> > > On Thu, Mar 24, 2016 at 03:53:25PM +0000, Li, Liang Z wrote:
> > > > > > > > Not very complex, we can implement like this:
> > > > > > > >
> > > > > > > > 1. Set all the bits in the migration_bitmap_rcu->bmap to 1 2.
> > > > > > > > Clear all the bits in ram_list.
> > > > > > > > dirty_memory[DIRTY_MEMORY_MIGRATION]
> > > > > > > > 3. Send the get_free_page_bitmap request 4. Start to send
> > > > > > > > pages to destination and check if the free_page_bitmap is ready
> > > > > > > > if (is_ready) {
> > > > > > > > filter out the free pages from migration_bitmap_rcu-
> >bmap;
> > > > > > > > migration_bitmap_sync();
> > > > > > > > }
> > > > > > > > continue until live migration complete.
> > > > > > > >
> > > > > > > >
> > > > > > > > Is that right?
> > > > > > >
> > > > > > > The order I'm trying to understand is something like:
> > > > > > >
> > > > > > > a) Send the get_free_page_bitmap request
> > > > > > > b) Start sending pages
> > > > > > > c) Reach the end of memory
> > > > > > > [ is_ready is false - guest hasn't made free map yet ]
> > > > > > > d) normal migration_bitmap_sync() at end of first pass
> > > > > > > e) Carry on sending dirty pages
> > > > > > > f) is_ready is true
> > > > > > > f.1) filter out free pages?
> > > > > > > f.2) migration_bitmap_sync()
> > > > > > >
> > > > > > > It's f.1 I'm worried about. If the guest started generating
> > > > > > > the free bitmap before (d), then a page marked as 'free' in
> > > > > > > f.1 might have become dirty before (d) and so (f.2) doesn't
> > > > > > > set the dirty again, and so we can't filter out pages in f.1.
> > > > > > >
> > > > > >
> > > > > > As you described, the order is incorrect.
> > > > > >
> > > > > > Liang
> > > > >
> > > > >
> > > > > So to make it safe, what is required is to make sure no free
> > > > > list us outstanding before calling migration_bitmap_sync.
> > > > >
> > > > > If one is outstanding, filter out pages before calling
> > > migration_bitmap_sync.
> > > > >
> > > > > Of course, if we just do it like we normally do with migration,
> > > > > then by the time we call migration_bitmap_sync dirty bitmap is
> > > > > completely empty, so there won't be anything to filter out.
> > > > >
> > > > > One way to address this is call migration_bitmap_sync in the IO
> > > > > handler, while VCPU is stopped, then make sure to filter out
> > > > > pages before the next migration_bitmap_sync.
> > > > >
> > > > > Another is to start filtering out pages upon IO handler, but
> > > > > make sure to flush the queue before calling migration_bitmap_sync.
> > > > >
> > > >
> > > > It's really complex, maybe we should switch to a simple start,
> > > > just skip the free page in the ram bulk stage and make it asynchronous?
> > > >
> > > > Liang
> > >
> > > You mean like your patches do? No, blocking bulk migration until
> > > guest response is basically a non-starter.
> > >
> >
> > No, don't wait anymore. Like below (copy from previous thread)
> > --------------------------------------------------------------
> > 1. Set all the bits in the migration_bitmap_rcu->bmap to 1 2. Clear
> > all the bits in ram_list.dirty_memory[DIRTY_MEMORY_MIGRATION]
> > 3. Send the get_free_page_bitmap request 4. Start to send pages to
> > destination and check if the free_page_bitmap is ready
> > if (is_ready) {
> > filter out the free pages from migration_bitmap_rcu->bmap;
> > migration_bitmap_sync();
> > }
> > continue until live migration complete.
> > ---------------------------------------------------------------
> > Can this work?
> >
> > Liang
>
> Not if you get the ready bit asynchronously like you wrote here since
> is_ready can get set while you called migration_bitmap_sync.
If the is_ready is not set just before the first calling of migration_bitmap_sync,
then we can ignore the free page bitmap and not filter the free pages.
Another thing we should do it to remove migration_bitmap_sync() from ram_save_setup().
> As I said previously,
> to make this work you need to filter out synchronously while VCPU is
> stopped and while free pages from list are not being used.
>
> Alternatively prevent getting free page list and filtering them out from guest
> from racing with migration_bitmap_sync.
>
> For example, flush the VQ after migration_bitmap_sync.
> So:
>
> lock
> migration_bitmap_sync();
> while (elem = virtqueue_pop) {
> virtqueue_push(elem)
> g_free(elem)
> }
> unlock
>
>
> while in handle_output
>
> lock
> while (elem = virtqueue_pop) {
> list = get_free_list(elem)
> filter_out_free(list)
> virtqueue_push(elem)
> free(elem)
> }
> unlock
>
>
> lock prevents migration_bitmap_sync from racing against handle_output
>
>
> This way you can actually use ioeventfd
> for this VQ so VCPU won't be blocked.
>
> I do not think this is so complex, and
> this way you can add requests for guest
> free bitmap at an arbitary interval
> either in host or in guest.
>
> For example, add a value that says how often should guest update the
> bitmap, set it to 0 to disable updates after migration done.
>
> Or, make guest resubmit a new one when we consume the old one, run
> handle_output about through a periodic timer on host.
>
I need some time to understand how your idea works.
Liang
> > > --
> > > MST
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