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

[Mike Snitzer]

On Mon, Sep 27, 2010 at 1:12 AM, Mike Snitzer <snitzer@xxxxxxxxxx> wrote:
> On Mon, Sep 27, 2010 at 12:59 AM, Jens Axboe <axboe@xxxxxxxxx> wrote:
>> 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.
>
> I'm not so sure... I think 2.6.35 is affected.  Jens, what do you hold
> to be the regression point?
...
> But things really broke once we started playing games with barrier
> flags associated with discards.  The regression point (relative to
> discard merging) seems to have occurred when we got away from using
> REQ_SOFTBARRIER with commit: fbbf055692aeb "block: fix DISCARD_BARRIER
> requests".  Which was still committed to v2.6.35...

OK I take that back, with commit fbbf055692aeb REQ_HARDBARRIER is used
for discards.. which is equally applicable to !rq_mergeable().

Anyway, I'd still like to understand what you feel is the regression point.

Mike
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