Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756891Ab0LAXoD (ORCPT ); Wed, 1 Dec 2010 18:44:03 -0500 Received: from mx2.netapp.com ([216.240.18.37]:62441 "EHLO mx2.netapp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756589Ab0LAXn6 convert rfc822-to-8bit (ORCPT ); Wed, 1 Dec 2010 18:43:58 -0500 X-IronPort-AV: E=Sophos;i="4.59,285,1288594800"; d="scan'208";a="489755159" Subject: Re: [PATCH v2 3/3] NFS: Fix a memory leak in nfs_readdir From: Trond Myklebust To: Linus Torvalds Cc: Andrew Morton , Hugh Dickins , Nick Piggin , Nick Bowler , Linux Kernel Mailing List , linux-nfs@vger.kernel.org, Rik van Riel , Christoph Hellwig , Al Viro In-Reply-To: References: <1291217804-11257-1-git-send-email-Trond.Myklebust@netapp.com> <1291217804-11257-2-git-send-email-Trond.Myklebust@netapp.com> <20101201150428.GA2879@elliptictech.com> <1291217804-11257-3-git-send-email-Trond.Myklebust@netapp.com> <1291217804-11257-4-git-send-email-Trond.Myklebust@netapp.com> <1291229669.6609.24.camel@heimdal.trondhjem.org> <1291234251.6609.39.camel@heimdal.trondhjem.org> <20101201123341.d12ef362.akpm@linux-foundation.org> <20101201133831.ea6ba10a.akpm@linux-foundation.org> <1291240272.6609.50.camel@heimdal.trondhjem.org> <20101201141351.8609140b.akpm@linux-foundation.org> <20101201143856.51f4f9d9.akpm@linux-foundation.org> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8BIT Organization: NetApp Inc Date: Wed, 01 Dec 2010 18:43:56 -0500 Message-ID: <1291247036.6609.76.camel@heimdal.trondhjem.org> Mime-Version: 1.0 X-Mailer: Evolution 2.32.1 (2.32.1-1.fc14) X-OriginalArrivalTime: 01 Dec 2010 23:43:58.0075 (UTC) FILETIME=[9F7624B0:01CB91B1] Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2217 Lines: 58 On Wed, 2010-12-01 at 15:31 -0800, Linus Torvalds wrote: > On Wed, Dec 1, 2010 at 2:38 PM, Andrew Morton wrote: > > > > OK, the stop_machine() plugs a lot of potential race-vs-module-unload > > things. But Trond is referring to races against vmscan inode reclaim, > > unmount, etc. > > So? > > A filesystem module cannot be unloaded while it's still mounted. > > And unmount doesn't succeed until all inodes are gone. > > And getting rid of an inode doesn't succeed until all pages associated > with it are gone. > > And getting rid of the pages involves locking them (whether in > truncate or vmscan) and removing them from all lists. > > Ergo: vmscan has a locked page leads to the filesystem being > guaranteed to not be unmounted. And that, in turn, guarantees that > the module won't be unloaded until the machine has gone through an > idle cycle. > > It really is that simple. There's nothing subtle there. The reason > spin_unlock(&mapping->tree_lock) is safe is exactly the above trivial > chain of dependencies. And it's also exactly why > mapping->a_ops->freepage() would also be safe. > > This is pretty much how all the module races are handled. Doing module > ref-counts per page (or per packet in flight for things like > networking) would be prohibitively expensive. There's no way we can > ever do that. Although the page is locked, it may no longer be visible to the lockless page lookup once the radix_tree_delete() completes in __remove_from_page_cache. Furthermore, if the same routine causes mapping->nr_pages to go to zero before iput_final() hits truncate_inode_pages(), then the latter exits immediately. Both these cases would appear to allow iput_final() to release the inode before vmscan gets round to unlocking the mapping->tree_lock since truncate_inode_pages() no longer thinks it has any work to do. -- Trond Myklebust Linux NFS client maintainer NetApp Trond.Myklebust@netapp.com www.netapp.com -- 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/