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=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS 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 07FA8C43381 for ; Thu, 7 Mar 2019 15:34:51 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D73922064A for ; Thu, 7 Mar 2019 15:34:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726641AbfCGPet (ORCPT ); Thu, 7 Mar 2019 10:34:49 -0500 Received: from mail-qt1-f193.google.com ([209.85.160.193]:40481 "EHLO mail-qt1-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726207AbfCGPet (ORCPT ); Thu, 7 Mar 2019 10:34:49 -0500 Received: by mail-qt1-f193.google.com with SMTP id f11so275726qti.7 for ; Thu, 07 Mar 2019 07:34:48 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:content-transfer-encoding :in-reply-to; bh=Wsfg0ok9ugLJ683R6tW39HiYXOlqVaFSZwOTghjsqWI=; b=NBHGSACvGfLHmdkc0yDZiGVpqvStiT/qFrG0PXNX2DYrqD+WbOOp12GfGs5sL0QiQs h1AFGIM9pFyBI6BcEBfets/+I8H7UHacu23e2r9X6Oy+fo27owyTKUzZNRS34GJ7ybNa Z/DneJLj5+1Zkr/st4lTYrhzR9BJISwsqbvELydPUlvYajb7cuPzjQUAl2IzS+smx4iV zpzTu2cYb8CFw6UAJf9CiNUZsciV0rubCF7zLFe/tGOauNAXQy7xKdjmO87lhMVgixwg mlaPZWze2218xSbvUswe9rNwgkng4K31E4uSe0vgRn9MiFOvEDV9m+yWJbwPd6e7+c63 FG/w== X-Gm-Message-State: APjAAAVo6GWqLgCOYu2AQyMoIyMBMH/VfXkaTUJiKvGHYme1QgA1r9Yn 2AUjY25OEkfMzyckr0eA3H2Q7Q== X-Google-Smtp-Source: APXvYqz69aAWMZo6tBGadWm6DMo8HXGSmtwBcs9kAOiIp3jiWs4Vy0Z3okP7DZSreWC/fa5WNW5dOA== X-Received: by 2002:ac8:23aa:: with SMTP id q39mr10380050qtq.82.1551972888059; Thu, 07 Mar 2019 07:34:48 -0800 (PST) Received: from redhat.com (pool-173-76-246-42.bstnma.fios.verizon.net. [173.76.246.42]) by smtp.gmail.com with ESMTPSA id x43sm3517230qtc.10.2019.03.07.07.34.45 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Thu, 07 Mar 2019 07:34:47 -0800 (PST) Date: Thu, 7 Mar 2019 10:34:39 -0500 From: "Michael S. Tsirkin" To: Jason Wang Cc: kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, peterx@redhat.com, linux-mm@kvack.org, aarcange@redhat.com Subject: Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address Message-ID: <20190307101708-mutt-send-email-mst@kernel.org> References: <1551856692-3384-1-git-send-email-jasowang@redhat.com> <1551856692-3384-6-git-send-email-jasowang@redhat.com> <20190306092837-mutt-send-email-mst@kernel.org> <15105894-4ec1-1ed0-1976-7b68ed9eeeda@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <15105894-4ec1-1ed0-1976-7b68ed9eeeda@redhat.com> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 07, 2019 at 10:45:57AM +0800, Jason Wang wrote: > > On 2019/3/7 上午12:31, Michael S. Tsirkin wrote: > > > +static void vhost_set_vmap_dirty(struct vhost_vmap *used) > > > +{ > > > + int i; > > > + > > > + for (i = 0; i < used->npages; i++) > > > + set_page_dirty_lock(used->pages[i]); > > This seems to rely on page lock to mark page dirty. > > > > Could it happen that page writeback will check the > > page, find it clean, and then you mark it dirty and then > > invalidate callback is called? > > > > > > Yes. But does this break anything? > The page is still there, we just remove a > kernel mapping to it. > > Thanks Yes it's the same problem as e.g. RDMA: we've just marked the page as dirty without having buffers. Eventually writeback will find it and filesystem will complain... So if the pages are backed by a non-RAM-based filesystem, it’s all just broken. one can hope that RDMA guys will fix it in some way eventually. For now, maybe add a flag in e.g. VMA that says that there's no writeback so it's safe to mark page dirty at any point? -- MST