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

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Message-ID: <CAL1RGDVnkLZU2Vge4o3BwDxnAfGv7TQRMqE6ha3MUt39CVp5NQ@mail.gmail.com>
Date:	Thu, 21 Mar 2013 02:13:38 -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

On Thu, Mar 21, 2013 at 1:51 AM, Michael S. Tsirkin <mst@...hat.com> wrote:
>> 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.
>
> What is the meaning of LOCAL_WRITE then? There are no local
> RDMA writes as far as I can see.

Umm, it means you're giving the local adapter permission to write to
that memory.  So you can use it as a receive buffer or as the target
for remote data from an RDMA read operation.

> OK then what we need is a new flag saying "I really do not
> intend to write into this memory please do not break
> COW or do anything else just in case I do".

Isn't that a shared read-only mapping?

 - R.
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