linux-kernel - Re: [PATCH 7/10] i386 sg: add support for chaining scatterlists

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Message-ID: <20070509102835.GQ4163@kernel.dk>
Date:	Wed, 9 May 2007 12:28:35 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Herbert Xu <herbert@...dor.apana.org.au>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 7/10] i386 sg: add support for chaining scatterlists

On Wed, May 09 2007, Herbert Xu wrote:
> Jens Axboe <jens.axboe@...cle.com> wrote:
> > 
> > diff --git a/include/asm-i386/scatterlist.h b/include/asm-i386/scatterlist.h
> > index d7e45a8..794b68c 100644
> > --- a/include/asm-i386/scatterlist.h
> > +++ b/include/asm-i386/scatterlist.h
> > @@ -8,8 +8,11 @@ struct scatterlist {
> >     unsigned int       offset;
> >     dma_addr_t         dma_address;
> >     unsigned int       length;
> > +    struct scatterlist *next;
> > };
> 
> BTW, the crypto layer's scatterlist already has a chaining mechanism
> using the existing structure.  The only difference is that the chained
> pointer is stored inside the 'struct page *' rather than a new pointer.
> Its existence is flagged by a zero value in the length field.
> 
> Now I'm not super-religious about this but we should at least consider
> whether forking out 4 bytes in every scatterlist member is worthwhile
> investment when we can use 12 bytes at the end instead.

>From a memory consumption POV, it's definitely a win to reuse the ->page
member as a pointer to the next sgtable. I didn't do that originally to
avoid complications in allocating and setting up the sg table, but I may
very well reconsider that decision soonish.

We can easily change this without disturbing the upper layers, so it's
not a big deal to change now or later.

-- 
Jens Axboe

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