linux-kernel - Re: [PATCH v4 06/11] md/raid5: get rid of bio_fits

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Message-ID: <20150527081549.GA25575@lst.de>
Date:	Wed, 27 May 2015 10:15:49 +0200
From:	Christoph Hellwig <hch@....de>
To:	NeilBrown <neilb@...e.de>
Cc:	Ming Lin <mlin@...nel.org>, Christoph Hellwig <hch@....de>,
	lkml <linux-kernel@...r.kernel.org>,
	Kent Overstreet <kent.overstreet@...il.com>,
	Jens Axboe <axboe@...nel.dk>, Dongsu Park <dpark@...teo.net>,
	linux-raid@...r.kernel.org
Subject: Re: [PATCH v4 06/11] md/raid5: get rid of bio_fits_rdev()

On Wed, May 27, 2015 at 09:03:09AM +1000, NeilBrown wrote:
> But I don't think the sequence is right.
> 
> You cannot remove ->merge_bvec_fn for *any* stacked device until *all* devices
> make use of blk_queue_split() (or otherwise handle arbitrarily large bios).
> 
> I think it would be easiest to:
>  - add blk_queue_split() and call it from common code before ->make_request_fn
>    is called.  The ensure all devices can accept arbitrarily large bios.
>  - driver-by-driver remove merge_bvec_fn and make sure the the driver can cope
>    with arbitrary bios themselve, calling blk_queue_split in the make_request
>    function only if needed
>  - finally remove the call to blk_queue_split from the common code.
> 
> Does that make sense to others?

Ok, sorry for leading in the wrong direction.  Because we stack
->merge_bvec_fn calls we do indeed need it until the end.

In that case I think it's better to just go back to something like the
original order and not split the patches up, everything else is just
getting too complicated unfrotunately.
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