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    SubjectRe: [PATCH 09/10] Change table chaining layout
    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
    -
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