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[209.132.183.28]) by mx.google.com with ESMTPS id c21si1322833qtc.197.2019.03.07.11.09.18 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 07 Mar 2019 11:09:18 -0800 (PST) Received-SPF: pass (google.com: domain of jglisse@redhat.com designates 209.132.183.28 as permitted sender) client-ip=209.132.183.28; Authentication-Results: mx.google.com; spf=pass (google.com: domain of jglisse@redhat.com designates 209.132.183.28 as permitted sender) smtp.mailfrom=jglisse@redhat.com; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from smtp.corp.redhat.com (int-mx01.intmail.prod.int.phx2.redhat.com [10.5.11.11]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mx1.redhat.com (Postfix) with ESMTPS id 137B9882F0; Thu, 7 Mar 2019 19:09:18 +0000 (UTC) Received: from redhat.com (ovpn-125-54.rdu2.redhat.com [10.10.125.54]) by smtp.corp.redhat.com (Postfix) with ESMTPS id BD4136013F; Thu, 7 Mar 2019 19:09:12 +0000 (UTC) Date: Thu, 7 Mar 2019 14:09:10 -0500 From: Jerome Glisse To: "Michael S. Tsirkin" Cc: Jason Wang , 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: <20190307190910.GE3835@redhat.com> 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> <20190307101708-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20190307101708-mutt-send-email-mst@kernel.org> User-Agent: Mutt/1.10.1 (2018-07-13) X-Scanned-By: MIMEDefang 2.79 on 10.5.11.11 X-Greylist: Sender IP whitelisted, not delayed by milter-greylist-4.5.16 (mx1.redhat.com [10.5.110.28]); Thu, 07 Mar 2019 19:09:18 +0000 (UTC) X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Thu, Mar 07, 2019 at 10:34:39AM -0500, Michael S. Tsirkin wrote: > 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? I thought this patch was only for anonymous memory ie not file back ? If so then set dirty is mostly useless it would only be use for swap but for this you can use an unlock version to set the page dirty. Cheers, Jérôme