Re: [PATCH 7/8] mm: reinstate ZERO_PAGE

Hugh Dickins
2009-09-08T12:17:43Z

On Tue, 8 Sep 2009, Nick Piggin wrote:
> On Mon, Sep 07, 2009 at 10:39:34PM +0100, Hugh Dickins wrote:
> > KAMEZAWA Hiroyuki has observed customers of earlier kernels taking
> > advantage of the ZERO_PAGE: which we stopped do_anonymous_page() from
> > using in 2.6.24. And there were a couple of regression reports on LKML.
> >
> > Following suggestions from Linus, reinstate do_anonymous_page() use of
> > the ZERO_PAGE; but this time avoid dirtying its struct page cacheline
> > with (map)count updates - let vm_normal_page() regard it as abnormal.
> >
> > Use it only on arches which __HAVE_ARCH_PTE_SPECIAL (x86, s390, sh32,
> > most powerpc): that's not essential, but minimizes additional branches
> > (keeping them in the unlikely pte_special case); and incidentally
> > excludes mips (some models of which needed eight colours of ZERO_PAGE
> > to avoid costly exceptions).
>
> Without looking closely, why is it a big problem to have a
> !HAVE PTE SPECIAL case? Couldn't it just be a check for
> pfn == zero_pfn that is conditionally compiled away for pte
> special architectures anyway?

Yes, I'm uncomfortable with that restriction too: it makes for
neater looking code in a couple of places, but it's not so good
for the architectures to diverge gratuitously there.

I'll give it a try without that restriction, see how it looks:
it was Linus who proposed the "special" approach, I'm sure he'll
speak up if he doesn't like how the alternative comes out.

Tucking the test away in an asm-generic macro, we can leave
the pain of a rangetest to the one mips case.

By the way, in compiling that list of "special" architectures,
I was surprised not to find ia64 amongst them. Not that it
matters to me, but I thought the Fujitsu guys were usually
keen on Itanium - do they realize that the special test is
excluding it, or do they have their own special patch for it?

>
> If zero page is such a good idea, I don't see the logic of
> limiting it like thisa. Your patch looks pretty clean though.
>
> At any rate, I think it might be an idea to cc linux-arch.

Yes, thanks.

Hugh