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=-14.3 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED,USER_AGENT_GIT autolearn=ham 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 15825C2D0E2 for ; Tue, 22 Sep 2020 11:27:09 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B955E23A1B for ; Tue, 22 Sep 2020 11:27:08 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=amazon.com header.i=@amazon.com header.b="F3MsGnXM" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726487AbgIVL1I (ORCPT ); Tue, 22 Sep 2020 07:27:08 -0400 Received: from smtp-fw-6001.amazon.com ([52.95.48.154]:65144 "EHLO smtp-fw-6001.amazon.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726454AbgIVL1G (ORCPT ); Tue, 22 Sep 2020 07:27:06 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=amazon.com; i=@amazon.com; q=dns/txt; s=amazon201209; t=1600774026; x=1632310026; h=from:to:cc:subject:date:message-id:mime-version: in-reply-to:content-transfer-encoding; bh=g3JRr23a6mY+Y62r/GZOjEp5HQn72fVEKBXRryfrcNw=; b=F3MsGnXMafsGQs0Ejhj7xoQe2BKE8oJj0h7BZLJhbOIPLaGt+9kDhQTD iDrWhVHin0ARXwnP0VCE7srHxvQp6th/4PhYeJWyJ2FTNvOvrYF3EWI6h XS9/pjtdc7rVqCEOaYk3EOgF78sE6BwX1MONp1ss+pylY+VPbElkUIJ1D g=; X-IronPort-AV: E=Sophos;i="5.77,290,1596499200"; d="scan'208";a="56930374" Received: from iad12-co-svc-p1-lb1-vlan3.amazon.com (HELO email-inbound-relay-1d-38ae4ad2.us-east-1.amazon.com) ([10.43.8.6]) by smtp-border-fw-out-6001.iad6.amazon.com with ESMTP; 22 Sep 2020 11:27:05 +0000 Received: from EX13D31EUA004.ant.amazon.com (iad12-ws-svc-p26-lb9-vlan2.iad.amazon.com [10.40.163.34]) by email-inbound-relay-1d-38ae4ad2.us-east-1.amazon.com (Postfix) with ESMTPS id C017BA20B0; Tue, 22 Sep 2020 11:27:02 +0000 (UTC) Received: from u3f2cd687b01c55.ant.amazon.com (10.43.160.244) by EX13D31EUA004.ant.amazon.com (10.43.165.161) with Microsoft SMTP Server (TLS) id 15.0.1497.2; Tue, 22 Sep 2020 11:26:55 +0000 From: SeongJae Park To: =?UTF-8?q?Roger=20Pau=20Monn=C3=A9?= CC: SeongJae Park , , , SeongJae Park , , , , , , , , Subject: Re: [PATCH v2 1/3] xen-blkback: add a parameter for disabling of persistent grants Date: Tue, 22 Sep 2020 13:26:38 +0200 Message-ID: <20200922112638.14238-1-sjpark@amazon.com> X-Mailer: git-send-email 2.17.1 MIME-Version: 1.0 In-Reply-To: <20200922111259.GJ19254@Air-de-Roger> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 8bit X-Originating-IP: [10.43.160.244] X-ClientProxiedBy: EX13D17UWC001.ant.amazon.com (10.43.162.188) To EX13D31EUA004.ant.amazon.com (10.43.165.161) Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Tue, 22 Sep 2020 13:12:59 +0200 "Roger Pau Monné" wrote: > On Tue, Sep 22, 2020 at 12:52:07PM +0200, SeongJae Park wrote: > > From: SeongJae Park > > > > Persistent grants feature provides high scalability. On some small > > systems, however, it could incur data copy overheads[1] and thus it is > > required to be disabled. But, there is no option to disable it. For > > the reason, this commit adds a module parameter for disabling of the > > feature. > > > > [1] https://wiki.xen.org/wiki/Xen_4.3_Block_Protocol_Scalability > > > > Signed-off-by: Anthony Liguori > > Signed-off-by: SeongJae Park > > --- > > .../ABI/testing/sysfs-driver-xen-blkback | 9 ++++++ > > drivers/block/xen-blkback/xenbus.c | 28 ++++++++++++++----- > > 2 files changed, 30 insertions(+), 7 deletions(-) > > > > diff --git a/Documentation/ABI/testing/sysfs-driver-xen-blkback b/Documentation/ABI/testing/sysfs-driver-xen-blkback > > index ecb7942ff146..ac2947b98950 100644 > > --- a/Documentation/ABI/testing/sysfs-driver-xen-blkback > > +++ b/Documentation/ABI/testing/sysfs-driver-xen-blkback > > @@ -35,3 +35,12 @@ Description: > > controls the duration in milliseconds that blkback will not > > cache any page not backed by a grant mapping. > > The default is 10ms. > > + > > +What: /sys/module/xen_blkback/parameters/feature_persistent > > +Date: September 2020 > > +KernelVersion: 5.10 > > +Contact: SeongJae Park > > +Description: > > + Whether to enable the persistent grants feature or not. Note > > + that this option only takes effect on newly created backends. > > + The default is Y (enable). > > diff --git a/drivers/block/xen-blkback/xenbus.c b/drivers/block/xen-blkback/xenbus.c > > index b9aa5d1ac10b..8a95ddd08b13 100644 > > --- a/drivers/block/xen-blkback/xenbus.c > > +++ b/drivers/block/xen-blkback/xenbus.c > > @@ -879,6 +879,12 @@ static void reclaim_memory(struct xenbus_device *dev) > > > > /* ** Connection ** */ > > > > +/* Enable the persistent grants feature. */ > > +static bool feature_persistent = true; > > +module_param(feature_persistent, bool, 0644); > > +MODULE_PARM_DESC(feature_persistent, > > + "Enables the persistent grants feature"); > > + > > /* > > * Write the physical details regarding the block device to the store, and > > * switch to Connected state. > > @@ -906,11 +912,15 @@ static void connect(struct backend_info *be) > > > > xen_blkbk_barrier(xbt, be, be->blkif->vbd.flush_support); > > > > - err = xenbus_printf(xbt, dev->nodename, "feature-persistent", "%u", 1); > > - if (err) { > > - xenbus_dev_fatal(dev, err, "writing %s/feature-persistent", > > - dev->nodename); > > - goto abort; > > + if (feature_persistent) { > > + err = xenbus_printf(xbt, dev->nodename, "feature-persistent", > > + "%u", feature_persistent); > > + if (err) { > > + xenbus_dev_fatal(dev, err, > > + "writing %s/feature-persistent", > > + dev->nodename); > > + goto abort; > > + } > > } > > > > err = xenbus_printf(xbt, dev->nodename, "sectors", "%llu", > > @@ -1093,8 +1103,12 @@ static int connect_ring(struct backend_info *be) > > xenbus_dev_fatal(dev, err, "unknown fe protocol %s", protocol); > > return -ENOSYS; > > } > > - pers_grants = xenbus_read_unsigned(dev->otherend, "feature-persistent", > > - 0); > > + if (feature_persistent) > > + pers_grants = xenbus_read_unsigned(dev->otherend, > > + "feature-persistent", 0); > > + else > > + pers_grants = 0; > > + > > Sorry for not realizing earlier, but looking at it again I think you > need to cache the value of feature_persistent when it's first used in > the blkback state data, so that it's consistent. > > What would happen for example with the following flow (assume a > persistent grants enabled frontend): > > feature_persistent = false > > connect(...) > feature-persistent is not written to xenstore > > User changes feature_persistent = true > > connect_ring(...) > pers_grants = true, because feature-persistent is set unconditionally > by the frontend and feature_persistent variable is now true. > > Then blkback will try to use persistent grants and the whole > connection will malfunction because the frontend won't. Ah, you're right. I should also catch this before but didn't, sorry. > > The other option is to prevent changing the variable when there are > blkback instances already running. I think storing the option value in xenstore would be simpler. That said, if you prefer this way, please let me know. Thanks, SeongJae Park