From: NeilBrown <neilb@suse.de>
Subject: Re: bio_add_page rw mode check by merge_bvec_fn
Date: Sat, 23 Nov 2013 16:12:34 +1100
Message-ID: <20131123161234.20ae4f9d@notabene.brown>
References: <528E4A50.7040609@itwm.fraunhofer.de>
	<528E4C19.70507@itwm.fraunhofer.de>
	<20131122153618.GB15143@infradead.org>
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Cc: Jens Axboe <axboe@kernel.dk>, xfs@oss.sgi.com,
	Bernd Schubert <bernd.schubert@itwm.fraunhofer.de>,
	Kent Overstreet <kmo@daterainc.com>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	linux-ext4@vger.kernel.org
To: Christoph Hellwig <hch@infradead.org>
In-Reply-To: <20131122153618.GB15143@infradead.org>
Errors-To: xfs-bounces@oss.sgi.com
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On Fri, 22 Nov 2013 07:36:18 -0800 Christoph Hellwig <hch@infradead.org>
wrote:

> While this is trivial to fix it's also fairly unexpected and easy
> to get wrong for new callers.  Neil, can you explain why you
> desperately need it?

Desperately?  Not at all?
Need?  Not really.  This is just in RAID5 and merge_bvec_fn is purely an
optimisation for RAID5.
Limiting read BIOs to one chunk allows us to bypass the stripe-cache, so can
be good.
Limiting write BIOs is completely unnecessary so we currently don't bother.

So I have no objection to bvm->bi_rw being removed.

Thanks,
NeilBrown

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