Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1762600Ab2KBBno (ORCPT ); Thu, 1 Nov 2012 21:43:44 -0400 Received: from mx1.redhat.com ([209.132.183.28]:26821 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1762542Ab2KBBnn (ORCPT ); Thu, 1 Nov 2012 21:43:43 -0400 Date: Thu, 1 Nov 2012 21:43:36 -0400 From: Dave Jones To: Hugh Dickins Cc: linux-mm@kvack.org, linux-kernel@vger.kernel.org Subject: Re: shmem_getpage_gfp VM_BUG_ON triggered. [3.7rc2] Message-ID: <20121102014336.GA1727@redhat.com> Mail-Followup-To: Dave Jones , Hugh Dickins , linux-mm@kvack.org, linux-kernel@vger.kernel.org References: <20121025023738.GA27001@redhat.com> <20121101191052.GA5884@redhat.com> <20121101232030.GA25519@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2026 Lines: 45 On Thu, Nov 01, 2012 at 04:48:41PM -0700, Hugh Dickins wrote: > On Thu, 1 Nov 2012, Dave Jones wrote: > > On Thu, Nov 01, 2012 at 04:03:40PM -0700, Hugh Dickins wrote: > > > > > > Except... earlier in the thread you explained how you hacked > > > #define VM_BUG_ON(cond) WARN_ON(cond) > > > to get this to come out as a warning instead of a bug, > > > and now it looks as if "a user" has here done the same. > > > > > > Which is very much a user's right, of course; but does > > > make me wonder whether that user might actually be davej ;) > > > > indirectly. I made the same change in the Fedora kernel a while ago > > to test a hypothesis that we weren't getting any VM_BUG_ON reports. > > Fedora turns on CONFIG_DEBUG_VM? Yes. > All mm developers should thank you for the wider testing exposure; > but I'm not so sure that Fedora users should thank you for turning > it on - really it's for mm developers to wrap around !assertions or > more expensive checks (e.g. checking calls) in their development. The last time I did some benchmarking the impact wasn't as ridiculous as say lockdep, or spinlock debug. Maybe the benchmarks I was using weren't pushing the VM very hard, but it seemed to me that the value in getting info in potential problems early was higher than a small performance increase. > Or did I read a few months ago that some change had been made to > such definitions, and VM_BUG_ON(contents) are evaluated even when > the config option is off? I do hope I'm mistaken on that. Pretty sure that isn't the case. I remember Andrew chastising people a few times for putting checks in VM_BUG_ON's that needed to stay around even when the config option was off. Perhaps you were thinking of one of those incidents ? 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/