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

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Date:	Thu, 21 Mar 2013 11:39:47 +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 Thu, Mar 21, 2013 at 02:13:38AM -0700, Roland Dreier wrote:
> 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.

Well RDMA read has it's own flag, IB_ACCESS_REMOTE_READ.
I don't see why do you need to give adapter permission
to use memory as a receive buffer given that
you must pass the address in the post receive verb,
but maybe that's what the IB spec says so that's what
the applications assumed?

> > 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.

Nothing to do with how the page is mapped.  We can and do write the page
before registration. BTW umem.c passes in force so it breaks COW
even for read-only mappings, right?

-- 
MST
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