On Mon, 9 Apr 2007 11:09:40 -0700 (PDT)
Christoph Lameter <[email protected]> wrote:
> Add PageTail / PageHead in order to avoid multiple branches when compound
> pages are checked.
>
> The patch adds PageTail(page) and PageHead(page) to check if a page is the
> head or the tail of a compound page. This is done by masking the two
> bits describing the state of a compound page and then comparing them. So
> one comparision and a branch instead of two bit checks and two branches.
>
OK. I'm still a bit concerned about bypassing the bitops synchronisation:
barriers, volatile, etc. We had lengthy ruminations on that a few years
ago, I think when working on free_pages_check().
> @@ -221,12 +215,24 @@ static inline void SetPageUptodate(struc
> #define __ClearPageCompound(page) __clear_bit(PG_compound, &(page)->flags)
>
> /*
> - * Note: PG_tail is an alias of another page flag. The result of PageTail()
> - * is only valid if PageCompound(page) is true.
> + * PG_reclaim is used in combination with PG_compound to mark the
> + * head and tail of a compound page
> + *
> + * PG_compound & PG_reclaim => Tail page
> + * PG_compound & ~PG_reclaim => Head page
> */
> -#define PageTail(page) test_bit(PG_tail, &(page)->flags)
> -#define __SetPageTail(page) __set_bit(PG_tail, &(page)->flags)
> -#define __ClearPageTail(page) __clear_bit(PG_tail, &(page)->flags)
> +
> +#define PG_head_tail_mask ((1L << PG_compound) | (1L << PG_reclaim))
> +
> +#define PageTail(page) ((page->flags & PG_head_tail_mask) \
> + == PG_head_tail_mask)
> +#define __SetPageTail(page) page->flags |= PG_head_tail_mask
> +#define __ClearPageTail(page) page->flags ~= PG_head_tail_mask
hm. The lack of parenthesisation here _might_ be OK, but I haven't
thought it through.
And I'd prefer not to have to, because I know that the do { } while (0)
thing works. As do static inline functions.
> +#define PageHead(page) ((page->flags & PG_head_tail_mask) \
> + == (1L << PG_compound))
> +#define __SetPageHead(page) __SetCompoundPage(page)
> +#define __ClearPageHead(page) __ClearCompoundPage(page)
You meant __SetPageCompound and __ClearPageCompound.
-
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