Race in shrink_cache

[Daniel Phillips <phillips@arcor.de>]

Hi Marcelo,

This looks really suspicious, vmscan.c#435:

	spin_unlock(&pagemap_lru_lock);
							if (put_page_testzero(page))
								__free_pages_ok(page, 0);
	/* avoid to free a locked page */
	page_cache_get(page);

	/* whoops, double free coming */

I suggest you bump the page count before releasing the lru lock.  The race
shown above may not in fact be possible, but the current code is fragile.

-- 
Daniel