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Date: Thu, 21 Mar 2013 09:03:57 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
Message-ID: <20130321070357.GD28328@redhat.com>
References: <20130321061838.GA28319@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] rdma: don't make pages writeable if not
	requiested
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To: Roland Dreier <roland@kernel.org>
Cc: qemu-devel@nongnu.org, "linux-rdma@vger.kernel.org" <linux-rdma@vger.kernel.org>, Yishai Hadas <yishaih@mellanox.com>, LKML <linux-kernel@vger.kernel.org>, "Michael R. Hines" <mrhines@linux.vnet.ibm.com>, Hal Rosenstock <hal.rosenstock@gmail.com>, Sean Hefty <sean.hefty@intel.com>, Christoph Lameter <cl@linux.com>

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 <mst@redhat.com> 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?