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=-2.3 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, 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 94016C43603 for ; Tue, 17 Dec 2019 06:23:18 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 711592072D for ; Tue, 17 Dec 2019 06:23:18 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1725856AbfLQGXR (ORCPT ); Tue, 17 Dec 2019 01:23:17 -0500 Received: from mx2.suse.de ([195.135.220.15]:46738 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725812AbfLQGXR (ORCPT ); Tue, 17 Dec 2019 01:23:17 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx2.suse.de (Postfix) with ESMTP id 06E4FB25B; Tue, 17 Dec 2019 06:23:14 +0000 (UTC) Subject: Re: [Xen-devel] [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected To: SeongJae Park Cc: axboe@kernel.dk, konrad.wilk@oracle.com, roger.pau@citrix.com, linux-block@vger.kernel.org, xen-devel@lists.xenproject.org, pdurrant@amazon.com, linux-kernel@vger.kernel.org References: <20191216194803.6294-1-sj38.park@gmail.com> From: =?UTF-8?B?SsO8cmdlbiBHcm/Dnw==?= Message-ID: Date: Tue, 17 Dec 2019 07:23:12 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.2.1 MIME-Version: 1.0 In-Reply-To: <20191216194803.6294-1-sj38.park@gmail.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On 16.12.19 20:48, SeongJae Park wrote: > On on, 16 Dec 2019 17:23:44 +0100, Jürgen Groß wrote: > >> On 16.12.19 17:15, SeongJae Park wrote: >>> On Mon, 16 Dec 2019 15:37:20 +0100 SeongJae Park wrote: >>> >>>> On Mon, 16 Dec 2019 13:45:25 +0100 SeongJae Park wrote: >>>> >>>>> From: SeongJae Park >>>>> >>> [...] >>>>> --- a/drivers/block/xen-blkback/xenbus.c >>>>> +++ b/drivers/block/xen-blkback/xenbus.c >>>>> @@ -824,6 +824,24 @@ static void frontend_changed(struct xenbus_device *dev, >>>>> } >>>>> >>>>> >>>>> +/* Once a memory pressure is detected, squeeze free page pools for a while. */ >>>>> +static unsigned int buffer_squeeze_duration_ms = 10; >>>>> +module_param_named(buffer_squeeze_duration_ms, >>>>> + buffer_squeeze_duration_ms, int, 0644); >>>>> +MODULE_PARM_DESC(buffer_squeeze_duration_ms, >>>>> +"Duration in ms to squeeze pages buffer when a memory pressure is detected"); >>>>> + >>>>> +/* >>>>> + * Callback received when the memory pressure is detected. >>>>> + */ >>>>> +static void reclaim_memory(struct xenbus_device *dev) >>>>> +{ >>>>> + struct backend_info *be = dev_get_drvdata(&dev->dev); >>>>> + >>>>> + be->blkif->buffer_squeeze_end = jiffies + >>>>> + msecs_to_jiffies(buffer_squeeze_duration_ms); >>>> >>>> This callback might race with 'xen_blkbk_probe()'. The race could result in >>>> __NULL dereferencing__, as 'xen_blkbk_probe()' sets '->blkif' after it links >>>> 'be' to the 'dev'. Please _don't merge_ this patch now! >>>> >>>> I will do more test and share results. Meanwhile, if you have any opinion, >>>> please let me know. > > I reduced system memory and attached bunch of devices in short time so that > memory pressure occurs while device attachments are ongoing. Under this > circumstance, I was able to see the race. > >>> >>> Not only '->blkif', but 'be' itself also coule be a NULL. As similar >>> concurrency issues could be in other drivers in their way, I suggest to change >>> the reclaim callback ('->reclaim_memory') to be called for each driver instead >>> of each device. Then, each driver could be able to deal with its concurrency >>> issues by itself. >> >> Hmm, I don't like that. This would need to be changed back in case we >> add per-guest quota. > > Extending this callback in that way would be still not too hard. We could use > the argument to the callback. I would keep the argument of the callback to > 'struct device *' as is, and will add a comment saying 'NULL' value of the > argument means every devices. As an example, xenbus would pass NULL-ending > array of the device pointers that need to free its resources. > > After seeing this race, I am now also thinking it could be better to delegate > detailed control of each device to its driver, as some drivers have some > complicated and unique relation with its devices. > >> >> Wouldn't a get_device() before calling the callback and a put_device() >> afterwards avoid that problem? > > I didn't used the reference count manipulation operations because other similar > parts also didn't. But, if there is no implicit reference count guarantee, it > seems those operations are indeed necessary. > > That said, as get/put operations only adjust the reference count, those will > not make the callback to wait until the linking of the 'backend' and 'blkif' to > the device (xen_blkbk_probe()) is finished. Thus, the race could still happen. > Or, am I missing something? No, I think we need a xenbus lock per device which will need to be taken in xen_blkbk_probe(), xenbus_dev_remove() and while calling the callback. Juergen