Re: [PATCH] Shrink rbtree

[Hugh Dickins <hugh@veritas.com>]

On Fri, 21 Apr 2006, David Woodhouse wrote:
> On Fri, 2006-04-21 at 23:06 +1000, Nick Piggin wrote:
> > How do we know the pointers are always going to be aligned? IIRC
> > struct address_space needed to be explicitly aligned when doing
> > this trick in page->mapping because some platform byte aligned it.
> 
> Really? I've been doing this kind of trick with the jffs2_raw_node_ref
> for years. We always allocate sufficiently aligned objects.

} __attribute__((aligned(sizeof(long))));
	/*
	 * On most architectures that alignment is already the case; but
	 * must be enforced here for CRIS, to let the least signficant bit
	 * of struct page's "mapping" pointer be used for PAGE_MAPPING_ANON.
	 */

You can often get away with it - I notice we never added the same
alignment to struct anon_vma, which in theory needed it just as much.
Some accident of how structures are packed into slabs on CRIS, I suppose.

Hugh
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