Re: [PATCH v1 2/2] virtio-balloon: disallow postcopy with VIRTIO_BALLOON_F_FREE_PAGE_HINT

[David Hildenbrand]

On 07.07.21 21:19, Michael S. Tsirkin wrote:
> On Wed, Jul 07, 2021 at 09:14:00PM +0200, David Hildenbrand wrote:
> > On 07.07.21 21:05, Michael S. Tsirkin wrote:
> > > On Wed, Jul 07, 2021 at 04:06:55PM +0200, David Hildenbrand wrote:
> > > > Postcopy never worked properly with 'free-page-hint=on', as there are
> > > > at least two issues:
> > > >
> > > > 1) With postcopy, the guest will never receive a VIRTIO_BALLOON_CMD_ID_DONE
> > > >      and consequently won't release free pages back to the OS once
> > > >      migration finishes.
> > > >
> > > >      The issue is that for postcopy, we won't do a final bitmap sync while
> > > >      the guest is stopped on the source and
> > > >      virtio_balloon_free_page_hint_notify() will only call
> > > >      virtio_balloon_free_page_done() on the source during
> > > >      PRECOPY_NOTIFY_CLEANUP, after the VM state was already migrated to
> > > >      the destination.
> > > >
> > > > 2) Once the VM touches a page on the destination that has been excluded
> > > >      from migration on the source via qemu_guest_free_page_hint() while
> > > >      postcopy is active, that thread will stall until postcopy finishes
> > > >      and all threads are woken up. (with older Linux kernels that won't
> > > >      retry faults when woken up via userfaultfd, we might actually get a
> > > >      SEGFAULT)
> > > >
> > > >      The issue is that the source will refuse to migrate any pages that
> > > >      are not marked as dirty in the dirty bmap -- for example, because the
> > > >      page might just have been sent. Consequently, the faulting thread will
> > > >      stall, waiting for the page to be migrated -- which could take quite
> > > >      a while and result in guest OS issues.
> > >
> > > OK so if source gets a request for a page which is not dirty
> > > it does not respond immediately? Why not just teach it to
> > > respond? It would seem that if destination wants a page we
> > > should just give it to the destination ...
> >
> > The source does not know if a page has already been sent (e.g., via the
> > background migration thread that moves all data over) vs. the page has not
> > been send because the page was hinted. This is the part where we'd need
> > additional tracking on the source to actually know that.
> >
> > We must not send a page twice, otherwise bad things can happen when placing
> > pages that already have been migrated, because that scenario can easily
> > happen with ordinary postcopy (page has already been sent and we're dealing
> > with a stale request from the destination).
>
> OK let me get this straight
>
> A. source sends page
> B. destination requests page
> C. destination changes page
> D. source sends page
> E. destination overwrites page
>
> this is what you are worried about right?

IIRC E. is with recent kernels:

E. placing the page fails with -EEXIST and postcopy migration fails

However, the man page (man ioctl_userfaultfd) doesn't describe what is 
actually supposed to happen when double-placing. Could be that it's 
"undefined behavior".

I did not try, though.


This is how it works today:

A. source sends page and marks it clean
B. destination requests page
C. destination receives page and places it
D. source ignores request as page is clean

> the fix is to mark page clean in A.
> then in D to not send page if it's clean?
>
> And the problem with hinting is this:
>
> A. page is marked clean
> B. destination requests page
> C. destination changes page
> D. source sends page <- does not happen, page is clean!
> E. destination overwrites page

Simplified it's

A. page is marked clean by hinting code
B. destination requests page
D. source ignores request as page is clean
E. destination stalls until postcopy unregisters uffd


Some thoughts

1. We do have a a recv bitmap where we track received pages on the 
destination (e.g., ramblock_recv_bitmap_test()), however we only use it 
to avoid sending duplicate requests to the hypervisor AFAIKs, and don't 
check it when placing pages.

2. Changing the migration behavior unconditionally on the source will 
break migration to old QEMU binaries that cannot handle this change.

3. I think the current behavior is in place to make debugging easier. If 
only a single instance of a page will ever be migrated from source to 
destination, there cannot be silent data corruption. Further, we avoid 
migrating unnecessarily pages twice.


Maybe Dave and Peter can spot any flaws in my understanding.

--
Thanks,

David / dhildenb