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=-3.9 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT 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 9F10FC43603 for ; Tue, 17 Dec 2019 16:17:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 6848621582 for ; Tue, 17 Dec 2019 16:17:57 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=amazon.com header.i=@amazon.com header.b="Qgw5V+G4" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729346AbfLQQR4 (ORCPT ); Tue, 17 Dec 2019 11:17:56 -0500 Received: from smtp-fw-9102.amazon.com ([207.171.184.29]:43915 "EHLO smtp-fw-9102.amazon.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728952AbfLQQRu (ORCPT ); Tue, 17 Dec 2019 11:17:50 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=amazon.com; i=@amazon.com; q=dns/txt; s=amazon201209; t=1576599470; x=1608135470; h=from:to:cc:subject:date:message-id:mime-version: in-reply-to:content-transfer-encoding; bh=cXvLi4yNQxt3p65AJ8lZq7BOonBnO1ATPy+SuVrgHOI=; b=Qgw5V+G4nw8D2NrShzEM+ip/dbLpYy2Ggw8ILbE/QN2aJoVf7Z0BSQy8 7s1V77I8tTJ8fcXF9rcX841hJD6Bnl3xCvQFnAfSbznyQ76Q277Hio+7z VRFnH2TweOQhwLSveIQWv2KurvGQnbAUJwAnyGpOxX8vEVkG/baDBOnj1 Q=; IronPort-SDR: SloTP35ntItl/SfHlYTc4zaNzogpagd/+j5Ygf9NYQzdLPnKww+k+nyZ6yb2Y5MwjvPTpmav48 mXrbXNg6QeAg== X-IronPort-AV: E=Sophos;i="5.69,326,1571702400"; d="scan'208";a="14052216" Received: from sea32-co-svc-lb4-vlan3.sea.corp.amazon.com (HELO email-inbound-relay-1a-67b371d8.us-east-1.amazon.com) ([10.47.23.38]) by smtp-border-fw-out-9102.sea19.amazon.com with ESMTP; 17 Dec 2019 16:17:49 +0000 Received: from EX13MTAUEA001.ant.amazon.com (iad55-ws-svc-p15-lb9-vlan3.iad.amazon.com [10.40.159.166]) by email-inbound-relay-1a-67b371d8.us-east-1.amazon.com (Postfix) with ESMTPS id 7BD78A2122; Tue, 17 Dec 2019 16:17:46 +0000 (UTC) Received: from EX13D31EUA001.ant.amazon.com (10.43.165.15) by EX13MTAUEA001.ant.amazon.com (10.43.61.243) with Microsoft SMTP Server (TLS) id 15.0.1367.3; Tue, 17 Dec 2019 16:17:45 +0000 Received: from u886c93fd17d25d.ant.amazon.com (10.43.161.179) by EX13D31EUA001.ant.amazon.com (10.43.165.15) with Microsoft SMTP Server (TLS) id 15.0.1367.3; Tue, 17 Dec 2019 16:17:40 +0000 From: SeongJae Park To: SeongJae Park CC: =?UTF-8?q?J=C3=BCrgen=20Gro=C3=9F?= , , SeongJae Park , , , , , , Subject: Re: Re: [Xen-devel] [PATCH v10 2/4] xen/blkback: Squeeze page pools if a memory pressure is detected Date: Tue, 17 Dec 2019 17:17:24 +0100 Message-ID: <20191217161724.3478-1-sjpark@amazon.com> X-Mailer: git-send-email 2.17.1 MIME-Version: 1.0 In-Reply-To: <20191217083032.19400-1-sjpark@amazon.com> (raw) Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit X-Originating-IP: [10.43.161.179] X-ClientProxiedBy: EX13D23UWC002.ant.amazon.com (10.43.162.22) To EX13D31EUA001.ant.amazon.com (10.43.165.15) Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Tue, 17 Dec 2019 09:30:32 +0100 SeongJae Park wrote: > On Tue, 17 Dec 2019 09:16:47 +0100 "Jürgen Groß" wrote: > > > On 17.12.19 08:59, SeongJae Park wrote: > > > On Tue, 17 Dec 2019 07:23:12 +0100 "Jürgen Groß" wrote: > > > > > >> 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. > > > > > > I also agree that locking should be used at last. But, as each driver manages > > > its devices and resources in their way, it could have its unique race > > > conditions. And, each unique race condition might have its unique efficient > > > way to synchronize it. Therefore, I think the synchronization should be done > > > by each driver, not by xenbus and thus we should make the callback to be called > > > per-driver. > > > > xenbus controls creation and removing of devices, so applying locking > > at xenbus level is the right thing to do in order to avoid races with > > device removal. > > > > In case a backend has further synchronization requirements those have to > > be handled at backend level, of course. > > > > In the end you'll need the xenbus level locking anyway in order to avoid > > a race when the last backend specific device is just being removed when > > the callback is about to be called for that device. Or you'd need to > > call try_get_module() before calling into each backend... > > Agreed. Thank you for your kind explanation of your concerns. Just posted the v11 patchset[1], which is based on your idea and passed my test. [1] https://lore.kernel.org/xen-devel/20191217160748.693-1-sjpark@amazon.com/ Thanks, SeongJae Park > > > Thanks, > SeongJae Park > > > > > > > Juergen