Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757966AbZIFNnP (ORCPT ); Sun, 6 Sep 2009 09:43:15 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1757950AbZIFNnL (ORCPT ); Sun, 6 Sep 2009 09:43:11 -0400 Received: from e39.co.us.ibm.com ([32.97.110.160]:60391 "EHLO e39.co.us.ibm.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757730AbZIFNnK (ORCPT ); Sun, 6 Sep 2009 09:43:10 -0400 Subject: Re: [patch 14/48] KVM: Reduce kvm stack usage in kvm_arch_vm_ioctl() From: Dave Hansen To: Pavel Machek Cc: Greg KH , linux-kernel@vger.kernel.org, stable@kernel.org, stable-review@kernel.org, torvalds@linux-foundation.org, akpm@linux-foundation.org, alan@lxorguk.ukuu.org.uk, Avi Kivity , Avi Kivity In-Reply-To: <20090906054749.GB1431@ucw.cz> References: <20090904200712.724048145@mini.kroah.org> <20090904200852.199656362@mini.kroah.org> <20090906054749.GB1431@ucw.cz> Content-Type: text/plain Date: Sun, 06 Sep 2009 06:43:09 -0700 Message-Id: <1252244589.14385.22402.camel@nimitz> Mime-Version: 1.0 X-Mailer: Evolution 2.26.1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2750 Lines: 77 On Sun, 2009-09-06 at 07:47 +0200, Pavel Machek wrote: > On Fri 2009-09-04 13:07:26, Greg KH wrote: > > On my machine with gcc 3.4, kvm uses ~2k of stack in a few > > select functions. This is mostly because gcc fails to > > notice that the different case: statements could have their > > stack usage combined. It overflows very nicely if interrupts > > happen during one of these large uses. > > > > This patch uses two methods for reducing stack usage. > > 1. dynamically allocate large objects instead of putting > > on the stack. > > 2. Use a union{} member for all of the case variables. This > > tricks gcc into combining them all into a single stack > > allocation. (There's also a comment on this) > > Are the 'reduce stack usage' patches suitable for stable? The rules > said that fix must be for 'serious problem', not 'theoretical > issue'... I guess some context got dropped at some point. I was getting really consistent oopses and goofy memory corruption when running KVM: http://lkml.org/lkml/2008/3/25/340 Not theoretical at all. I think it cost me a few new gray hairs. > > Signed-off-by: Dave Hansen > > Signed-off-by: Avi Kivity > > Signed-off-by: Greg Kroah-Hartman > > --- > > arch/x86/kvm/x86.c | 72 +++++++++++++++++++++++++++++++++-------------------- > > 1 file changed, 45 insertions(+), 27 deletions(-) > > > > --- a/arch/x86/kvm/x86.c > > +++ b/arch/x86/kvm/x86.c > > @@ -1630,6 +1630,15 @@ long kvm_arch_vm_ioctl(struct file *filp > > struct kvm *kvm = filp->private_data; > > void __user *argp = (void __user *)arg; > > int r = -EINVAL; > > + /* > > + * This union makes it completely explicit to gcc-3.x > > + * that these two variables' stack usage should be > > + * combined, not added together. > > + */ > > + union { > > + struct kvm_pit_state ps; > > + struct kvm_memory_alias alias; > > + } u; > > > > switch (ioctl) { > > case KVM_SET_TSS_ADDR: > > ...plus this is really ugly hack. Just declare the variable inside the > case block that needs it? Do we need to give that a better comment? It's explained a bit better here: http://lkml.org/lkml/2008/7/17/12 http://lkml.org/lkml/2008/7/17/16 Would this comment help? /* * gcc-3.x will sum the stack usage of two stack variables * if they are declared in two different case blocks. This * union makes it explicit that their stack space can be * shared which greatly reduces stack usage. */ -- Dave -- 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/