From mboxrd@z Thu Jan 1 00:00:00 1970 From: Roland Dreier Subject: Re: [PATCH] rdma: don't make pages writeable if not requiested Date: Thu, 21 Mar 2013 00:15:33 -0700 Message-ID: References: <20130321061838.GA28319@redhat.com> <20130321070357.GD28328@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Return-path: In-Reply-To: <20130321070357.GD28328-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> Sender: linux-rdma-owner-u79uwXL29TY76Z2rM5mHXA@public.gmane.org To: "Michael S. Tsirkin" 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 >> 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? We're talking about the permissions for the register MR operation, right? (That's what the kernel RDMA driver code that does get_user_pages() sees) In that case, no, I don't see any reason for LOCAL_WRITE, since the only RDMA operations that will access this memory are remote reads. The writing (that triggers COW) is coming from normal process access triggering a page fault, etc. This is a pretty standard way of using RDMA... For example, I allocate some memory and register it for RDMA read (and pass the R_Key to the remote system) with only REMOTE_READ permission. Then I fill in the memory with the results of some computation and the remote system does an RDMA read to get those results. - R. -- 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 S1757688Ab3CUHP5 (ORCPT ); Thu, 21 Mar 2013 03:15:57 -0400 Received: from mail-wg0-f44.google.com ([74.125.82.44]:44836 "EHLO mail-wg0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757071Ab3CUHP4 (ORCPT ); Thu, 21 Mar 2013 03:15:56 -0400 MIME-Version: 1.0 In-Reply-To: <20130321070357.GD28328@redhat.com> References: <20130321061838.GA28319@redhat.com> <20130321070357.GD28328@redhat.com> From: Roland Dreier Date: Thu, 21 Mar 2013 00:15:33 -0700 X-Google-Sender-Auth: hvQ-DchnR9TnfPQdbG5J8CJKWkY Message-ID: Subject: Re: [PATCH] rdma: don't make pages writeable if not requiested To: "Michael S. Tsirkin" Cc: "Michael R. Hines" , Sean Hefty , Hal Rosenstock , Yishai Hadas , Christoph Lameter , "linux-rdma@vger.kernel.org" , LKML , qemu-devel@nongnu.org Content-Type: text/plain; charset=ISO-8859-1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org >> 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? We're talking about the permissions for the register MR operation, right? (That's what the kernel RDMA driver code that does get_user_pages() sees) In that case, no, I don't see any reason for LOCAL_WRITE, since the only RDMA operations that will access this memory are remote reads. The writing (that triggers COW) is coming from normal process access triggering a page fault, etc. This is a pretty standard way of using RDMA... For example, I allocate some memory and register it for RDMA read (and pass the R_Key to the remote system) with only REMOTE_READ permission. Then I fill in the memory with the results of some computation and the remote system does an RDMA read to get those results. - R. From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([208.118.235.92]:59579) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UIZjE-0000sO-OZ for qemu-devel@nongnu.org; Thu, 21 Mar 2013 03:16:04 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1UIZj9-0000wT-VD for qemu-devel@nongnu.org; Thu, 21 Mar 2013 03:16:00 -0400 Received: from mail-wg0-f45.google.com ([74.125.82.45]:40667) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1UIZj9-0000wC-Po for qemu-devel@nongnu.org; Thu, 21 Mar 2013 03:15:55 -0400 Received: by mail-wg0-f45.google.com with SMTP id dq12so1938030wgb.0 for ; Thu, 21 Mar 2013 00:15:54 -0700 (PDT) MIME-Version: 1.0 Sender: roland.dreier@gmail.com In-Reply-To: <20130321070357.GD28328@redhat.com> References: <20130321061838.GA28319@redhat.com> <20130321070357.GD28328@redhat.com> From: Roland Dreier Date: Thu, 21 Mar 2013 00:15:33 -0700 Message-ID: Content-Type: text/plain; charset=ISO-8859-1 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: "Michael S. Tsirkin" Cc: qemu-devel@nongnu.org, "linux-rdma@vger.kernel.org" , Yishai Hadas , LKML , "Michael R. Hines" , Hal Rosenstock , Sean Hefty , Christoph Lameter >> 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? We're talking about the permissions for the register MR operation, right? (That's what the kernel RDMA driver code that does get_user_pages() sees) In that case, no, I don't see any reason for LOCAL_WRITE, since the only RDMA operations that will access this memory are remote reads. The writing (that triggers COW) is coming from normal process access triggering a page fault, etc. This is a pretty standard way of using RDMA... For example, I allocate some memory and register it for RDMA read (and pass the R_Key to the remote system) with only REMOTE_READ permission. Then I fill in the memory with the results of some computation and the remote system does an RDMA read to get those results. - R.