[jdk16] RFR: 8259032: MappedMemorySegmentImpl#makeMappedSegment() ignores Unmapper#pagePosition

[Uwe Schindler]

On Tue, 5 Jan 2021 15:44:50 GMT, Alan Bateman <alanb at openjdk.org> wrote:

>> Hi,
>> this patch fixes an oversight where the address() method in FileChannelImpl.Unmapper does not take into account the value of `pagePosition` - this leads to a situation in which, effectively, the specified offset when mapping the segment is ignored by the runtime (because base address will always be aligned to some known quantity - which is OS/platform dependent).
>> 
>> To test this I had to break open into FileChannelImpl and ready the granularity.
>
> test/jdk/java/foreign/TestByteBuffer.java line 28:
> 
>> 26:  * @modules java.base/sun.nio.ch
>> 27:  *          jdk.incubator.foreign/jdk.internal.foreign
>> 28:  * @run testng/othervm --illegal-access=permit -Dforeign.restricted=permit TestByteBuffer
> 
> Can you change java.base/sun.nio.ch to java.base/sun.nio.ch:+open instead? That would avoid the --illegal-access=permit.

I am not sure if a test like this is really needed.
The alignment is pageSize on Linux and some arbitrary value (65536) on Windows. If you have some test file that writes like a few bytes (1, 2, 3, 4,...) To a file and then maps it with offsets other than zero, you just have to get the first byte and compare it to offset.

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PR: https://git.openjdk.java.net/jdk16/pull/84