From: Hugh Dickins <email@example.com>
To: Roland Dreier <firstname.lastname@example.org>
Andrea Arcangeli <email@example.com>,
Subject: Re: [PATCH/RFC G-U-P experts] IB/umem: Modernize our get_user_pages() parameters
Date: Wed, 8 Feb 2012 15:10:17 -0800 (PST) [thread overview]
Message-ID: <alpine.LSU.firstname.lastname@example.org> (raw)
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.
next prev parent reply other threads:[~2012-02-08 23:10 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-01-26 5:59 [PATCH/RFC G-U-P experts] IB/umem: Modernize our get_user_pages() parameters Roland Dreier
2012-01-26 20:01 ` Hugh Dickins
2012-01-26 22:45 ` Roland Dreier
2012-01-27 17:28 ` Roland Dreier
2012-01-28 2:31 ` Hugh Dickins
2012-01-28 19:25 ` Jason Gunthorpe
2012-01-30 19:19 ` Roland Dreier
2012-01-28 2:19 ` Hugh Dickins
2012-01-30 19:16 ` Roland Dreier
2012-01-30 20:20 ` Andrea Arcangeli
2012-02-06 17:46 ` Roland Dreier
2012-01-30 20:34 ` Hugh Dickins
2012-02-06 17:39 ` Roland Dreier
2012-02-07 20:39 ` Hugh Dickins
2012-02-08 23:10 ` Hugh Dickins [this message]
2012-02-09 17:50 ` Roland Dreier
2012-02-09 22:57 ` Hugh Dickins
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