RFR(XS): 8178497: Bug in MutableNUMASpace::ensure_parsability

[Thomas Schatzl]

Hi Sangheon,

On Mon, 2017-11-20 at 11:05 -0800, sangheon.kim wrote:
> Hi all,
> 
> Can I have some reviews for this tiny pointer arithmetic change?
> Current code doesn't use pointer arithmetic, so the the resulting
> values are wrong(too small). i.e. adding two values first and then
> typecast to HeapWord* which is wrong here.
> e.g.
> intptr_t cur_top = X;
> size_t touched_words = XX;
> ...
> align_down((HeapWord*)(cur_top + touched_words), XXX);
> 
> This should be 'align_down( (HeapWord*)cur_top + touched_words,
> XXXX);'.
> 
> As I don't see any good reason of using 'intptr_t', changed to use 
> 'HeapWord*' and changed related stuff.
> 
> I didn't add regression test or some further investigation as this
> is clear that the calculation is wrong. And hard to provoke the
> problem outside.
> 
> CR: https://bugs.openjdk.java.net/browse/JDK-8178497
> Webrev: http://cr.openjdk.java.net/~sangheki/8178497/webrev.0
> Testing: local building

  actually after reading this code a bit in more detail I think the
failure mode is "only" performance loss. I.e. the "invalid" part of a
MutableNUMASpace is freed and reallocated for better NUMA locality.

With a too small invalid size what happens is that this locality
improvement will not happen for that part of the MutableNUMASpace. I do
not think there is a useful way of creating a reproducer.

Looks good.

Thanks,
  Thomas