RFR: 8207851 JEP Draft: Support ByteBuffer mapped over non-volatile memory

Andrew Dinn adinn at redhat.com
Fri Jul 27 07:39:19 UTC 2018

On 26/07/18 20:05, Florian Weimer wrote:
> * Andrew Dinn:
>>> +    // TODO - remove the following defines
>>> +    // for testing we need to define the extra MAP_XXX flags used for
>>> +    // persistent memory. they will eventually get defined by the
>>> +    // system and included via sys/mman.h
>>> Do you really want to change implementation behavior based on which
>>> glibc headers (or kernel headers) were used to build the JDK?  It's
>>> likely more appropriate to define the constants yourself if they are
>>> not available.  There is some architecture variance for the MAP_*
>>> constants, but in case of MAP_SHARED_VALIDATE, even HPPA has the same
>>> definition.
>> No, I guess I don't really want to change implementation behaviour based
>> on which glibc headers (or kernel headers) were used to build the JDK.
>> So, I think I need to modify this so the constants are always defined by
>> FileChannelImpl.c
>> I have followed the Intel libpmem code in defining the values for these
>> constants. Can you provide details of the 'architecture variance' you
>> refer to above?
> Just a quick note on that: Some of the constants in <sys/mman.h> vary
> between the Linux architectures for historic reasons.  However, for
> MAP_SHARED_VALIDATE, the constant is consistent across all
> architectures supported by glibc and in the mainline kernel.
> You could add something like this to be on the safe side:
> #error Unexpected value for MAP_SHARED_VALIDATE
> #endif
> #else /* !MAP_SHARED_VALIDATE */
> #endif
> But I doubt that this is required for this constant.
Thanks for the update, Florian. I agree that the above tweak is
unnecessary at present -- given that the current code is targeted at
Linux on x86_64 and AArch64 where a change is extremely unlikely. That
decision may need revisiting if/when the implementation is extended to
other ports.


Andrew Dinn
Senior Principal Software Engineer
Red Hat UK Ltd
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