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SubjectRe: [PATCH 17/33] readahead: context based method
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On Wed, 2006-05-24 at 19:13 +0800, Wu Fengguang wrote:

> +#define PAGE_REFCNT_0 0
> +#define PAGE_REFCNT_1 (1 << PG_referenced)
> +#define PAGE_REFCNT_2 (1 << PG_active)
> +#define PAGE_REFCNT_3 ((1 << PG_active) | (1 << PG_referenced))
> +#define PAGE_REFCNT_MASK PAGE_REFCNT_3
> +
> +/*
> + * STATUS REFERENCE COUNT
> + * __ 0
> + * _R PAGE_REFCNT_1
> + * A_ PAGE_REFCNT_2
> + * AR PAGE_REFCNT_3
> + *
> + * A/R: Active / Referenced
> + */
> +static inline unsigned long page_refcnt(struct page *page)
> +{
> + return page->flags & PAGE_REFCNT_MASK;
> +}
> +
> +/*
> + * STATUS REFERENCE COUNT TYPE
> + * __ 0 fresh
> + * _R PAGE_REFCNT_1 stale
> + * A_ PAGE_REFCNT_2 disturbed once
> + * AR PAGE_REFCNT_3 disturbed twice
> + *
> + * A/R: Active / Referenced
> + */
> +static inline unsigned long cold_page_refcnt(struct page *page)
> +{
> + if (!page || PageActive(page))
> + return 0;
> +
> + return page_refcnt(page);
> +}
> +

Why all of this if all you're ever going to use is cold_page_refcnt.
What about something like this:

static inline int cold_page_referenced(struct page *page)
{
if (!page || PageActive(page))
return 0;
return !!PageReferenced(page);
}
> +
> +/*
> + * Count/estimate cache hits in range [first_index, last_index].
> + * The estimation is simple and optimistic.
> + */
> +static int count_cache_hit(struct address_space *mapping,
> + pgoff_t first_index, pgoff_t last_index)
> +{
> + struct page *page;
> + int size = last_index - first_index + 1;
> + int count = 0;
> + int i;
> +
> + cond_resched();
> + read_lock_irq(&mapping->tree_lock);
> +
> + /*
> + * The first page may well is chunk head and has been accessed,
> + * so it is index 0 that makes the estimation optimistic. This
> + * behavior guarantees a readahead when (size < ra_max) and
> + * (readahead_hit_rate >= 16).
> + */
> + for (i = 0; i < 16;) {
> + page = __find_page(mapping, first_index +
> + size * ((i++ * 29) & 15) / 16);
> + if (cold_page_refcnt(page) >= PAGE_REFCNT_1 && ++count >= 2)
cold_page_referenced(page) && ++count >= 2
> + break;
> + }
> +
> + read_unlock_irq(&mapping->tree_lock);
> +
> + return size * count / i;
> +}


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