From: Jens Axboe <axboe@kernel.dk>
To: Kyle McMartin <kyle@mcmartin.ca>
Cc: Adrian Hunter <adrian.hunter@nokia.com>,
linux-kernel Mailing List <linux-kernel@vger.kernel.org>,
stable@kernel.org
Subject: Re: [PATCH] block: prevent merges of discard and write requests
Date: Mon, 27 Sep 2010 13:59:52 +0900 [thread overview]
Message-ID: <4CA024C8.3000203@kernel.dk> (raw)
In-Reply-To: <20100927033030.GR13116@bombadil.infradead.org>
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
next prev parent reply other threads:[~2010-09-27 4:59 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2010-09-25 10:36 [PATCH] block: prevent merges of discard and write requests Adrian Hunter
2010-09-25 10:40 ` Jens Axboe
2010-09-27 3:30 ` Kyle McMartin
2010-09-27 4:59 ` Jens Axboe [this message]
2010-09-27 5:02 ` Kyle McMartin
2010-09-27 5:12 ` Mike Snitzer
2010-09-27 5:26 ` Mike Snitzer
2010-09-27 6:08 ` Jens Axboe
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