linux-kernel - Re: [PATCH] block: prevent merges of discard and write requests

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Date:	Mon, 27 Sep 2010 13:59:52 +0900
From:	Jens Axboe <axboe@...nel.dk>
To:	Kyle McMartin <kyle@...artin.ca>
CC:	Adrian Hunter <adrian.hunter@...ia.com>,
	linux-kernel Mailing List <linux-kernel@...r.kernel.org>,
	stable@...nel.org
Subject: Re: [PATCH] block: prevent merges of discard and write requests

On 2010-09-27 12:30, Kyle McMartin wrote:
> On Sat, Sep 25, 2010 at 12:40:48PM +0200, Jens Axboe wrote:
>> On 2010-09-25 12:36, Adrian Hunter wrote:
>>> Add logic to prevent two I/O requests being merged if
>>> only one of them is a discard.  Ditto secure discard.
>>>
>>> Without this fix, it is possible for write requests
>>> to transform into discard requests.  For example:
>>>
>>>   Submit bio 1 to discard 8 sectors from sector n
>>>   Submit bio 2 to write 8 sectors from sector n + 16
>>>   Submit bio 3 to write 8 sectors from sector n + 8
>>>
>>> Bio 1 becomes request 1.  Bio 2 becomes request 2.
>>> Bio 3 is merged with request 2, and then subsequently
>>> request 2 is merged with request 1 resulting in just
>>> one I/O request which discards all 24 sectors.
>>
>> Wow, that's a disaster. We can now have requests in the
>> same direction and of the same type (fs), but not mergeable.
>>
>> I would move the check up above the position calculations.
>> I will apply this and upstream it right away. Thanks a lot!
>>
> 
> Jens, is this (the REQ_DISCARD hunk) required for stable as well? It
> appears there's not much change relating to merging requests between
> HEAD and v2.6.35, so I assume it is?

No, 2.6.35 and earlier is safe, it's only 2.6.36-rc that is
affected by this bug.

-- 
Jens Axboe

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