linux-kernel - Re: [PATCH] rdma: don't make pages writeable if not requiested

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Date:	Thu, 21 Mar 2013 09:03:57 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Roland Dreier <roland@...nel.org>
Cc:	"Michael R. Hines" <mrhines@...ux.vnet.ibm.com>,
	Sean Hefty <sean.hefty@...el.com>,
	Hal Rosenstock <hal.rosenstock@...il.com>,
	Yishai Hadas <yishaih@...lanox.com>,
	Christoph Lameter <cl@...ux.com>,
	"linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
	LKML <linux-kernel@...r.kernel.org>, qemu-devel@...gnu.org
Subject: Re: [PATCH] rdma: don't make pages writeable if not requiested

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@...hat.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?
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