From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH] rdma: don't make pages writeable if not requiested Date: Thu, 21 Mar 2013 09:03:57 +0200 Message-ID: <20130321070357.GD28328@redhat.com> References: <20130321061838.GA28319@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: Sender: linux-rdma-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: Roland Dreier Cc: "Michael R. Hines" , Sean Hefty , Hal Rosenstock , Yishai Hadas , Christoph Lameter , "linux-rdma-u79uwXL29TY76Z2rM5mHXA@public.gmane.org" , LKML , qemu-devel-qX2TKyscuCcdnm+yROfE0A@public.gmane.org List-Id: linux-rdma@vger.kernel.org On Wed, Mar 20, 2013 at 11:55:54PM -0700, Roland Dreier wrote: > On Wed, Mar 20, 2013 at 11:18 PM, Michael S. Tsirkin wrote: > > core/umem.c seems to get the arguments to get_user_pages > > in the reverse order: it sets writeable flag and > > breaks COW for MAP_SHARED if and only if hardware needs to > > write the page. > > > > This breaks memory overcommit for users such as KVM: > > each time we try to register a page to send it to remote, this > > breaks COW. It seems that for applications that only have > > REMOTE_READ permission, there is no reason to break COW at all. > > I proposed a similar (but not exactly the same, see below) patch a > while ago: https://lkml.org/lkml/2012/1/26/7 but read the thread, > especially https://lkml.org/lkml/2012/2/6/265 > > I think this change will break the case where userspace tries to > register an MR with read-only permission, but intends locally through > the CPU to write to the memory. Shouldn't it set LOCAL_WRITE then? -- To unsubscribe from this list: send the line "unsubscribe linux-rdma" in the body of a message to majordomo-u79uwXL29TY76Z2rM5mHXA@public.gmane.org More majordomo info at http://vger.kernel.org/majordomo-info.html From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1757535Ab3CUHDa (ORCPT ); Thu, 21 Mar 2013 03:03:30 -0400 Received: from mx1.redhat.com ([209.132.183.28]:51489 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752198Ab3CUHD3 (ORCPT ); Thu, 21 Mar 2013 03:03:29 -0400 Date: Thu, 21 Mar 2013 09:03:57 +0200 From: "Michael S. Tsirkin" To: Roland Dreier Cc: "Michael R. Hines" , Sean Hefty , Hal Rosenstock , Yishai Hadas , Christoph Lameter , "linux-rdma@vger.kernel.org" , LKML , qemu-devel@nongnu.org Subject: Re: [PATCH] rdma: don't make pages writeable if not requiested Message-ID: <20130321070357.GD28328@redhat.com> References: <20130321061838.GA28319@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Mar 20, 2013 at 11:55:54PM -0700, Roland Dreier wrote: > On Wed, Mar 20, 2013 at 11:18 PM, Michael S. Tsirkin wrote: > > core/umem.c seems to get the arguments to get_user_pages > > in the reverse order: it sets writeable flag and > > breaks COW for MAP_SHARED if and only if hardware needs to > > write the page. > > > > This breaks memory overcommit for users such as KVM: > > each time we try to register a page to send it to remote, this > > breaks COW. It seems that for applications that only have > > REMOTE_READ permission, there is no reason to break COW at all. > > I proposed a similar (but not exactly the same, see below) patch a > while ago: https://lkml.org/lkml/2012/1/26/7 but read the thread, > especially https://lkml.org/lkml/2012/2/6/265 > > I think this change will break the case where userspace tries to > register an MR with read-only permission, but intends locally through > the CPU to write to the memory. Shouldn't it set LOCAL_WRITE then? From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:55350) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UIZXA-0003wB-2b for qemu-devel@nongnu.org; Thu, 21 Mar 2013 03:03:34 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UIZX6-0004Jj-VU for qemu-devel@nongnu.org; Thu, 21 Mar 2013 03:03:31 -0400 Received: from mx1.redhat.com ([209.132.183.28]:21152) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UIZX6-0004Jd-O9 for qemu-devel@nongnu.org; Thu, 21 Mar 2013 03:03:28 -0400 Date: Thu, 21 Mar 2013 09:03:57 +0200 From: "Michael S. Tsirkin" Message-ID: <20130321070357.GD28328@redhat.com> References: <20130321061838.GA28319@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: Subject: Re: [Qemu-devel] [PATCH] rdma: don't make pages writeable if not requiested List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Roland Dreier Cc: qemu-devel@nongnu.org, "linux-rdma@vger.kernel.org" , Yishai Hadas , LKML , "Michael R. Hines" , Hal Rosenstock , Sean Hefty , Christoph Lameter On Wed, Mar 20, 2013 at 11:55:54PM -0700, Roland Dreier wrote: > On Wed, Mar 20, 2013 at 11:18 PM, Michael S. Tsirkin wrote: > > core/umem.c seems to get the arguments to get_user_pages > > in the reverse order: it sets writeable flag and > > breaks COW for MAP_SHARED if and only if hardware needs to > > write the page. > > > > This breaks memory overcommit for users such as KVM: > > each time we try to register a page to send it to remote, this > > breaks COW. It seems that for applications that only have > > REMOTE_READ permission, there is no reason to break COW at all. > > I proposed a similar (but not exactly the same, see below) patch a > while ago: https://lkml.org/lkml/2012/1/26/7 but read the thread, > especially https://lkml.org/lkml/2012/2/6/265 > > I think this change will break the case where userspace tries to > register an MR with read-only permission, but intends locally through > the CPU to write to the memory. Shouldn't it set LOCAL_WRITE then?