Re: [RFC][PATCH] Cross Memory Attach

[Avi Kivity <avi@redhat.com>]

  On 09/16/2010 03:18 AM, Christopher Yeoh wrote:
> On Wed, 15 Sep 2010 23:46:09 +0900
> Bryan Donlan<bdonlan@gmail.com>  wrote:
>
> >  On Wed, Sep 15, 2010 at 19:58, Avi Kivity<avi@redhat.com>  wrote:
> >
> >  >  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.
> >
> >  Rather than introducing a new vmfd() API for this, why not just add
> >  implementations for these more efficient operations to the existing
> >  /proc/$pid/mem interface?
>
> Perhaps I'm misunderstanding something here, but
> accessing /proc/$pid/mem requires ptracing the target process.
> We can't really have all these MPI processes ptraceing each other
> just to send/receive a message....
>

You could have each process open /proc/self/mem and pass the fd using 
SCM_RIGHTS.

That eliminates a race; with copy_to_process(), by the time the pid is 
looked up it might designate a different process.

-- 
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