linux-kernel - Re: [PATCH v2] lib/scatterlist: add simple page iterator

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Date:	Wed, 13 Feb 2013 16:51:21 +0200
From:	Imre Deak <imre.deak@...el.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Maxim Levitsky <maximlevitsky@...il.com>,
	Daniel Vetter <daniel.vetter@...ll.ch>,
	linaro-mm-sig@...ts.linaro.org
Subject: Re: [PATCH v2] lib/scatterlist: add simple page iterator

On Tue, 2013-02-12 at 09:13 -0800, Tejun Heo wrote:
> Hello,
> 
> On Tue, Feb 12, 2013 at 07:07:20PM +0200, Imre Deak wrote:
> > It's the one implemented by sg_miter_{start,stop} in scatterlist.c. It
> > also iterates through a scatterlist a page at a time, but it also kmaps
> > these pages. Since in our use case we don't need to map the pages we
> > needed a solution without this overhead.
> 
> I'm not against having non-mapping iterator but please consider that
> kmaps are no-ops on many configurations.  It matters only for archs w/
> high memory.

Ok, I haven't thought about that. But in any case we care about those
archs too and would like to avoid the mapping there as well.

> > where each entry on the sglist contained 16 consecutive pages. This
> > takes ~10% more time for the uninlined version to run. This is a rather
> > artificial test and I couldn't come up with something more real-life
> > using only the i915 driver's ioctl interface that would show a
> > significant change in speed.
> > 
> > So at least for now I'm ok with just uninlining all the helpers.
> 
> Can we reimplement mapping iters using the new ones?

Yes I think it's a good idea, I will follow up with a new patchset
addressing this and Andrew's comments.

Thanks,
Imre


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