linux-kernel - Re: [PATCH 09/10] Change table chaining layout

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Message-ID: <20071023183321.GA14671@kernel.dk>
Date:	Tue, 23 Oct 2007 20:33:21 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Boaz Harrosh <bharrosh@...asas.com>
Cc:	torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
	mingo@...e.hu
Subject: Re: [PATCH 09/10] Change table chaining layout

On Tue, Oct 23 2007, Boaz Harrosh wrote:
> On Mon, Oct 22 2007 at 20:11 +0200, Jens Axboe <jens.axboe@...cle.com> wrote:
> > Change the page member of the scatterlist structure to be an unsigned
> > long, and encode more stuff in the lower bits:
> > 
> > - Bits 0 and 1 zero: this is a normal sg entry. Next sg entry is located
> >   at sg + 1.
> > - Bit 0 set: this is a chain entry, the next real entry is at ->page_link
> >   with the two low bits masked off.
> > - Bit 1 set: this is the final entry in the sg entry. sg_next() will return
> >   NULL when passed such an entry.
> > 
> > It's thus important that sg table users use the proper accessors to get
> > and set the page member.
> > 
> > Signed-off-by: Jens Axboe <jens.axboe@...cle.com>
> > ---
> <snip>
> >  
> >  /**
> >   * sg_set_page - Set sg entry to point at given page
> > @@ -20,11 +37,20 @@
> >   **/
> >  static inline void sg_set_page(struct scatterlist *sg, struct page *page)
> >  {
> > -	sg->page = page;
> > +	unsigned long page_link = sg->page_link & 0x3;
> > +
> You might want to put a BUG_ON(page & 0x3); Make sure
> you're not loosing information. (The m68k problem)

That's a really good idea, thanks Boaz! I'll add that.

-- 
Jens Axboe

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