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

[Wu Fengguang]

On Fri, May 26, 2006 at 10:27:16AM -0700, Andrew Morton wrote:
> Wu Fengguang <wfg@xxxxxxxxxxxxxxxx> 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 */
-
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