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

[John Cuthbertson]

Hi Bengt,

This looks good to me.

Thanks for the explanation regarding hs24 and hs25.

JohnC

On 4/25/2013 8:14 AM, Bengt Rutisson wrote:
>
> Hi again,
>
> It turns out that the code that I was fixing is actually not in use. 
> Thanks Coleen for triggering me to figure that out!
>
> Here is an updated webrev where the code is removed instead:
> http://cr.openjdk.java.net/~brutisso/8012915/webrev.01/
>
> Thanks,
> Bengt
>
> On 4/25/13 1:09 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.
>>
>> Thanks,
>> Bengt
>

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