From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail202.messagelabs.com (mail202.messagelabs.com [216.82.254.227]) by kanga.kvack.org (Postfix) with ESMTP id 8EEA26B0085 for ; Tue, 8 Sep 2009 11:34:55 -0400 (EDT) Date: Tue, 8 Sep 2009 17:34:41 +0200 From: Nick Piggin Subject: Re: [PATCH 7/8] mm: reinstate ZERO_PAGE Message-ID: <20090908153441.GB29902@wotan.suse.de> References: <20090908073119.GA29902@wotan.suse.de> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: owner-linux-mm@kvack.org To: Hugh Dickins Cc: Andrew Morton , KAMEZAWA Hiroyuki , KOSAKI Motohiro , Linus Torvalds , Rik van Riel , linux-kernel@vger.kernel.org, linux-mm@kvack.org, linux-arch@vger.kernel.org List-ID: 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, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org