernel - Problem with DISCARD and RAID5

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Message-ID: <20121101173854.62061307@notabene.brown>
Date:	Thu, 1 Nov 2012 17:38:54 +1100
From:	NeilBrown <neilb@...e.de>
To:	Shaohua Li <shli@...nel.org>
Cc:	linux RAID <linux-raid@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Problem with DISCARD and RAID5

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 loop.

 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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