RFR (M): 8012915: ReservedSpace::align_reserved_region() broken on Windows

[Bengt Rutisson]

Hi everyone,

Could I have a couple of reviews of this change?
http://cr.openjdk.java.net/~brutisso/8012915/webrev.00/

Background from the CR:

The method ReservedSpace::align_reserved_region() does not work on 
Windows. It tries to free parts of the previously allocated memory by 
doing two calls to os::release_memory(). However, on Windows 
os::release_memory() is implemented as:

  VirtualFree(addr, 0, MEM_RELEASE)

which will always free up all the allocated memory:

http://msdn.microsoft.com/en-gb/library/windows/desktop/aa366892%28v=vs.85%29.aspx 


"The function frees the entire region that is reserved in the initial 
allocation call to VirtualAlloc."

This means that if ReservedSpace::align_reserved_region() is executed on 
Windows and we try to trim the beginning of the memory that we had 
allocated we will free all of the allocated memory. Subsequent calls to 
os::commit_memory() which will end up as:

VirtualAlloc(addr, bytes, MEM_COMMIT, PAGE_READWRITE)

will fail with 487, ERROR_INVALID_ADDRESS - Attempt to access invalid 
address.

The solution is to use the existing method os::reserve_memory_aligned() 
which does the same thing but works on all platforms.

I assume that it is unusual that we enter this code path since I haven't 
seen a lot of crashes that seem related to this issue. But when we tried 
a patch that plays a bit with the alignments we crash on Windows. To 
have a simple reproducer I added a unit test. The test is not 100% fool 
proof, but on Windows without my proposed fix it fails 999 times out of 
1000 runs. With my fix it passes 100% of the time.

Thanks,
Bengt
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