From mboxrd@z Thu Jan  1 00:00:00 1970
From: "David S. Miller" <davem@davemloft.net>
Subject: Re: [PATCH] struct file cleanup : the very large file_ra_state is
 now allocated only on demand.
Date: Thu, 18 Aug 2005 00:18:54 -0700 (PDT)
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From: Eric Dumazet <dada1@cosmosbay.com>
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.