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

[Mikael Gerdin]

Bengt,

On 04/25/2013 01:09 PM, Bengt Rutisson wrote:
>
> 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.

The fix looks good and I like that you added a test for this even though 
the test is not 100% deterministic.

Ship it!
/Mikael

>
> Thanks,
> Bengt