Re: [Qemu-devel] [PATCH for-2.8] migration: Fix return code of ram_save_iterate()

[David Gibson <david@gibson.dropbear.id.au>]

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On Wed, Nov 09, 2016 at 08:46:34AM +0100, Thomas Huth wrote:
> On 09.11.2016 08:18, Amit Shah wrote:
> > On (Fri) 04 Nov 2016 [14:10:17], Thomas Huth wrote:
> >> qemu_savevm_state_iterate() expects the iterators to return 1
> >> when they are done, and 0 if there is still something left to do.
> >> However, ram_save_iterate() does not obey this rule and returns
> >> the number of saved pages instead. This causes a fatal hang with
> >> ppc64 guests when you run QEMU like this (also works with TCG):
> > 
> > "works with" -- does that mean reproduces with?
> 
> Yes, that's what I've meant: You can reproduce it with TCG (e.g. running
> on a x86 system), too, there's no need for a real POWER machine with KVM
> here.
> 
> >>  qemu-img create -f qcow2  /tmp/test.qcow2 1M
> >>  qemu-system-ppc64 -nographic -nodefaults -m 256 \
> >>                    -hda /tmp/test.qcow2 -serial mon:stdio
> >>
> >> ... then switch to the monitor by pressing CTRL-a c and try to
> >> save a snapshot with "savevm test1" for example.
> >>
> >> After the first iteration, ram_save_iterate() always returns 0 here,
> >> so that qemu_savevm_state_iterate() hangs in an endless loop and you
> >> can only "kill -9" the QEMU process.
> >> Fix it by using proper return values in ram_save_iterate().
> >>
> >> Signed-off-by: Thomas Huth <thuth@redhat.com>
> >> ---
> >>  migration/ram.c | 6 +++---
> >>  1 file changed, 3 insertions(+), 3 deletions(-)
> >>
> >> diff --git a/migration/ram.c b/migration/ram.c
> >> index fb9252d..a1c8089 100644
> >> --- a/migration/ram.c
> >> +++ b/migration/ram.c
> >> @@ -1987,7 +1987,7 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
> >>      int ret;
> >>      int i;
> >>      int64_t t0;
> >> -    int pages_sent = 0;
> >> +    int done = 0;
> >>  
> >>      rcu_read_lock();
> >>      if (ram_list.version != last_version) {
> >> @@ -2007,9 +2007,9 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
> >>          pages = ram_find_and_save_block(f, false, &bytes_transferred);
> >>          /* no more pages to sent */
> >>          if (pages == 0) {
> >> +            done = 1;
> >>              break;
> >>          }
> >> -        pages_sent += pages;
> >>          acct_info.iterations++;
> >>  
> >>          /* we want to check in the 1st loop, just in case it was the 1st time
> >> @@ -2044,7 +2044,7 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
> >>          return ret;
> >>      }
> >>  
> >> -    return pages_sent;
> >> +    return done;
> >>  }
> > 
> > I agree with David, we can just remove the return value.  The first
> > patch of the series can do that; and this one could become the 2nd
> > patch.  Should be OK for the soft freeze.
> 
> Sorry, I still did not quite get it - if I'd change the return type of
> ram_save_iterate() and the other iterate functions to "void", how is
> qemu_savevm_state_iterate() supposed to know whether all iterators are
> done or not?

It doesn't - it's return value is, in turn, mostly ignored by the
caller.

On the migration path we already determine whether to proceed or not
based purely on the separate state_pending callbacks.

For the savevm path, we don't really need the iteration phase at all -
we can jump straight to the completion phase, since downtime is not an
issue.

> And other iterators also use negative return values to
> signal errors

Ah.. that's a good point.  Possibly we should leave in the negative
codes for errors and just remove all positive return values.

> - should that then be handled via an "Error **" parameter
> instead? ... my gut feeling still says that such a bigger rework (we've
> got to touch all iterators for this!) should rather not be done right in
> the middle of the freeze period...

Yeah the errors could - and probably should - be handled with Error **
instead of return codes, but I also wonder if that's too much for soft
freeze.  I guess that's the call of the migration guys.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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