linux-kernel - Re: [PATCH/RFC G-U-P experts] IB/umem: Modernize our get_user

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Message-ID: <CAL1RGDWm6q9SxO_X5PR8Z7_V6wiYmoHqdPfX++8=Ph1v5HiZ6Q@mail.gmail.com>
Date:	Mon, 30 Jan 2012 11:19:18 -0800
From:	Roland Dreier <roland@...nel.org>
To:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
Cc:	Hugh Dickins <hughd@...gle.com>, linux-rdma@...r.kernel.org,
	Andrea Arcangeli <aarcange@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH/RFC G-U-P experts] IB/umem: Modernize our get_user_pages() parameters

On Sat, Jan 28, 2012 at 11:25 AM, Jason Gunthorpe
<jgunthorpe@...idianresearch.com> wrote:
> I know accessing system memory (eg obtained via mmap on
> /sys/bus/pci/devices/0000:00:02.0/resource0) has been asked for in the
> past, and IIRC, the problem was that some of the common code, (GUP?)
> errored on these maps. I don't know if Roland's case is similar.

I think the problem there is that this is done via remap_pfn_range()
or similar, and the mapping has no underlying pages at all.  So we
would need a new interface that gives us different information for
such cases.

This is quite a bit trickier since I don't think the DMA API even has
a way to express getting a "device A" bus address for some memory
that is in a BAR for "device B".  So I'm not trying to address this case
(yet).  First I'd like to deal with as many flavors of page-backed
mappings as I can.

 - R.
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