RFR 8252500: ZGC on aarch64: Unable to allocate heap for certain Linux kernel configurations

[Christoph Göttschkes]

Hi,

please review and test the following patch:

Bug: https://bugs.openjdk.java.net/browse/JDK-8252500
Webrev: https://cr.openjdk.java.net/~cgo/8252500/webrev.00

The patch introduces a new function to probe for the highest valid bit 
in the virtual address space for userspace programs on Linux.

I guarded the whole implementation to only probe on Linux, other 
platforms will remain unaffected. Possibly, it would be nicer to move 
the probing code into an OS+ARCH specific source file. But since this is 
only a single function, I thought it would be better to put it right 
next to the caller and guard it with an #ifdef LINUX.

The probing mechanism uses a combination of msync + mmap, to first check 
if the address is valid using msync (if msync succeeds, the address was 
valid). If msync fails, mmap is used to check if msync failed because 
the memory wasn't mapped, or if it failed because the address is 
invalid. Due to some undefined behavior (documented in the msync man 
page), I also use a single mmap at the end, if the msync approach failed 
before.

I tested msync with different combinations of mappings, and also with 
sbrk, and it always succeeded, or failed with ENOMEM. I never got back 
any other error code.

The specified minimum value has been chosen "randomly". The JVM 
terminates (unable to allocate heap), if this minimum value is smaller 
than the requested Java Heap size, so it might be better to make the 
minimum dependent on the MaxHeapSize and not a compile time constant? I 
didn't want to make the minimum too big, since for aarch64 on Linux, the 
documented minimum would be 38 (see [1]).

I am about to execute the HotSpot tier1 JTreg tests on two different 
aarch64 machines, one with 4KB pages and 3 levels, the other with 4KB 
pages and 4 levels (see [1]).

Please test the patch, review the implementation and give feedback.

Thanks,
Christoph

[1] https://www.kernel.org/doc/Documentation/arm64/memory.txt