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

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Message-ID: <4C9DD1B0.9020902@kernel.dk>
Date:	Sat, 25 Sep 2010 12:40:48 +0200
From:	Jens Axboe <axboe@...nel.dk>
To:	Adrian Hunter <adrian.hunter@...ia.com>
CC:	linux-kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] block: prevent merges of discard and write requests

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 Axboe

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