From: Linus Torvalds Subject: Re: 2.6.25-git2: BUG: unable to handle kernel paging request at ffffffffffffffff Date: Mon, 21 Apr 2008 10:30:19 -0700 (PDT) Message-ID: References: <200804191522.54334.rjw@sisk.pl> <200804202104.24037.rjw@sisk.pl> <20080421011855.GA6243@gondor.apana.org.au> <20080421020806.GL20138@linux.vnet.ibm.com> <20080421170526.GC9153@linux.vnet.ibm.com> Mime-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Cc: Herbert Xu , "Rafael J. Wysocki" , LKML , Ingo Molnar , Andrew Morton , linux-ext4@vger.kernel.org To: "Paul E. McKenney" Return-path: Received: from smtp1.linux-foundation.org ([140.211.169.13]:41310 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755851AbYDURbn (ORCPT ); Mon, 21 Apr 2008 13:31:43 -0400 In-Reply-To: <20080421170526.GC9153@linux.vnet.ibm.com> Sender: linux-ext4-owner@vger.kernel.org List-ID: On Mon, 21 Apr 2008, Paul E. McKenney wrote: > > I did take a quick look for improperly freeing dentries -- unhashed > dentries are freed directly, so if there is a code path that somehow > unhashes dentries and then d_free()s them without a grace period, we > have a problem. No, not even then. We *always* unhash the dentries before freeing them, but we very consciously use "hlist_del_rcu()" on them, not "hlist_del_init()". That, in turn, will mean that the "pprev" pointer will still be set, so the "hlist_unhashed()" thing will *not* trigger. IOW, when we do that direct-free with: if (hlist_unhashed(&dentry->d_hash)) __d_free(dentry); the "hlist_unhashed()" will literally guarantee that i has *never* been on a hash-list at all! (If you want to test whether it is currently unhashed or not, you actually have to use "d_unhashed()" on the dentry under the dentry lock, which tests the DCACHE_UNHASHED bit). Of course, there could be some bug in there, but the thing is, none of this has even changed in a long time, certainly not since 2.6.25. Which is why I think the dcache code is all fine, and the bug comes from somewhere else corrupting the data structures. Linus