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 11:11:15AM -0600, Jason Gunthorpe wrote:
> On Thu, Mar 21, 2013 at 11:39:47AM +0200, Michael S. Tsirkin wrote:
> > 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@redhat.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
> 
> The access flags have to do with what independent access remote nodes
> get. There are four major cases:
> 
> access = IBV_ACCESS_REMOTE_READ says the adaptor will let remote nodes
> read the memory.
> 
> access = 0 (ie IBV_ACCESS_LOCAL_READ) says that only the adaptor, under
> the direct control of the application, can read this memory. Remote
> nodes are barred.
> 
> access = IBV_ACCESS_REMOTE_WRITE|IBV_ACCESS_LOCAL_WRITE says the adaptor
> will let remote nodes write the memory
> 
> access = IBV_ACCESS_LOCAL_WRITE bars remote nodes from writing to that
> memory. Only the adaptor, under the direct control of the application,
> can write the memory.
> 
> The fact LOCAL_READ/REMOTE_READ exists makes it possible to do what
> you want - it guarentees the adaptor will never write to this memory
> under any circumstances, so you can leave the page COW'd. If
> LOCAL_WRITE was implied then you'd have to COW everything..
> 
> Would it be better to drive the COW break decision off the region's MM
> flags? Ie if the memory is mapped read only into the process then you
> can keep the COW at the RDMA layer, otherwise you should break
> it. That seems more natural than a new flag?
> 
> Jason

No because application does this:
init page

...

after a lot of time

..

register
send
unregister

so it can not be read only.

-- 
MST