RFR (S): 8196485: FromCardCache default card index can cause crashes

[Thomas Schatzl]

Ping?

On Thu, 2018-03-08 at 09:59 +0100, Thomas Schatzl wrote:
> Hi all,
> 
>   can I have reviews for the following change that fixes a potential
> crash due to missing remembered set entries on huge (2TB+) heaps, or
> heaps that cross the 2TB address boundary (set via HeapBaseAddress)?
> 
> The reason is that in the FromCardCache that is used to filter out
> repeated additions of the same card G1 uses 32 bits to represent that
> card, with a default value of 2^32-1.
> 
> So if the first card we add to the remembered set has the index -1,
> G1 assumes that we already added that value and wrongly bails out.
> 
> 2TB+ because 2^32 + 2^9 (card size) = 2^41 which is 2TB.
> 
> Note that some assertions in that code already catch this issue.
> 
> The fix is to extend the size of a FromCardCacheEntry to 64 bits,
> which allows 2^73 byte sized heaps - which hopefully is sufficient
> for some time :)
> 
> On my machine the passing test takes 8s (in debug mode), so I added
> it to the regular tests run everytime.
> 
> CR:
> https://bugs.openjdk.java.net/browse/JDK-8196485
> Webrev:
> http://cr.openjdk.java.net/~tschatzl/8196485/webrev/
> Testing:
> hs-tier1+2, new jtreg test asserting without the patch, running
> through with the patch.
> 
> Thanks,
>   Thomas