Re: [Qemu-devel] [PATCH 0/2][RFC] postcopy migration: Linux char device for postcopy

[Isaku Yamahata]

On Thu, Dec 29, 2011 at 01:24:32PM +0200, Avi Kivity wrote:
> On 12/29/2011 03:26 AM, Isaku Yamahata wrote:
> > This is Linux kernel driver for qemu/kvm postcopy live migration.
> > This is used by qemu/kvm postcopy live migration patch.
> >
> > TODO:
> > - Consider FUSE/CUSE option
> >   So far several mmap patches for FUSE/CUSE are floating around. (their
> >   purpose isn't different from our purpose, though). They haven't merged
> >   into the upstream yet.
> >   The driver specific part in qemu patches is modularized. So I expect it
> >   wouldn't be difficult to switch kernel driver to CUSE based driver.
> 
> It would be good to get more input about this, please involve lkml and
> the FUSE/CUSE people.

Okay.


> > ioctl commands:
> >
> > UMEM_DEV_CRATE_UMEM: create umem device for qemu
> > UMEM_DEV_LIST: list created umem devices
> > UMEM_DEV_REATTACH: re-attach the created umem device
> >               UMEM_DEV_LIST and UMEM_DEV_REATTACH are used when
> >               the process that services page fault disappears or get stack.
> >               Then, administrator can list the umem devices and unblock
> >               the process which is waiting for page.
> 
> Ah, I asked about this in my patch comments.  I think this is done
> better by using SCM_RIGHTS to pass fds along, or asking qemu to launch a
> new process.

Can you please elaborate? I think those ways you are suggesting doesn't solve
the issue. Let me clarify the problem.

  process A (typically incoming qemu)
     |
     | mmap("/dev/umem") and access those pages triggering page faults
     | (the file descriptor might be closed after mmap() before page faults)
     |
     V
   /dev/umem
     ^
     |
     |
   daemon X resolving page faults triggered by process A
   (typically this daemon forked from incoming qemu:process A)

If daemon X disappears accidentally, there is no one that resolves
page faults of process A. At this moment process A is blocked due to page
fault. There is no file descriptor available corresponding to the VMA.
Here there is no way to kill process A, but system reboot.


> Introducing a global namespace has a lot of complications attached.
> 
> >
> > UMEM_GET_PAGE_REQUEST: retrieve page fault of qemu process
> > UMEM_MARK_PAGE_CACHED: mark the specified pages pulled from the source
> >                        for daemon
> >
> > UMEM_MAKE_VMA_ANONYMOUS: make the specified vma in the qemu process
> >                      This is _NOT_ implemented yet.
> >                          anonymous I'm not sure whether this can be 
> > implemented
> >                          or not.
> 
> How do we find out?  This is fairly important, stuff like transparent
> hugepages and ksm only works on anonymous memory.

I agree that this is important.
At KVM-forum 2011, Andrea said THP and KSM works with non-anonymous VMA.
(Or at lease he'll look into those stuff. My memory is vague, though.
 Please correct me if I'm wrong)
-- 
yamahata