From: Roland Dreier <firstname.lastname@example.org>
To: Andrea Arcangeli <email@example.com>
Cc: Hugh Dickins <firstname.lastname@example.org>,
Subject: Re: [PATCH/RFC G-U-P experts] IB/umem: Modernize our get_user_pages() parameters
Date: Mon, 6 Feb 2012 09:46:23 -0800 [thread overview]
Message-ID: <CAL1RGDUzYTVJJNwYzraObNvkZmOT=1oR4gBL2hKhB2harAiLLw@mail.gmail.com> (raw)
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 <email@example.com> 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
> MAP_SHARED instead of MAP_PRIVATE.
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.
next prev parent reply other threads:[~2012-02-06 17:46 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 [this message]
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
2012-02-09 17:50 ` Roland Dreier
2012-02-09 22:57 ` Hugh Dickins
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