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

[Boaz Harrosh <bharrosh@panasas.com>]

On Mon, Oct 22 2007 at 20:11 +0200, Jens Axboe <jens.axboe@oracle.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@oracle.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)

> +	sg->page_link = page_link | (unsigned long) page;
>  }
>  
> -#define sg_page(sg)	((sg)->page)
> +#define sg_page(sg)	((struct page *) ((sg)->page_link & ~0x3))
>  
<snip>

Boaz