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

From: Michael S. Tsirkin
Date: Thu Mar 21 2013 - 04:50:55 EST

On Thu, Mar 21, 2013 at 12:15:33AM -0700, Roland Dreier wrote:
> >> 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.

What is the meaning of LOCAL_WRITE then? There are no local
RDMA writes as far as I can see.

> 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.

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".

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