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SubjectRe: [rfc] increase struct page size?!
On Fri, May 18, 2007 at 04:42:10PM +0100, Hugh Dickins wrote:
> On Fri, 18 May 2007, Nick Piggin wrote:
> >
> > If we add 8 bytes to struct page on 64-bit machines, it becomes 64 bytes,
> > which is quite a nice number for cache purposes.
> >
> > However we don't have to let those 8 bytes go to waste: we can use them
> > to store the virtual address of the page, which kind of makes sense for
> > 64-bit, because they can likely to use complicated memory models.
> Sooner rather than later, don't we need those 8 bytes to expand from
> atomic_t to atomic64_t _count and _mapcount? Not that we really need
> all 64 bits of both, but I don't know how to work atomically with less.

Yeah, that would be a very good use of it.

> (Why do I have this sneaking feeling that you're actually wanting
> to stick something into the lower bits of page->virtual?)

No, I just thought I would get even more flamed if I was just adding
padding that wasn't even doing _anything_ :) I remembered Ken had a
benchmark where page_address was slow, and the rest is history...
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