From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-qt0-f197.google.com (mail-qt0-f197.google.com [209.85.216.197]) by kanga.kvack.org (Postfix) with ESMTP id F298E6B0033 for ; Mon, 11 Dec 2017 12:04:55 -0500 (EST) Received: by mail-qt0-f197.google.com with SMTP id 18so21948136qtt.10 for ; Mon, 11 Dec 2017 09:04:55 -0800 (PST) Received: from mx0a-001b2d01.pphosted.com (mx0a-001b2d01.pphosted.com. [148.163.156.1]) by mx.google.com with ESMTPS id x15si119628qth.269.2017.12.11.09.04.54 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 11 Dec 2017 09:04:54 -0800 (PST) Received: from pps.filterd (m0098410.ppops.net [127.0.0.1]) by mx0a-001b2d01.pphosted.com (8.16.0.21/8.16.0.21) with SMTP id vBBH44vY082761 for ; Mon, 11 Dec 2017 12:04:53 -0500 Received: from e14.ny.us.ibm.com (e14.ny.us.ibm.com [129.33.205.204]) by mx0a-001b2d01.pphosted.com with ESMTP id 2eswub0tvf-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Mon, 11 Dec 2017 12:04:52 -0500 Received: from localhost by e14.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Mon, 11 Dec 2017 12:04:51 -0500 Date: Mon, 11 Dec 2017 09:04:49 -0800 From: "Paul E. McKenney" Subject: Re: [PATCH -mm] mm, swap: Fix race between swapoff and some swap operations Reply-To: paulmck@linux.vnet.ibm.com References: <20171207011426.1633-1-ying.huang@intel.com> <20171207162937.6a179063a7c92ecac77e44af@linux-foundation.org> <20171208014346.GA8915@bbox> <87po7pg4jt.fsf@yhuang-dev.intel.com> <20171208082644.GA14361@bbox> <87k1xxbohp.fsf@yhuang-dev.intel.com> <20171208140909.4e31ba4f1235b638ae68fd5c@linux-foundation.org> <87609dvnl0.fsf@yhuang-dev.intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <87609dvnl0.fsf@yhuang-dev.intel.com> Message-Id: <20171211170449.GS7829@linux.vnet.ibm.com> Sender: owner-linux-mm@kvack.org List-ID: To: "Huang, Ying" Cc: Andrew Morton , Minchan Kim , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Hugh Dickins , Johannes Weiner , Tim Chen , Shaohua Li , Mel Gorman , =?utf-8?B?Su+/vXLvv71tZQ==?= Glisse , Michal Hocko , Andrea Arcangeli , David Rientjes , Rik van Riel , Jan Kara , Dave Jiang , Aaron Lu On Mon, Dec 11, 2017 at 01:30:03PM +0800, Huang, Ying wrote: > Andrew Morton writes: > > > On Fri, 08 Dec 2017 16:41:38 +0800 "Huang\, Ying" wrote: > > > >> > Why do we need srcu here? Is it enough with rcu like below? > >> > > >> > It might have a bug/room to be optimized about performance/naming. > >> > I just wanted to show my intention. > >> > >> Yes. rcu should work too. But if we use rcu, it may need to be called > >> several times to make sure the swap device under us doesn't go away, for > >> example, when checking si->max in __swp_swapcount() and > >> add_swap_count_continuation(). And I found we need rcu to protect swap > >> cache radix tree array too. So I think it may be better to use one > >> calling to srcu_read_lock/unlock() instead of multiple callings to > >> rcu_read_lock/unlock(). > > > > Or use stop_machine() ;) It's very crude but it sure is simple. Does > > anyone have a swapoff-intensive workload? > > Sorry, I don't know how to solve the problem with stop_machine(). > > The problem we try to resolved is that, we have a swap entry, but that > swap entry can become invalid because of swappoff between we check it > and we use it. So we need to prevent swapoff to be run between checking > and using. > > I don't know how to use stop_machine() in swapoff to wait for all users > of swap entry to finish. Anyone can help me on this? You can think of stop_machine() as being sort of like a reader-writer lock. The readers can be any section of code with preemption disabled, and the writer is the function passed to stop_machine(). Users running real-time applications on Linux don't tend to like stop_machine() much, but perhaps it is nevertheless the right tool for this particular job. Thanx, Paul -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org