linux-kernel - Re: [patch] blk-flush: fix flush policy calculation

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Date:	Tue, 02 Aug 2011 15:46:49 -0400
From:	Jeff Moyer <jmoyer@...hat.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Shaohua Li <shli@...nel.org>, linux-kernel@...r.kernel.org,
	Tejun Heo <tj@...nel.org>, Jens Axboe <jaxboe@...ionio.com>,
	msnitzer@...hat.com
Subject: Re: [patch] blk-flush: fix flush policy calculation

Vivek Goyal <vgoyal@...hat.com> writes:

>> In testing, I did this:
>> 
>> @@ -1817,6 +1817,14 @@ int blk_insert_cloned_request(struct
>> request_queue *q, struct request *rq)
>>                 return -EIO;
>>  #endif
>>  
>> +       if ((rq->cmd_flags & (REQ_FLUSH|REQ_FUA)) && !q->flush_flags) {
>> +               rq->cmd_flags &= ~(REQ_FLUSH|REQ_FUA);
>> +               if (!blk_rq_bytes(rq)) {
>> +                       blk_end_request(rq, 0, 0);
>> +                       return 0;
>> +               }
>> +       }
>> +
>
> Will it make more sense to take care resetting flush/fua flags in
> blk_insert_flush()

Yes, that's much cleaner.

> and also the part which will end the request if request is empty.

Mmm-hmm.

I tried this, and it blew up in my face.  ;-)  Cloned requests from
device-mapper-land have rq->end_io filled in, which causes
blk_insert_flush to BUG here:

        BUG_ON(rq->end_io);

So, we can take the usual route of trying to squirrel away a pointer,
and reinstate that after the flush_data_end_io.  Or, perhaps someone has
a more clever idea, that might keep us from further expanding struct
request...?

Cheers,
Jeff
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