Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758057Ab1DAPwW (ORCPT ); Fri, 1 Apr 2011 11:52:22 -0400 Received: from smtp1.linux-foundation.org ([140.211.169.13]:41497 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754723Ab1DAPwS convert rfc822-to-8bit (ORCPT ); Fri, 1 Apr 2011 11:52:18 -0400 MIME-Version: 1.0 In-Reply-To: References: From: Linus Torvalds Date: Fri, 1 Apr 2011 08:44:50 -0700 Message-ID: Subject: Re: [PATCH] mm: fix possible cause of a page_mapped BUG To: =?UTF-8?B?Um9iZXJ0IMWad2nEmWNraQ==?= 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: 1914 Lines: 47 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. 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 -- 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/