RFR(S) 8153843: G1CardLiveDataHelper incorrectly sets next_live_bytes on dead humongous regions

[Thomas Schatzl]

Hi,

On Fri, 2016-04-08 at 12:09 -0400, Tom Benson wrote:
> Hi Mikael,
> It looks like there is an indentation problem at lines 214/215.
> Dumb question:  The code now extends the range down to the object 
> start, if current region continues_humongous.  Do you want to extend 
> to the true end region of the object, which might not be the current
> one?

  neither "hr" nor "start" are reassigned as they are used for the
actual marking.

Other issues:
  - start_region can always use humongous_start_region(). For humongous
starts regions, humongous_start_region() points to itself.

  - the change misses that during humongous object registration for
eager reclaim the klass we access to determine the kind of the object
can already be invalid.

I saw some of the crashes were during humongous candidate region
selection. My suggestion on fixing this is to replace
g1CollectedHeap.cpp:~3003 with

bool is_candidate = !g1h->is_obj_dead((oop)(r->bottom())) &&
humongous_region_is_candidate(g1h, r);

These two changes also seem to make JDK-8153834 disappear at least when
running previously failing tests locally too :)

> On 4/8/2016 11:47 AM, Mikael Gerdin wrote:
> > Hi all,
> > 
> > Please review this small change to solve an integration blocker in 
> > hs-rt which surfaced with the latest integration from main.
> > 
> > The problem is that humongous regions are kept around for longer 
> > (possibly indefinitely) because the card live data helper 

Eager reclaim will catch them, as it only considers reachability.

Thanks,
  Thomas