Date: Wed, 17 Oct 2007 18:36:34 -0700 (PDT)
From: Linus Torvalds <torvalds@linux-foundation.org>
To: David Miller <davem@davemloft.net>
cc: fujita.tomonori@lab.ntt.co.jp, jens.axboe@oracle.com, mingo@elte.hu,
       linux-kernel@vger.kernel.org, jgarzik@pobox.com,
       alan@lxorguk.ukuu.org.uk, tomof@acm.org
Subject: Re: [bug] ata subsystem related crash with latest -git
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On Wed, 17 Oct 2007, David Miller wrote:
>
> I believe that we have enough of a limited set of accessors to
> sg->page that we can more aggressively encode things in the lower
> bits.
> 
> I'm thinking of encoding the low two bits of sg->page as
> follows:
> 
> 1) bits == 0
> 
>    then the SG list is linear and sg_next() is sg++
> 
> 2) bits == 1
> 
>    the nest SG is an indirect chunk, sg_next() is
>    therefore something like:
> 
> 	next = *((struct scatterlist **)(sg + 1));
> 
> 3) bits == 2
> 
>    this is the last entry in the scatterlist, sg_next() is NULL
> 
> So for the cases of ARCH_HAS_SG_CHAIN not being set (ie. back
> compatible), we can do no bit encoding in page->flags and just do
> sg_next() == sg++, as is done now.

Yes, that sounds sane.

Although I also wonder whether we want one global per-arch 
ARCH_HAS_SG_CHAIN, or perhaps have something more dynamic, which allows a 
per-SG-list choice (which in turn would require some kind of "head" entry 
that is passed into sg_next(), and in general descibes the SG list)

		Linus
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