On Thu, 16 Sep 2010 11:26:36 +0200 Avi Kivity <avi@xxxxxxxxxx> wrote: > On 09/16/2010 03:18 AM, Christopher Yeoh wrote: > > On Wed, 15 Sep 2010 23:46:09 +0900 > > Bryan Donlan<bdonlan@xxxxxxxxx> wrote: > > > > > On Wed, Sep 15, 2010 at 19:58, Avi Kivity<avi@xxxxxxxxxx> 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@xxxxxxxxxx -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxxx For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>