From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dan Williams Subject: Re: [PATCH v3 0/5] kvm "virtio pmem" device Date: Mon, 14 Jan 2019 13:35:57 -0800 Message-ID: References: <20190109144736.17452-1-pagupta@redhat.com> <20190110012617.GA4205@dastard> <1326478078.61913951.1547192704870.JavaMail.zimbra@redhat.com> <20190113232902.GD4205@dastard> <20190113233820.GX6310@bombadil.infradead.org> <942065073.64011540.1547450140670.JavaMail.zimbra@redhat.com> <20190114212501.GG4205@dastard> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Cc: Jan Kara , KVM list , David Hildenbrand , linux-nvdimm , Jason Wang , Qemu Developers , virtualization-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org, adilger kernel , Ross Zwisler , Eric Blake , darrick wong , "Michael S. Tsirkin" , Matthew Wilcox , Christoph Hellwig , Linux ACPI , linux-ext4 , Rik van Riel , Stefan Hajnoczi , Igor Mammedov , lcapitulino-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org, Kevin Wolf , Nitesh Narayan Lal , Theodore Ts'o , xiaoguangrong eric Return-path: In-Reply-To: <20190114212501.GG4205@dastard> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: linux-nvdimm-bounces-hn68Rpc1hR1g9hUCZPvPmw@public.gmane.org Sender: "Linux-nvdimm" List-Id: linux-fsdevel.vger.kernel.org On Mon, Jan 14, 2019 at 1:25 PM Dave Chinner wrote: > > On Mon, Jan 14, 2019 at 02:15:40AM -0500, Pankaj Gupta wrote: > > > > > > Until you have images (and hence host page cache) shared between > > > > multiple guests. People will want to do this, because it means they > > > > only need a single set of pages in host memory for executable > > > > binaries rather than a set of pages per guest. Then you have > > > > multiple guests being able to detect residency of the same set of > > > > pages. If the guests can then, in any way, control eviction of the > > > > pages from the host cache, then we have a guest-to-guest information > > > > leak channel. > > > > > > I don't think we should ever be considering something that would allow a > > > guest to evict page's from the host's pagecache [1]. The guest should > > > be able to kick its own references to the host's pagecache out of its > > > own pagecache, but not be able to influence whether the host or another > > > guest has a read-only mapping cached. > > > > > > [1] Unless the guest is allowed to modify the host's file; obviously > > > truncation, holepunching, etc are going to evict pages from the host's > > > page cache. > > > > This is so correct. Guest does not not evict host page cache pages directly. > > They don't right now. > > But someone is going to end up asking for discard to work so that > the guest can free unused space in the underlying spares image (i.e. > make use of fstrim or mount -o discard) because they have workloads > that have bursts of space usage and they need to trim the image > files afterwards to keep their overall space usage under control. > > And then.... ...we reject / push back on that patch citing the above concern. > > In case of virtio-pmem & DAX, guest clears guest page cache exceptional entries. > > Its solely decision of host to take action on the host page cache pages. > > > > In case of virtio-pmem, guest does not modify host file directly i.e don't > > perform hole punch & truncation operation directly on host file. > > ... this will no longer be true, and the nuclear landmine in this > driver interface will have been armed.... I agree with the need to be careful when / if explicit cache control is added, but that's not the case today. 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=-1.0 required=3.0 tests=DKIMWL_WL_MED,DKIM_SIGNED, DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS, URIBL_BLOCKED autolearn=unavailable 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 D42E2C43387 for ; Mon, 14 Jan 2019 21:36:16 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 9CD1A20659 for ; Mon, 14 Jan 2019 21:36:16 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=intel-com.20150623.gappssmtp.com header.i=@intel-com.20150623.gappssmtp.com header.b="anLbxKCk" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727085AbfANVgK (ORCPT ); Mon, 14 Jan 2019 16:36:10 -0500 Received: from mail-ot1-f68.google.com ([209.85.210.68]:44501 "EHLO mail-ot1-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726994AbfANVgK (ORCPT ); Mon, 14 Jan 2019 16:36:10 -0500 Received: by mail-ot1-f68.google.com with SMTP id f18so511920otl.11 for ; Mon, 14 Jan 2019 13:36:09 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=intel-com.20150623.gappssmtp.com; s=20150623; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=d6Q1Z9siWlt75gYyENAulZ+E6jB+huYePgaEmzfAUcs=; b=anLbxKCkq4twJco8nMO6LA9dDtWLUW8xuzTqHuyq2S7qx8GgCaIgSIu1+mVzASm0hp Zv8s9VHCebErD+983hj5ARy84J0uGjoqExlqloxpMFxLs0mQ6DfsStebBTaQjHpZjv/4 9mdCrcYyYBaTU9dhw7wlVbaapxE129qoR0/AIEEzozYhtCO+0nvD7bcuT8p4hPLqYHCq yWCKGqVpChI3nYHCWAzWQGo0IUR3idIHu05P+h6iYynCu+k+wWdrRMNs9wop6toj1MdM zJmeSv4UVH/vipERJ3wMDF0TDCYm3Tx4wVp5C2TcKYbAcFR3JokvIYzgpBhilY2T07YO qJ8w== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=d6Q1Z9siWlt75gYyENAulZ+E6jB+huYePgaEmzfAUcs=; b=njgiBr8iY8n3NL08XxiLOUIAWM3MoUPLueC94AnWt16JBA2JHki+Uf5rIeiheIqKgH AgcHJ07Tb8TjPMbd+1/IUaqIA7TmahMQQIEFihNv4QLHDHzxnJItfniK4PKDhWKH9YIV FkFWJ8c9qCqo8gJJJEKxuIzbbKR1EBehOeGEYIlGdqNiALYbd/Y7kijJZaU4tw8wy92Z TrZkHxUBNuKMA/al2fuiXPCn8yP85qnoQsxS6oowt20yHYhqkv33BSPo2vEABP8yaFXo 1xgBv4n6wq1MnJdu5uhttaAhxQddgmvjMBdqPFhT6ojZ2K3jJ1Pej5OYZxTO7dDyLqPi ijIg== X-Gm-Message-State: AJcUukcCqFV0IONe2U7qBXYW97ZhMOL0kpB8NSyJnEfb/5LRWwPXchBg zfLDNiChqD/HpmO/PIkvewI7b34nMJVAikTuNO4XRA== X-Google-Smtp-Source: ALg8bN7sM5lUsMnFtCgg/ctrMEAGuqRlx7k6Ir7v6j19ZiKoSn0zC6FcuSNALADb4aA2aRTqnS0RXhYlQxTbKE1FvW4= X-Received: by 2002:a9d:5cc2:: with SMTP id r2mr313672oti.367.1547501769412; Mon, 14 Jan 2019 13:36:09 -0800 (PST) MIME-Version: 1.0 References: <20190109144736.17452-1-pagupta@redhat.com> <20190110012617.GA4205@dastard> <1326478078.61913951.1547192704870.JavaMail.zimbra@redhat.com> <20190113232902.GD4205@dastard> <20190113233820.GX6310@bombadil.infradead.org> <942065073.64011540.1547450140670.JavaMail.zimbra@redhat.com> <20190114212501.GG4205@dastard> In-Reply-To: <20190114212501.GG4205@dastard> From: Dan Williams Date: Mon, 14 Jan 2019 13:35:57 -0800 Message-ID: Subject: Re: [PATCH v3 0/5] kvm "virtio pmem" device To: Dave Chinner Cc: Pankaj Gupta , Matthew Wilcox , Linux Kernel Mailing List , KVM list , Qemu Developers , linux-nvdimm , linux-fsdevel , virtualization@lists.linux-foundation.org, Linux ACPI , linux-ext4 , linux-xfs , Jan Kara , Stefan Hajnoczi , Rik van Riel , Nitesh Narayan Lal , Kevin Wolf , Paolo Bonzini , Ross Zwisler , vishal l verma , dave jiang , David Hildenbrand , jmoyer , xiaoguangrong eric , Christoph Hellwig , "Michael S. Tsirkin" , Jason Wang , lcapitulino@redhat.com, Igor Mammedov , Eric Blake , "Theodore Ts'o" , adilger kernel , darrick wong , "Rafael J. Wysocki" Content-Type: text/plain; charset="UTF-8" Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org Message-ID: <20190114213557.51nRRVQSuMTZAc1aQrflDEiE0jaHSQBXvoifhfXeOnc@z> On Mon, Jan 14, 2019 at 1:25 PM Dave Chinner wrote: > > On Mon, Jan 14, 2019 at 02:15:40AM -0500, Pankaj Gupta wrote: > > > > > > Until you have images (and hence host page cache) shared between > > > > multiple guests. People will want to do this, because it means they > > > > only need a single set of pages in host memory for executable > > > > binaries rather than a set of pages per guest. Then you have > > > > multiple guests being able to detect residency of the same set of > > > > pages. If the guests can then, in any way, control eviction of the > > > > pages from the host cache, then we have a guest-to-guest information > > > > leak channel. > > > > > > I don't think we should ever be considering something that would allow a > > > guest to evict page's from the host's pagecache [1]. The guest should > > > be able to kick its own references to the host's pagecache out of its > > > own pagecache, but not be able to influence whether the host or another > > > guest has a read-only mapping cached. > > > > > > [1] Unless the guest is allowed to modify the host's file; obviously > > > truncation, holepunching, etc are going to evict pages from the host's > > > page cache. > > > > This is so correct. Guest does not not evict host page cache pages directly. > > They don't right now. > > But someone is going to end up asking for discard to work so that > the guest can free unused space in the underlying spares image (i.e. > make use of fstrim or mount -o discard) because they have workloads > that have bursts of space usage and they need to trim the image > files afterwards to keep their overall space usage under control. > > And then.... ...we reject / push back on that patch citing the above concern. > > In case of virtio-pmem & DAX, guest clears guest page cache exceptional entries. > > Its solely decision of host to take action on the host page cache pages. > > > > In case of virtio-pmem, guest does not modify host file directly i.e don't > > perform hole punch & truncation operation directly on host file. > > ... this will no longer be true, and the nuclear landmine in this > driver interface will have been armed.... I agree with the need to be careful when / if explicit cache control is added, but that's not the case today.