From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id F1BB7C38A2A for ; Thu, 7 May 2020 17:54:29 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C957721835 for ; Thu, 7 May 2020 17:54:29 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1588874069; bh=QJaiSjCjLbAWFDcmuf/Pneq2du1jY9r042RVJds1hO0=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:List-ID: From; b=y2o5uI0AbVLo5bvd95ZGOk1vUzJg0CeCDzE6QGfumhuoCbaMRCL/TlR245lUD7UoB +R1UPfNoRMIqzu0hj3TSoOEBvTio+QqkVv8Ptm8i5ouQE72uGviK+F94wq1YFS2O2G sRKIsDXan4ZNy4BE9YnI33x9O2mEtlt6/lW2R7MM= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728097AbgEGRy2 (ORCPT ); Thu, 7 May 2020 13:54:28 -0400 Received: from mail.kernel.org ([198.145.29.99]:44704 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726320AbgEGRy1 (ORCPT ); Thu, 7 May 2020 13:54:27 -0400 Received: from paulmck-ThinkPad-P72.home (50-39-105-78.bvtn.or.frontiernet.net [50.39.105.78]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPSA id 5B1D52145D; Thu, 7 May 2020 17:54:27 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1588874067; bh=QJaiSjCjLbAWFDcmuf/Pneq2du1jY9r042RVJds1hO0=; h=Date:From:To:Cc:Subject:Reply-To:References:In-Reply-To:From; b=pKD8QCZKdNeR4cgjo9ugXAKZQS8MIUuvWNTGi+44zq2pg862dujYCMPY35hu47AYD O6+hmKJ8TYF7QejWc8yOJ8ND9UfRKhg0yxPYnkEFAzRiYc86ZGk0CbtsKRZqwxRTJR +DLTSFziy5UChxjljtQe1VYZCq/s9RWAKawRIo20= Received: by paulmck-ThinkPad-P72.home (Postfix, from userid 1000) id 4493C35231BF; Thu, 7 May 2020 10:54:27 -0700 (PDT) Date: Thu, 7 May 2020 10:54:27 -0700 From: "Paul E. McKenney" To: Catalin Marinas Cc: Qian Cai , Linux-MM , LKML Subject: Re: Kmemleak infrastructure improvement for task_struct leaks and call_rcu() Message-ID: <20200507175427.GT2869@paulmck-ThinkPad-P72> Reply-To: paulmck@kernel.org References: <45D2D811-C3B0-442B-9744-415B4AC5CCDB@lca.pw> <20200506174019.GA2869@paulmck-ThinkPad-P72> <20200507171418.GC3180@gaia> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200507171418.GC3180@gaia> User-Agent: Mutt/1.9.4 (2018-02-28) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, May 07, 2020 at 06:14:19PM +0100, Catalin Marinas wrote: > On Wed, May 06, 2020 at 10:40:19AM -0700, Paul E. McKenney wrote: > > On Wed, May 06, 2020 at 12:22:37PM -0400, Qian Cai wrote: > > > == call_rcu() leaks == > > > Another issue that might be relevant is that it seems sometimes, > > > kmemleak will give a lot of false positives (hundreds) because the > > > memory was supposed to be freed by call_rcu() (for example, in > > > dst_release()) but for some reasons, it takes a long time probably > > > waiting for grace periods or some kind of RCU self-stall, but the > > > memory had already became an orphan. I am not sure how we are going > > > to resolve this properly until we have to figure out why call_rcu() > > > is taking so long to finish? > > > > I know nothing about kmemleak, but I won't let that stop me from making > > random suggestions... > > > > One approach is to do an rcu_barrier() inside kmemleak just before > > printing leaked blocks, and check to see if any are still leaked after > > the rcu_barrier(). > > The main issue is that kmemleak doesn't stop the world when scanning > (which can take over a minute, depending on your hardware), so we get > lots of transient pointer misses. There are some heuristics but > obviously they don't always work. > > With RCU, objects are queued for RCU freeing later and chained via > rcu_head.next (IIUC). Under load, this list can be pretty volatile and > if this happen during kmemleak scanning, it's sufficient to lose track > of a next pointer and the rest of the list would be reported as a leak. > > I think rcu_barrier() just before the starting the kmemleak scanning may > help if it reduces the number of objects queued. It might, especially if the call_rcu() rate is lower after the rcu_barrier() than it was beforehand. Which might well be the case when a large cleanup activity ended just before rcu_barrier() was invoked. > Now, I wonder whether kmemleak itself can break this RCU chain. The > kmemleak metadata is allocated on a slab alloc callback. The freeing, > however, is done using call_rcu() because originally calling back into > the slab freeing from kmemleak_free() didn't go well. Since the > kmemleak_object structure is not tracked by kmemleak, I wonder whether > its rcu_head would break this directed pointer reference graph. It is true that kmemleak could decide that being passed to call_rcu() as being freed. However, it would need to know the rcu_head offset. And there are (or were) a few places that pass linked structures to call_rcu(), and kmemleak would presumably need to mark them all free at that point. Or maybe accept the much lower false-positive rate from not marking them. > Let's try the rcu_barrier() first and I'll think about the metadata case > over the weekend. Looking forward to hearing how it goes! Thanx, Paul