linux-kernel - Re: [PATCH 1/2] scatterlist: use sg

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Message-ID: <20150611065056.GG20384@8bytes.org>
Date:	Thu, 11 Jun 2015 08:50:56 +0200
From:	Joerg Roedel <joro@...tes.org>
To:	Dan Williams <dan.j.williams@...el.com>
Cc:	Jens Axboe <axboe@...nel.dk>, Michal Simek <monstr@...str.eu>,
	Russell King <linux@....linux.org.uk>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Julia Lawall <Julia.Lawall@...6.fr>,
	"dmaengine@...r.kernel.org" <dmaengine@...r.kernel.org>,
	David Woodhouse <dwmw2@...radead.org>,
	Christoph Hellwig <hch@....de>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH 1/2] scatterlist: use sg_phys()

On Wed, Jun 10, 2015 at 09:00:31AM -0700, Dan Williams wrote:
> > So sg_phys() turns out to be 'page_to_phys(sg_page(s)) + s->offset',
> > which makes the above statement to:
> >
> >         page_to_phys(sg_page(s)) + s->offset - s->offset;
> >
> > The compiler will probably optimize that away, but it still doesn't look
> > like an improvement.
> 
> The goal is to eventually stop leaking struct page deep into the i/o
> stack.  Anything that relies on being able to retrieve a struct page
> out of an sg entry needs to be converted.  I think we need a new
> helper for this case "sg_phys_aligned()?".

You still have a reference to a struct page, because sg_phys() calls
sg_page() too. If you want to get rid of sg_page() something like
sg_pfn() migth be a more workable solution than sg_phys_(page_)aligned.

But maybe I am just missing the bigger scope of this, so I agree with
Russell that it is better so see a patch series which shows the
direction you want to go with this.


	Joerg

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