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

[Roland Dreier <roland@kernel.org>]

On Wed, Feb 8, 2012 at 3:10 PM, Hugh Dickins <hughd@google.com> wrote:
> 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()?

The access flags are:

enum ibv_access_flags {
        IBV_ACCESS_LOCAL_WRITE          = 1,
        IBV_ACCESS_REMOTE_WRITE         = (1<<1),
        IBV_ACCESS_REMOTE_READ          = (1<<2),
        IBV_ACCESS_REMOTE_ATOMIC        = (1<<3),
        IBV_ACCESS_MW_BIND              = (1<<4)
};

pretty much the only one of interest is IBV_ACCESS_REMOTE_READ --
all the others imply the possibility of RDMA HW writing to the page.

So basically if any flags other than IBV_ACCESS_REMOTE_READ are
set, we pass FOLL_WRITE to __get_user_pages(), otherwise we pass
the new FOLL_FOLLOW.  [does "Marcia, Marcia, Marcia" mean anything
to a Brit? ;)]

ie the change from the status quo would be:

[read-only]  write=1, force=1 --> FOLL_FOLLOW
[writeable]  wrote=1, force=0 --> FOLL_WRITE (equivalent)

> 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.

First of all, I kind of like FOLL_FOLLOW as the name :)

Now you're confusing me: I think we do need FOLL_FOLLOW to
set up read-only access -- we want to trigger the COWs that userspace
might trigger by touching the memory up front.  This is to handle
a case like

    [userspace]
    int *buf = malloc(16 * 4096);
    // buf now points to 16 anonymous zero_pages
    mr = ibv_reg_mr(pd, buf, 16 * 4096, IBV_ACCESS_REMOTE_READ);
    // RDMA HW will only ever read buf, but...
    buf[0] = 2012;
    // COW triggered, first page of buf changed, RDMA HW has wrong mapping!

For something the RDMA HW might write to, then I agree we don't want
FOLL_FOLLOW -- we just would use FOLL_WRITE as we currently do.

When I get around to coding this up, I think I'm going to spend a lot
of time on the comments and on the commit log :)

 - R.