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

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Date:	Tue, 09 Aug 2011 13:29:02 -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:

> On Tue, Aug 02, 2011 at 03:46:49PM -0400, Jeff Moyer wrote:
>> 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 how does it work when underlying device supports FLUSH/FUA? That means
> that dm-multipath requets never go through flush machinery? So if a

Yes, they do.  They go through it *above* dm-multipath, and dm-multipath
just hands the requests down to the device via whatever path it selects.

Cheers,
Jeff
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