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

[Kyle McMartin]

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?

--Kyle
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