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

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Date:	Thu, 21 Mar 2013 21:15:41 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
Cc:	Roland Dreier <roland@...nel.org>,
	"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 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?

> 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.
> 
> Jason

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.

-- 
MST
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