Re: [PATCH 17/33] readahead: context based method

Andrew Morton
2006-05-26T17:24:24Z

Wu Fengguang <wfg@mail.ustc.edu.cn> 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;
> +}
> +

This assumes that PG_referenced < PG_active. Nobody knows that this
assumption was made and someone might go and reorder the page flags and
subtly break readahead.

We need to either not do it this way, or put a big comment in page-flags.h,
or even redefine PG_active to be PG_referenced+1.