Suggested fix for JDK-4724038 (Add unmap method to MappedByteBuffer)

[Andrew Haley]

On 09/08/2015 12:37 PM, David M. Lloyd wrote:
> On 09/08/2015 04:30 AM, Andrew Haley wrote:
>> On 09/08/2015 09:58 AM, Paul Sandoz wrote:
> 
>> However, I think that some cleverness in HotSpot could make that cost
>> go away.  For example, we could associate with every
>> MappedByteBufferForwardingObject a protection page in memory.  When
>> the forwarding object is unmapped that page is write-protected.  Every
>> access to the mapped file is preceded by a write to the page; there
>> don't have to be any memory fence instructions.  The protection page
>> would stay until the forwarding object was unmapped.

Sorry, I meant "until the forwarding object was collected".

> If you're already doing this, why not skip the level of indirection and 
> mprotect the entire mapped region to PROT_NONE when the user unmaps?

Because one page has to hang around until the forwarding object is
garbage collected.  The entire mapping doesn't.

Andrew.