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=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT 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 B37E9C43387 for ; Mon, 14 Jan 2019 09:55:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 8E7EC20821 for ; Mon, 14 Jan 2019 09:55:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726763AbfANJzY (ORCPT ); Mon, 14 Jan 2019 04:55:24 -0500 Received: from mx2.suse.de ([195.135.220.15]:33288 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726609AbfANJzX (ORCPT ); Mon, 14 Jan 2019 04:55:23 -0500 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.220.254]) by mx1.suse.de (Postfix) with ESMTP id 51D1EAF15; Mon, 14 Jan 2019 09:55:21 +0000 (UTC) Received: by quack2.suse.cz (Postfix, from userid 1000) id 7D5A91E157A; Mon, 14 Jan 2019 10:55:20 +0100 (CET) Date: Mon, 14 Jan 2019 10:55:20 +0100 From: Jan Kara To: Pankaj Gupta Cc: Dan Williams , Jan Kara , KVM list , David Hildenbrand , linux-nvdimm , Jason Wang , Dave Chinner , Qemu Developers , virtualization@lists.linux-foundation.org, adilger kernel , Ross Zwisler , dave jiang , darrick wong , vishal l verma , "Michael S. Tsirkin" , Matthew Wilcox , Christoph Hellwig , Linux ACPI , jmoyer , linux-ext4 , Rik van Riel , Stefan Hajnoczi , Igor Mammedov , lcapitulino@redhat.com, Kevin Wolf , Nitesh Narayan Lal , Theodore Ts'o , xiaoguangrong eric , "Rafael J. Wysocki" , Linux Kernel Mailing List , linux-xfs , linux-fsdevel , Paolo Bonzini Subject: Re: [Qemu-devel] [PATCH v3 0/5] kvm "virtio pmem" device Message-ID: <20190114095520.GC13316@quack2.suse.cz> References: <20190109144736.17452-1-pagupta@redhat.com> <20190110012617.GA4205@dastard> <20190110101757.GC15790@quack2.suse.cz> <1354249849.63357171.1547343519970.JavaMail.zimbra@redhat.com> <540171952.63371441.1547345866585.JavaMail.zimbra@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <540171952.63371441.1547345866585.JavaMail.zimbra@redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-fsdevel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-fsdevel@vger.kernel.org Message-ID: <20190114095520.suiG6gBMpQWf6NhcNrXZWA1xTw0c0PeEQg3U6VpwImA@z> On Sat 12-01-19 21:17:46, Pankaj Gupta wrote: > > > > Right. Thinking about this I would be more concerned about the fact that > > > > guest can effectively pin amount of host's page cache upto size of the > > > > device/file passed to guest as PMEM, can't it Pankaj? Or is there some > > > > QEMU > > > > magic that avoids this? > > > > > > Yes, guest will pin these host page cache pages using 'get_user_pages' by > > > elevating the page reference count. But these pages can be reclaimed by > > > host > > > at any time when there is memory pressure. > > > > Wait, how can the guest pin the host pages? I would expect this to > > happen only when using vfio and device assignment. Otherwise, no the > > host can't reclaim a pinned page, that's the whole point of a pin to > > prevent the mm from reclaiming ownership. > > yes. You are right I just used the pin word but it does not actually pin pages > permanently. I had gone through the discussion on existing problems with > get_user_pages and DMA e.g [1] to understand Jan's POV. It does mention GUP > pin pages so I also used the word 'pin'. But guest does not permanently pin > these pages and these pages can be reclaimed by host. OK, then I was just confused how virtio-pmem is going to work. Thanks for explanation! So can I imagine this as guest mmaping the host file and providing the mapped range as "NVDIMM pages" to the kernel inside the guest? Or is it more complex? Honza -- Jan Kara SUSE Labs, CR