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SubjectRe: [PATCH/RFC G-U-P experts] IB/umem: Modernize our get_user_pages() parameters
Hi Andrea, sorry for the slow reply, had to work on other stuff for a bit.

On Mon, Jan 30, 2012 at 12:20 PM, Andrea Arcangeli <> wrote:
> If you map it with an mmap(PROT_READ|PROT_WRITE), force or not force
> won't change a thing in terms of cows. Just make sure you map your
> control memory right, then safely remove force=1 and you won't get the
> control page cowed by mistake. Then if you map it with MAP_SHARED it
> won't be mapped read-only by fork() (leading to either parent or child
> losing the control on the device), Hugh already suggested you to use

Actually this isn't about control memory for the RDMA adapter...
as you mentioned that typically is MMIO and mapped with remap_pfn
stuff, without using any GUP stuff.

I'm talking about the registration of other memory for reading/writing
by a remote system via RDMA.

The reason I'm talking about exporting kernel memory is that I wanted
to do a debugging trick where a kernel module exposed some state
into an mmap'able buffer. And I wanted to be able to read that state
even if my broken module killed the whole system (in fact exactly
when things crash I want to be able to read the state to figure out
why I crashed!).

So I wrote a trivial userspace program that does nothing but mmap
the buffer, accept RDMA connections from remote systems, and
map the buffer for reading over those connections. Then I can have
a second system that connects to that process and polls the buffer.

Because all the RDMA state is setup in advance, I can keep polling
even after the first system panics. It's sort of like that firewire remote
debugging, except I only get access to a limited memory buffer.

The only difficulty is the problem that started this thread, ie a bogus
COW so the remote system ends up polling the wrong pages. So with
my original patch, I'm able to debug but I guess we agree it's the
wrong fix for the general problem, and I'll write up a patch that adds
what I think is the correct fix (the new FOLL flag) soon.

- R.

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