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=-2.6 required=3.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,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 51DE3C49ED8 for ; Tue, 10 Sep 2019 14:47:19 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 287DD20872 for ; Tue, 10 Sep 2019 14:47:19 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=default; t=1568126839; bh=Sg+UE7kKycJMZyBm4O26iFfOQKua77pluElMev43Wk4=; h=Date:From:To:Cc:Subject:References:In-Reply-To:List-ID:From; b=cO51U2gcwZQjDNk1/A3bGxCiPjMLdxI6K/jagg8IqwEBl7IU7rMesPk+XBrM/qT20 5FoKrZA7oNWNwMkQ+g4DA26cjRwJ5QHWU4CK8q++1ME2A/RcAdLgiIN/cts8coTgAW Bxe2DwRGpM4NojkdLwI+N1A4vp8xOXcqvAMLuKnQ= Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2393668AbfIJOrS (ORCPT ); Tue, 10 Sep 2019 10:47:18 -0400 Received: from mx2.suse.de ([195.135.220.15]:48628 "EHLO mx1.suse.de" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1726132AbfIJOrR (ORCPT ); Tue, 10 Sep 2019 10:47:17 -0400 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 AA491ABD2; Tue, 10 Sep 2019 14:47:14 +0000 (UTC) Date: Tue, 10 Sep 2019 16:47:13 +0200 From: Michal Hocko To: Alexander Duyck Cc: virtio-dev@lists.oasis-open.org, kvm list , "Michael S. Tsirkin" , Catalin Marinas , David Hildenbrand , Dave Hansen , LKML , Matthew Wilcox , linux-mm , Andrew Morton , will@kernel.org, linux-arm-kernel@lists.infradead.org, Oscar Salvador , Yang Zhang , Pankaj Gupta , Konrad Rzeszutek Wilk , Nitesh Narayan Lal , Rik van Riel , lcapitulino@redhat.com, "Wang, Wei W" , Andrea Arcangeli , ying.huang@intel.com, Paolo Bonzini , Dan Williams , Fengguang Wu , Alexander Duyck , "Kirill A. Shutemov" Subject: Re: [PATCH v9 0/8] stg mail -e --version=v9 \ Message-ID: <20190910144713.GF2063@dhcp22.suse.cz> References: <20190907172225.10910.34302.stgit@localhost.localdomain> <20190910124209.GY2063@dhcp22.suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue 10-09-19 07:42:43, Alexander Duyck wrote: > On Tue, Sep 10, 2019 at 5:42 AM Michal Hocko wrote: > > > > I wanted to review "mm: Introduce Reported pages" just realize that I > > have no clue on what is going on so returned to the cover and it didn't > > really help much. I am completely unfamiliar with virtio so please bear > > with me. > > > > On Sat 07-09-19 10:25:03, Alexander Duyck wrote: > > [...] > > > This series provides an asynchronous means of reporting to a hypervisor > > > that a guest page is no longer in use and can have the data associated > > > with it dropped. To do this I have implemented functionality that allows > > > for what I am referring to as unused page reporting > > > > > > The functionality for this is fairly simple. When enabled it will allocate > > > statistics to track the number of reported pages in a given free area. > > > When the number of free pages exceeds this value plus a high water value, > > > currently 32, it will begin performing page reporting which consists of > > > pulling pages off of free list and placing them into a scatter list. The > > > scatterlist is then given to the page reporting device and it will perform > > > the required action to make the pages "reported", in the case of > > > virtio-balloon this results in the pages being madvised as MADV_DONTNEED > > > and as such they are forced out of the guest. After this they are placed > > > back on the free list, > > > > And here I am reallly lost because "forced out of the guest" makes me > > feel that those pages are no longer usable by the guest. So how come you > > can add them back to the free list. I suspect understanding this part > > will allow me to understand why we have to mark those pages and prevent > > merging. > > Basically as the paragraph above mentions "forced out of the guest" > really is just the hypervisor calling MADV_DONTNEED on the page in > question. So the behavior is the same as any userspace application > that calls MADV_DONTNEED where the contents are no longer accessible > from userspace and attempting to access them will result in a fault > and the page being populated with a zero fill on-demand page, or a > copy of the file contents if the memory is file backed. As I've said I have no idea about virt so this doesn't really tell me much. Does that mean that if somebody allocates such a page and tries to access it then virt will handle a fault and bring it back? -- Michal Hocko SUSE Labs 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=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED, 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 CD2B2C49ED7 for ; Tue, 10 Sep 2019 14:47:26 +0000 (UTC) Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id A0A1521019 for ; Tue, 10 Sep 2019 14:47:26 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="qLqOlPUe" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org A0A1521019 Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=kernel.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:Cc:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References: Message-ID:Subject:To:From:Date:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=oU/7W3kyDqlmrPeTOJNQRL/kpumXiSsyzXAfM5w9qbw=; b=qLqOlPUemVF5Df CoEvr2M+4vdlUoNX1Rp+t8N8XrZqrB+GBlqfA8QoO5QDM2RZT6pKfrWlVBHKnakiwldedL0wk71T4 LD7cDCWU/SjbHAEOrqrozYtIhN44Z7IdkzYJwGld6abnyaqRbqRGVyXtICAq0y/OARg0hhZEjN4+u C/34iQ4XGvNxVVB1KMlSb+jb7OOGWAxi6/+r7mjfKJyJYVZZ6moTyJgSXKFrK6gOYk3ylPIiyepB6 A/N5mjbMXRtXMCN3ogDBj7dnMKZPAgtPE4kuv/ZkWdBCrnxj2Sv44nthmitlJ7veYKPUsJnan/Cua UgnSVQOO4nHEIJAcquZQ==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.92 #3 (Red Hat Linux)) id 1i7hQP-0006NU-Ug; Tue, 10 Sep 2019 14:47:22 +0000 Received: from mx2.suse.de ([195.135.220.15] helo=mx1.suse.de) by bombadil.infradead.org with esmtps (Exim 4.92 #3 (Red Hat Linux)) id 1i7hQK-0006I6-89 for linux-arm-kernel@lists.infradead.org; Tue, 10 Sep 2019 14:47:17 +0000 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 AA491ABD2; Tue, 10 Sep 2019 14:47:14 +0000 (UTC) Date: Tue, 10 Sep 2019 16:47:13 +0200 From: Michal Hocko To: Alexander Duyck Subject: Re: [PATCH v9 0/8] stg mail -e --version=v9 \ Message-ID: <20190910144713.GF2063@dhcp22.suse.cz> References: <20190907172225.10910.34302.stgit@localhost.localdomain> <20190910124209.GY2063@dhcp22.suse.cz> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20190910_074716_649750_267FDFB6 X-CRM114-Status: GOOD ( 22.36 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Yang Zhang , Pankaj Gupta , kvm list , David Hildenbrand , Catalin Marinas , lcapitulino@redhat.com, linux-mm , Alexander Duyck , will@kernel.org, Andrea Arcangeli , virtio-dev@lists.oasis-open.org, "Michael S. Tsirkin" , Matthew Wilcox , "Wang, Wei W" , ying.huang@intel.com, Rik van Riel , Konrad Rzeszutek Wilk , Dan Williams , linux-arm-kernel@lists.infradead.org, Oscar Salvador , Nitesh Narayan Lal , Dave Hansen , LKML , Paolo Bonzini , Andrew Morton , Fengguang Wu , "Kirill A. Shutemov" Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+infradead-linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Tue 10-09-19 07:42:43, Alexander Duyck wrote: > On Tue, Sep 10, 2019 at 5:42 AM Michal Hocko wrote: > > > > I wanted to review "mm: Introduce Reported pages" just realize that I > > have no clue on what is going on so returned to the cover and it didn't > > really help much. I am completely unfamiliar with virtio so please bear > > with me. > > > > On Sat 07-09-19 10:25:03, Alexander Duyck wrote: > > [...] > > > This series provides an asynchronous means of reporting to a hypervisor > > > that a guest page is no longer in use and can have the data associated > > > with it dropped. To do this I have implemented functionality that allows > > > for what I am referring to as unused page reporting > > > > > > The functionality for this is fairly simple. When enabled it will allocate > > > statistics to track the number of reported pages in a given free area. > > > When the number of free pages exceeds this value plus a high water value, > > > currently 32, it will begin performing page reporting which consists of > > > pulling pages off of free list and placing them into a scatter list. The > > > scatterlist is then given to the page reporting device and it will perform > > > the required action to make the pages "reported", in the case of > > > virtio-balloon this results in the pages being madvised as MADV_DONTNEED > > > and as such they are forced out of the guest. After this they are placed > > > back on the free list, > > > > And here I am reallly lost because "forced out of the guest" makes me > > feel that those pages are no longer usable by the guest. So how come you > > can add them back to the free list. I suspect understanding this part > > will allow me to understand why we have to mark those pages and prevent > > merging. > > Basically as the paragraph above mentions "forced out of the guest" > really is just the hypervisor calling MADV_DONTNEED on the page in > question. So the behavior is the same as any userspace application > that calls MADV_DONTNEED where the contents are no longer accessible > from userspace and attempting to access them will result in a fault > and the page being populated with a zero fill on-demand page, or a > copy of the file contents if the memory is file backed. As I've said I have no idea about virt so this doesn't really tell me much. Does that mean that if somebody allocates such a page and tries to access it then virt will handle a fault and bring it back? -- Michal Hocko SUSE Labs _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel