Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756626AbZF3Teb (ORCPT ); Tue, 30 Jun 2009 15:34:31 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1752939AbZF3TeY (ORCPT ); Tue, 30 Jun 2009 15:34:24 -0400 Received: from hera.kernel.org ([140.211.167.34]:49261 "EHLO hera.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752723AbZF3TeX (ORCPT ); Tue, 30 Jun 2009 15:34:23 -0400 Message-ID: <4A4A6888.30001@kernel.org> Date: Tue, 30 Jun 2009 12:33:28 -0700 From: Yinghai Lu User-Agent: Thunderbird 2.0.0.19 (X11/20081227) MIME-Version: 1.0 To: "H. Peter Anvin" CC: Mikael Pettersson , Linus Torvalds , Ingo Molnar , Matthew Wilcox , Grant Grundler , Linux Kernel Mailing List , linux-pci@vger.kernel.org Subject: Re: [BUG 2.6.31-rc1] HIGHMEM64G causes hang in PCI init on 32-bit x86 References: <200906261559.n5QFxJH8027336@pilspetsen.it.uu.se> <19013.29264.623540.275538@pilspetsen.it.uu.se> <4A45A5C1.5080701@zytor.com> <19013.59956.144640.331854@pilspetsen.it.uu.se> <20090629022911.GC20297@lackof.org> <4A484A8A.9020704@zytor.com> <19016.41349.636663.515540@pilspetsen.it.uu.se> <20090629112155.GJ5480@parisc-linux.org> <19016.44061.600652.676183@pilspetsen.it.uu.se> <4A490804.3040609@zytor.com> <4A494478.7020304@kernel.org> <4A494E3C.70304@kernel.org> <4A495C0D.2020807@zytor.com> <4A4966EF.6010809@kernel.org> <4A496D4B.3040608@kernel.org> <19017.53428.834539.389495@pilspetsen.it.uu.se> <4A4A25B1.5010102@zytor.com> In-Reply-To: <4A4A25B1.5010102@zytor.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1998 Lines: 52 H. Peter Anvin wrote: > Mikael Pettersson wrote: >> Thanks, 2.6.31-rc1 vanilla (which didn't boot) plus this one does boot. >> /proc/iomem now looks as follows: >> > > ... as it should. So far so good, and this is a real problem. > > However, there is something that really bothers me: *why does this help > on Mikael's system, which is PAE and therefore has a 64-bit > resource_size_t*? This whole patch should be a no-op! There is still > something that doesn't make sense. > > The use of "unsigned long" in ram_alignment() will overflow after 2^52 > bytes, but again, that's not the issue here, since the highest "start" > value we have is (0x2 << 32). > > By process of elimination, the culprit must be round_up(), which reveals > that the macro definition of round_up() has a *very* sublte behavior > with mixed types: > > #define round_up(x, y) (((x) + (y) - 1) & ~((y) - 1)) > > ram_alignment() returns unsigned long, which becomes (y). This means > that the mask word on the right hand of the & gets truncated to 32 bits > *before* the masking happens -- since ((y) - 1) is still unsigned long, > inverting it will not set bits [63..32] to on. > > I think this macro is actively dangerous. Better would be: > > ({ __typeof__(x) __mask = (y)-1; ((x)+__mask) & ~__mask; }) > > ... which is also multiple-inclusion-free at the cost of using gcc > ({...}) constructs. > > The deep irony in this is that in our particular case is perhaps that > align_up(x,y)-1 is the same thing as x | (y-1) which would have avoided > the problem... agreed, that is why we change round_up to take u64. wonder if we should kill round_up and use roundup instead. in include/linux/kernel.h #define roundup(x, y) ((((x) + ((y) - 1)) / (y)) * (y)) YH -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/