RFR: 8252500: ZGC on aarch64: Unable to allocate heap for certain Linux kernel configurations [v2]

[Erik Österlund]

On Mon, 7 Sep 2020 13:12:48 GMT, Christoph Göttschkes <cgo at openjdk.org> wrote:

>> Not sure how to comment a few lines below where you conditionally return MINIMUM_MAX_ADDRESS_BIT in this new cool GUI.
>> Anyway, you could use MIN2 to return MIN2(MINIMUM_MAX_ADDRESS_BIT, max_address_bit) to make the intention more clear.
>
> We are not processing MINIMUM_MAX_ADDRESS_BIT in the loop, because this is the absolute minimum that should be
> returned. So, regardless of the probing result, we are going to return that value. Therefor, it is a waste of time to
> actually perform the probe.  The check `max_address_bit < MINIMUM_MAX_ADDRESS_BIT` is done, because after the probing
> (if it failed and we didn't find any valid page), right after the for loop, we do a single mmap and use the returned
> address to calculate the `max_address_bit`. So `max_address_bit` could then be lower than `MINIMUM_MAX_ADDRESS_BIT`.
> For the return part, you are right, we could simplify it by using `MAX2`, since we are interested in the highest bit,
> and `MINIMUM_MAX_ADDRESS_BIT` is the absolute minimum we want to return.  I will push an update for the return part.

I see. Incremental looks good.

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PR: https://git.openjdk.java.net/jdk/pull/40