Re: [Qemu-devel] [PATCH] migratioin/ram.c: reset complete_round when we gets a queued page

[Juan Quintela <quintela@redhat.com>]

Peter Xu <peterx@redhat.com> wrote:
> On Wed, Jun 05, 2019 at 04:52:07PM +0800, Wei Yang wrote:
>> On Wed, Jun 05, 2019 at 02:41:08PM +0800, Peter Xu wrote:
>> >On Wed, Jun 05, 2019 at 09:08:28AM +0800, Wei Yang wrote:
>> >> In case we gets a queued page, the order of block is interrupted. We may
>> >> not rely on the complete_round flag to say we have already searched the
>> >> whole blocks on the list.
>> >> 
>> >> Signed-off-by: Wei Yang <richardw.yang@linux.intel.com>
>> >> ---
>> >>  migration/ram.c | 6 ++++++
>> >>  1 file changed, 6 insertions(+)
>> >> 
>> >> diff --git a/migration/ram.c b/migration/ram.c
>> >> index d881981876..e9b40d636d 100644
>> >> --- a/migration/ram.c
>> >> +++ b/migration/ram.c
>> >> @@ -2290,6 +2290,12 @@ static bool get_queued_page(RAMState *rs, PageSearchStatus *pss)
>> >>           */
>> >>          pss->block = block;
>> >>          pss->page = offset >> TARGET_PAGE_BITS;
>> >> +
>> >> +        /*
>> >> +         * This unqueued page would break the "one round" check, even is
>> >> +         * really rare.
>> >


> Ah I see your point, but I don't think there is a problem - note that
> complete_round will be reset for each ram_find_and_save_block(), so
> even if we have that iteration of ram_find_and_save_block() to return
> we'll still know we have dirty pages to migrate and in the next call
> we'll be fine, no?

Reviewed-by: Juan Quintela <quintela@redhat.com>

I *think* that peter is perhaps right, but it is not clear at all, and
it is easier to be safe.  I think that the only case that this could
matter is if:
- all pages are clean (so complete_round will get as true)
- we went a queue_page request

Is that possible?  I am not completely sure after looking at the code.
It *could* be if the page that got queued is the last page remaining,
but ......  I fully agree that the case that _almost all_ pages are
clean and we get a request for a queued page is really rare, so it
should not matter in real life, but ....

Later, Juan.