Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752770Ab3CUUJk (ORCPT ); Thu, 21 Mar 2013 16:09:40 -0400 Received: from quartz.orcorp.ca ([184.70.90.242]:41866 "EHLO quartz.orcorp.ca" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751967Ab3CUUJi (ORCPT ); Thu, 21 Mar 2013 16:09:38 -0400 Date: Thu, 21 Mar 2013 14:09:22 -0600 From: Jason Gunthorpe To: "Michael S. Tsirkin" Cc: Roland Dreier , "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: <20130321200922.GA8109@obsidianresearch.com> References: <20130321070357.GD28328@redhat.com> <20130321085107.GE28328@redhat.com> <20130321093946.GG28328@redhat.com> <20130321171115.GA653@obsidianresearch.com> <20130321181633.GC4366@redhat.com> <20130321184135.GA8044@obsidianresearch.com> <20130321191541.GB5272@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20130321191541.GB5272@redhat.com> User-Agent: Mutt/1.5.21 (2010-09-15) X-Broken-Reverse-DNS: no host name found for IP address 10.0.0.162 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2547 Lines: 53 On Thu, Mar 21, 2013 at 09:15:41PM +0200, Michael S. Tsirkin wrote: > On Thu, Mar 21, 2013 at 12:41:35PM -0600, Jason Gunthorpe wrote: > > On Thu, Mar 21, 2013 at 08:16:33PM +0200, Michael S. Tsirkin wrote: > > > > > This is the one I find redundant. Since the write will be done by > > > the adaptor under direct control by the application, why does it > > > make sense to declare this beforehand? If you don't want to allow > > > local write access to memory, just do not post any receive WRs with > > > this address. If you posted and regret it, reset the QP to cancel. > > > > This is to support your COW scenario - the app declares before hand to > > the kernel that it will write to the memory and the kernel ensures > > pages are dedicated to the app at registration time. Or the app says > > it will only read and the kernel could leave them shared. > > Someone here is confused. LOCAL_WRITE/absence of it does not address > COW, it breaks COW anyway. Are you now saying we should change rdma so > without LOCAL_WRITE it will not break COW? I am talking about 'from a spec' perspective - not what Linux does today. The absence of LOCAL_WRITE is part of the specification to support shared pages. Pages can only be kept shared if all the ACCESS WRITE bits are clear - today Linux always breaks the COW, but if you patch in the ability to keep things shared then it must only happen when *all* the ACCESS WRITE bits are clear. > > The adaptor enforces the access control to prevent a naughty app from > > writing to shared memory - think about mmap'ing libc.so and then using > > RDMA to write to the shared pages. It is necessary to ensure that is > > impossible. > That's why it's redundant: we can't trust an application to tell us > 'this page is writeable', we must get this info from kernel. And so > there's apparently no need for application to tell adaptor about > LOCAL_WRITE. The API design gives user space maximum flexibility, if it wants to create an enforced no-write MR in otherwise writable pages by skipping LOCAL_WRITE then it can do so. The kernel's role in this should be to deny ibv_reg_mr with WRITE bits set if the pages are not writable by the app - I don't know if it does this today, it isn't critically important as long as the pages are unshared. Jason -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/