Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S964999Ab3GRT7G (ORCPT ); Thu, 18 Jul 2013 15:59:06 -0400 Received: from mail-pa0-f52.google.com ([209.85.220.52]:49688 "EHLO mail-pa0-f52.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S934054Ab3GRT7A (ORCPT ); Thu, 18 Jul 2013 15:59:00 -0400 Date: Thu, 18 Jul 2013 12:59:10 -0700 (PDT) From: Hugh Dickins X-X-Sender: hugh@eggly.anvils To: Dave Jones cc: Dave Hansen , Andrew Morton , "Kirill A. Shutemov" , Andrea Arcangeli , Al Viro , Wu Fengguang , Jan Kara , Mel Gorman , linux-mm@kvack.org, Andi Kleen , Matthew Wilcox , "Kirill A. Shutemov" , Hillf Danton , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 5/8] thp, mm: locking tail page is a bug In-Reply-To: <20130718020414.GA1786@redhat.com> Message-ID: References: <1373885274-25249-1-git-send-email-kirill.shutemov@linux.intel.com> <1373885274-25249-6-git-send-email-kirill.shutemov@linux.intel.com> <20130717140953.7560e88e607f8f5df1b1fdd8@linux-foundation.org> <51E71E29.6030703@sr71.net> <20130718020414.GA1786@redhat.com> User-Agent: Alpine 2.00 (LNX 1167 2008-08-23) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2144 Lines: 45 On Wed, 17 Jul 2013, Dave Jones wrote: > On Wed, Jul 17, 2013 at 05:58:13PM -0700, Hugh Dickins wrote: > > On Wed, 17 Jul 2013, Dave Hansen wrote: > > > On 07/17/2013 02:09 PM, Andrew Morton wrote: > > > > lock_page() is a pretty commonly called function, and I assume quite a > > > > lot of people run with CONFIG_DEBUG_VM=y. > > > > > > > > Is the overhead added by this patch really worthwhile? > > > > > > I always thought of it as a developer-only thing. I don't think any of > > > the big distros turn it on by default. > > > > That's how I think of it too (and the problem is often that too few mm > > developers turn it on); but Dave Jones did confirm last November that > > Fedora turns it on. > > > > I believe Fedora turns it on to help us all, and wouldn't mind a mere > > VM_BUG_ON(PageTail(page)) in __lock_page() if it's helpful to Kirill. > > > > But if VM_BUG_ONs become expensive, I do think it's for Fedora to > > turn off CONFIG_DEBUG_VM, rather than for mm developers to avoid it. > > I'm ambivalent about whether we keep it on or off, we have no shortage > of bugs to fix already, though I think as mentioned above, very few people > actually enable it, so we're going to lose a lot of testing. > > Another idea, perhaps is an extra config option for more expensive debug options ? I can't think of any expensive debug under CONFIG_DEBUG_VM at present, just a little overhead as you'd expect. I don't think checking a page flag or two in __lock_page() will change that much. We do tend to make specific debug options for the more expensive ones, rather than a generic heavy debug option. For example CONFIG_DEBUG_VM_RB, which checks an mm's entire vma tree whenever a change is made there. We could group the heavy debug options, like that and CONFIG_PROVE_LOCKING, into a menu of their own; but I've no appetite for such a change myself! Hugh -- 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/