bio_add_page rw mode check by merge_bvec_fn

[NeilBrown]

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