Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932218Ab1DAQV0 (ORCPT ); Fri, 1 Apr 2011 12:21:26 -0400 Received: from mail-ey0-f174.google.com ([209.85.215.174]:35304 "EHLO mail-ey0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752839Ab1DAQVZ convert rfc822-to-8bit (ORCPT ); Fri, 1 Apr 2011 12:21:25 -0400 MIME-Version: 1.0 In-Reply-To: References: Date: Fri, 1 Apr 2011 18:21:23 +0200 Message-ID: Subject: Re: [PATCH] mm: fix possible cause of a page_mapped BUG From: =?UTF-8?B?Um9iZXJ0IMWad2nEmWNraQ==?= To: Linus Torvalds Cc: Hugh Dickins , Andrew Morton , Miklos Szeredi , Michel Lespinasse , "Eric W. Biederman" , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Peter Zijlstra , Rik van Riel Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8BIT Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2776 Lines: 70 On Fri, Apr 1, 2011 at 5:44 PM, Linus Torvalds wrote: > On Fri, Apr 1, 2011 at 7:34 AM, Robert Święcki wrote: >> >> Hey, I'll apply your patch and check it out. In the meantime I >> triggered another Oops (NULL-ptr deref via sys_mprotect). >> >> The oops is here: >> >> http://alt.swiecki.net/linux_kernel/sys_mprotect-2.6.38.txt > > That's not a NULL pointer dereference. That's a BUG_ON(). > > And for some reason you've turned off the BUG_ON() messages, saving > some tiny amount of memory. Is it possible to turn it off via config flags? Looking into arch/x86/include/asm/bug.h it seems it's unconditional (as in "it always manifests itself somehow") and I have CONFIG_DEBUG_BUGVERBOSE=y. This BUG/Oopps was triggered before I applied Hugh's patch on a vanilla kernel. Anything that could help you debugging this? Uploading kernel image (unfortunately I've overwritten this one), dumping more kgdb data? I must admit I'm not up-to-date with current linux kernel debugging techniques. The kernel config is here: http://alt.swiecki.net/linux_kernel/ise-test-2.6.38-kernel-config.txt For now I'll compile with -O0 -fno-inline (are you sure you'd like -Os?) > Anyway, it looks like the first BUG_ON() in vma_prio_tree_add(), so it > would be this one: > >        BUG_ON(RADIX_INDEX(vma) != RADIX_INDEX(old)); > > but it is possible that gcc has shuffled things around (so it _might_ > be the HEAP_INDEX() one). If you had CONFIG_DEBUG_BUGVERBOSE=y, you'd > get a filename and line number. One reason I hate -O2 in cases like > this is that the basic block movement makes it way harder to actually > debug things. I would suggest using -Os too (CONFIG_OPTIMIZE_FOR_SIZE > or whatever it's called). > > Anyway, I do find it worrying. The vma code shouldn't be this fragile.  Hugh? > > I do wonder what triggers this. Is it a huge-page vma? We seem to be > lacking the check to see that mprotect() is on a hugepage boundary - > and that seems bogus. Or am I missing some check? The new transparent > hugepage support splits the page, but what if it's a _static_ hugepage > thing? > > But why would that affect the radix_index thing? I have no idea. I'd > like to blame the anon_vma rewrites last year, but I can't see why > that should matter either. Again, hugepages had some special rules, I > think (and that would explain why nobody normal sees this). > > Guys, please give this one a look. > >                          Linus > -- Robert Święcki -- 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/