On Tue, 7 Feb 2012, Hugh Dickins wrote:

On Mon, 6 Feb 2012, Roland Dreier wrote:

quoted

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.

quoted

quoted

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?

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