Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1756772AbZD1XGw (ORCPT ); Tue, 28 Apr 2009 19:06:52 -0400 Received: (majordomo@vger.kernel.org) by vger.kernel.org id S1753596AbZD1XGn (ORCPT ); Tue, 28 Apr 2009 19:06:43 -0400 Received: from smtp1.linux-foundation.org ([140.211.169.13]:56547 "EHLO smtp1.linux-foundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753408AbZD1XGm (ORCPT ); Tue, 28 Apr 2009 19:06:42 -0400 Date: Tue, 28 Apr 2009 16:02:19 -0700 From: Andrew Morton To: Matt Mackall Cc: fengguang.wu@intel.com, linux-kernel@vger.kernel.org, kosaki.motohiro@jp.fujitsu.com, andi@firstfloor.org, adobriyan@gmail.com, linux-mm@kvack.org Subject: Re: [PATCH 5/5] proc: export more page flags in /proc/kpageflags Message-Id: <20090428160219.ca0123a1.akpm@linux-foundation.org> In-Reply-To: <1240958794.938.1045.camel@calx> References: <20090428010907.912554629@intel.com> <20090428014920.769723618@intel.com> <20090428143244.4e424d36.akpm@linux-foundation.org> <1240958794.938.1045.camel@calx> X-Mailer: Sylpheed version 2.2.4 (GTK+ 2.8.20; i486-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1475 Lines: 39 On Tue, 28 Apr 2009 17:46:34 -0500 Matt Mackall wrote: > > > +/* a helper function _not_ intended for more general uses */ > > > +static inline int page_cap_writeback_dirty(struct page *page) > > > +{ > > > + struct address_space *mapping; > > > + > > > + if (!PageSlab(page)) > > > + mapping = page_mapping(page); > > > + else > > > + mapping = NULL; > > > + > > > + return mapping && mapping_cap_writeback_dirty(mapping); > > > +} > > > > If the page isn't locked then page->mapping can be concurrently removed > > and freed. This actually happened to me in real-life testing several > > years ago. > > We certainly don't want to be taking locks per page to build the flags > data here. As we don't have any pretense of being atomic, it's ok if we > can find a way to do the test that's inaccurate when a race occurs, so > long as it doesn't dereference null. > > But if there's not an obvious way to do that, we should probably just > drop this flag bit for this iteration. trylock_page() could be used here, perhaps. Then again, why _not_ just do lock_page()? After all, few pages are ever locked. There will be latency if the caller stumbles across a page which is under read I/O, but so be it? -- 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/