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

[Roland Dreier]

>> 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.