From: Eric Dumazet <redacted>
Date: Thu, 18 Aug 2005 09:14:47 +0200

After reading your suggestions, I understand we still need two slabs.
One (filp_cachep) without the readahead data, the other one
(filp_ra_cachep) with it.

Correct.

static inline struct file_ra_state *get_ra_state(struct file *f)
{
#ifdef CONFIG_DEBUG_READAHEAD
	BUG_ON(filp_ra_cachep != GET_PAGE_CACHE(virt_to_page(f)));
#endif
	return (struct file_ra_state *)(f+1);
}

Or, instead of mucking with SLAB internals, just put something like
a "filp_ra_present" debugging binary state into filp while it gets
debugged.