On Tue, Sep 08, 2009 at 01:17:01PM +0100, Hugh Dickins wrote: > 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. I guess using special is pretty neat and doesn't require an additional branch in vm_normal_page paths. But I think it is important to allow other architectures at least the _option_ to have equivalent behaviour as x86 here. So it would be great if you would look into it. > 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? I don't understand your question. Are you asking whether they know your patch will not enable zero pages on ia64? I guess pte special was primarily driven by gup_fast, which in turn was driven primarily by DB2 9.5, which I think might be only available on x86 and ibm's architectures. But I admit to being a curious as to when I'll see a gup_fast patch come out of SGI or HP or Fujitsu :) -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
