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

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Message-ID: <CAG4TOxPkhOhGmzeA1K4a0Zw8HxS-QkOr-PCx7mJgA+KkuH3ZiQ@mail.gmail.com>
Date:	Thu, 21 Mar 2013 00:15:33 -0700
From:	Roland Dreier <roland@...nel.org>
To:	"Michael S. Tsirkin" <mst@...hat.com>
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

>> 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.
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