linux-ext4 - Re: bio_add_page rw mode check by merge_bvec

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Message-ID: <20131123161234.20ae4f9d@notabene.brown>
Date:	Sat, 23 Nov 2013 16:12:34 +1100
From:	NeilBrown <neilb@...e.de>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	Bernd Schubert <bernd.schubert@...m.fraunhofer.de>,
	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
	xfs@....sgi.com, linux-ext4@...r.kernel.org,
	Jens Axboe <axboe@...nel.dk>,
	Kent Overstreet <kmo@...erainc.com>
Subject: Re: bio_add_page rw mode check by merge_bvec_fn

On Fri, 22 Nov 2013 07:36:18 -0800 Christoph Hellwig <hch@...radead.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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