linux-kernel - Re: next bio iters break discard?

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Message-ID: <20140114044841.GO9037@kmo>
Date:	Mon, 13 Jan 2014 20:48:41 -0800
From:	Kent Overstreet <kmo@...erainc.com>
To:	"Martin K. Petersen" <martin.petersen@...cle.com>
Cc:	Hugh Dickins <hughd@...gle.com>, Jens Axboe <axboe@...nel.dk>,
	Shaohua Li <shli@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: next bio iters break discard?

On Mon, Jan 13, 2014 at 11:06:33PM -0500, Martin K. Petersen wrote:
> >>>>> "Kent" == Kent Overstreet <kmo@...erainc.com> writes:
> 
> Kent,
> 
> Kent> I think for discards we can deal with this easily enough -
> Kent> __blk_recalc_rq_segments() will have to special case them - but
> Kent> there's a similar (but worse) issue with WRITE_SAME, and looking
> Kent> at the code it does attempt to merge WRITE_SAME requests too.
> 
> DISCARD bios have no payload going down the stack. They get a payload
> attached in the sd driver and will therefore have a single bvec at
> completion time.
> 
> WRITE_SAME bios have a single bvec payload throughout their lifetime.
> 
> For both these types of requests we never attempt to merge the actual
> payloads. But the block range worked on may shrink or grow as the bio is
> split or merged going down the stack.
> 
> IOW, DISCARD, WRITE SAME and the impending COPY requests do not have a
> 1:1 mapping between the block range worked on and the size of any bvecs
> attached. Your recent changes must have changed the way we handled that
> in the past.

Yeah - but with WRITE_SAME bios, wouldn't we at least have to check that they're
writing the same data to merge them?
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