RFR(S): 7069863: G1: SIGSEGV running SPECjbb2011 and -UseBiasedLocking

[Vladimir Kozlov]

g1CollectedHeap.cpp:

   // When compressed oops are used the preferred heap base is calculated
   // by subtracting the requested size from the 32Gb boundary and using
   // the result as the base address for heap reservation. If the size is
   // not aligned to HeapRegion::GrainBytes passed into the ReservedHeapSpace
   // constructor then the base of the actual reserved heap may end up
   // differing from the requested base address. If this happens then we
   // could end up using a non-optimal compressed oops mode.


virtualspace.cpp:

  213   // prefix_align == suffix_align).
                        ^ prefix_align < suffix_align

ReservedSpace::ReservedSpace() also misses the call to 
failed_to_reserve_as_requested() after the code which handle wrong alignment.

Add assert into ReservedSpace::initialize()

   // Assert that if noaccess_prefix is used, it is the same as alignment.
   assert(noaccess_prefix == 0 ||
          noaccess_prefix == alignment, "noaccess prefix wrong");

Why we have next code? Should we do this alignment adjustment at the beginning 
of the method?

355   _alignment = MAX2(alignment, (size_t) os::vm_page_size());

Vladimir

John Cuthbertson wrote:
> Hi Everyone,
> 
> Can I have a couple of volunteers look over these changes - the webrev 
> can be found at: http://cr.openjdk.java.net/~johnc/7069863/webrev.0/
> 
> The issue was caused by an implicit null check in generated code not 
> firing. The implicit null check did not trap as a result of a mismatch 
> between the compressed oops mode and the calculated heap base. Also the 
> page below heap base was not being protected. The mismatch between the 
> compressed oops mode and heap base was the result of the G1 heap 
> initialization code passing in a total heap size (G1 heap and perm) that 
> was not a multiple of the alignment passed into the ReservedSpace 
> constructor. Hence the preferred heap base address was not correctly 
> aligned causing the ReservedSpace to be allocated at an address other 
> than the preferred heap base. The page below the heap base was not 
> protected because the G1 heap initialization code was calling the wrong 
> ReservedSpace constructor.
> 
> Testing: the failing test case; various small tests with various 
> collectors, large heaps, and +PrintCompressedOopsMode.
> 
> Many thanks to Vladimir and Igor for helping to diagnose the issue.
> 
> Thanks,
> 
> JohnC