Re: [RFC][PATCH] Cross Memory Attach

[Christopher Yeoh <cyeoh@au1.ibm.com>]

On Thu, 16 Sep 2010 11:26:36 +0200
Avi Kivity <avi@redhat.com> wrote:
>   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.

Just to revive an old thread (I've been on holidays), but this doesn't
work either. the ptrace check is done by mem_read (eg on each read) so
even if you do pass the fd using SCM_RIGHTS, reads on the fd still
fail. 

So unless there's good reason to believe that the ptrace permission
check is no longer needed, the /proc/pid/mem interface doesn't seem to
be an option for what we want to do.

Oh and interestingly reading from /proc/pid/mem involves a double copy
- copy to a temporary kernel page and then out to userspace. But that is
fixable.

Regards,

Chris
-- 
cyeoh@ozlabs.org