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

Wu Fengguang
2006-05-27T08:04:37Z

On Fri, May 26, 2006 at 10:27:16AM -0700, Andrew Morton wrote:
> Wu Fengguang <wfg@mail.ustc.edu.cn> wrote:
> >
> > This is the slow code path of adaptive read-ahead.
> >
> > ...
> >
> > +
> > +/*
> > + * 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;
>
> `size' might overflow.

It does. Fixed the caller:
@@query_page_cache_segment()
index = radix_tree_scan_hole_backward(&mapping->page_tree,
- offset, ra_max);
+ offset - 1, ra_max);
Here (offset >= 1) always holds.

> > + 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);
>
> 29?

It's a prime number. Should be made obvious by the following macro:

#define CACHE_HIT_HASH_KEY 29 /* some prime number */