linux-kernel - Re: [RFC][PATCH] Cross Memory Attach

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Date:	Wed, 15 Sep 2010 12:58:15 +0200
From:	Avi Kivity <avi@...hat.com>
To:	Christopher Yeoh <cyeoh@....ibm.com>
CC:	linux-kernel@...r.kernel.org,
	Linux Memory Management List <linux-mm@...ck.org>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC][PATCH] Cross Memory Attach

  On 09/15/2010 03:18 AM, Christopher Yeoh wrote:
> The basic idea behind cross memory attach is to allow MPI programs doing
> intra-node communication to do a single copy of the message rather than
> a double copy of the message via shared memory.

If the host has a dma engine (many modern ones do) you can reduce this 
to zero copies (at least, zero processor copies).

> The following patch attempts to achieve this by allowing a
> destination process, given an address and size from a source process, to
> copy memory directly from the source process into its own address space
> via a system call. There is also a symmetrical ability to copy from
> the current process's address space into a destination process's
> address space.
>
>

Instead of those two syscalls, how about a vmfd(pid_t pid, ulong start, 
ulong len) system call which returns an file descriptor that represents 
a portion of the process address space.  You can then use preadv() and 
pwritev() to copy memory, and io_submit(IO_CMD_PREADV) and 
io_submit(IO_CMD_PWRITEV) for asynchronous variants (especially useful 
with a dma engine, since that adds latency).

With some care (and use of mmu_notifiers) you can even mmap() your vmfd 
and access remote process memory directly.

A nice property of file descriptors is that you can pass them around 
securely via SCM_RIGHTS.  So a process can create a window into its 
address space and pass it to other processes.

(or you could just use a shared memory object and pass it around)

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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