Re: [PATCH/RFC G-U-P experts] IB/umem: Modernize our get_user_pages() parameters

[Hugh Dickins <hughd@google.com>]

On Tue, 7 Feb 2012, Hugh Dickins wrote:
> On Mon, 6 Feb 2012, Roland Dreier wrote:
> > Which I think explains why the code is the way it is.  But clearly
> > we could do better if we had a better way of telling GUP our real
> > intentions -- ie the FOLL_READONLY_COW flag.
> 
> You've persuaded me.  Yes, you have been using force because that was
> the only tool available at the time, to get close to the sensible
> behaviour you are now asking for.
> 
> > 
> > > Can you, for example, enforce the permissions set up by the user?
> > > I mean, if they do the ibv_reg_mr() on a private readonly area,
> > > so __get_user_pages with the FOLL_APPROPRIATELY flag will fault
> > > in ZERO_PAGEs, can you enforce that RDMA will never spray data
> > > into those pages?
> > 
> > Yes, the access flags passed into ibv_reg_mr() are enforced by
> > the RDMA hardware, so if no write access is request, no write
> > access is possible.
> 
> Okay, if you enforce the agreed permissions in hardware, that's fine.

A doubt assaulted me overnight: sorry, I'm back to not understanding.

What are these access flags passed into ibv_reg_mr() that are enforced?
What relation do they bear to what you will pass to __get_user_pages()?

You are asking for a FOLL_FOLLOW ("follow permissions of the vma") flag,
which automatically works for read-write access to a VM_READ|VM_WRITE vma,
but read-only access to a VM_READ-only vma, without you having to know
which permission applies to which range of memory in the area specified.

But you don't need that new flag to set up read-only access, and if you
use that new flag to set up read-write access to an area which happens to
contain VM_READ-only ranges, you have set it up to write into ZERO_PAGEs.

?Hugh?