From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address Date: Thu, 7 Mar 2019 10:34:39 -0500 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-Transfer-Encoding: 8bit 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 To: Jason Wang Return-path: Content-Disposition: inline In-Reply-To: <15105894-4ec1-1ed0-1976-7b68ed9eeeda@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.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