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

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Mar 21, 2013 at 12:41:35PM -0600, Jason Gunthorpe wrote:
> On Thu, Mar 21, 2013 at 08:16:33PM +0200, Michael S. Tsirkin wrote:
> 
> > This is the one I find redundant. Since the write will be done by
> > the adaptor under direct control by the application, why does it
> > make sense to declare this beforehand?  If you don't want to allow
> > local write access to memory, just do not post any receive WRs with
> > this address.  If you posted and regret it, reset the QP to cancel.
> 
> This is to support your COW scenario - the app declares before hand to
> the kernel that it will write to the memory and the kernel ensures
> pages are dedicated to the app at registration time. Or the app says
> it will only read and the kernel could leave them shared.

Someone here is confused. LOCAL_WRITE/absence of it does not address
COW, it breaks COW anyway.  Are you now saying we should change rdma so
without LOCAL_WRITE it will not break COW?

> The adaptor enforces the access control to prevent a naughty app from
> writing to shared memory - think about mmap'ing libc.so and then using
> RDMA to write to the shared pages. It is necessary to ensure that is
> impossible.
> 
> Jason

That's why it's redundant: we can't trust an application to tell us
'this page is writeable', we must get this info from kernel.  And so
there's apparently no need for application to tell adaptor about
LOCAL_WRITE.

-- 
MST