RFR: 8330275: Crash in XMark::follow_array [v3]

[Ashutosh Mehra]

On Thu, 25 Apr 2024 14:28:47 GMT, Ashutosh Mehra <asmehra at openjdk.org> wrote:

>> This PR addresses the issue in ZGC where the number of address offset bits can go beyond the limit imposed by the encoding scheme in mark stack, thereby causing the encoding to fail.
>> Encoding of partial array offset in mark stack requires that the address offset be no more than 44 bits. But the current mechanism to probe maximum address offset bits on aarch64, riscv and ppc platforms can return value larger that 44 bits. This patch sets the maximum address offset bits to 44.
>> 
>> I have updated the generational mode to avoid subtracting 3 bits from the maximum address offset bit probed by the system, as the generational mode does not use multi-mapping.
>> 
>> I have also updated the code to set MarkPartialArrayMinSizeShift dynamically depending on the number of address offset bits used. This would avoid running into such problem again if in future maximum address offset bits is increased beyond 44.
>> 
>> For some reason (that I can't comprehend from the code) the existing implementation for probing the max addressable bit for ppc in non-generation ZGC is very different from other platforms and from generational mode as well. I have kept the existing implementation as is and just fixed it to ensure it does not return value greater than 44 bits.
>> 
>> Testing: test/hotspot/jtreg/gc/z and test/hotspot/jtreg/gc/x on x86
>
> Ashutosh Mehra has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Fix typos
>   
>   Signed-off-by: Ashutosh Mehra <asmehra at redhat.com>

I agree from the point of view of backporting, point-fix is all we need in this PR.

@tstuefe  As for the other platforms (riscv and ppc), looking at their code they seem to be broken in the same way as aarch64 but then the problem only happens if the user runs with > 1TB heap size with more than 48 addressable bits.
Again, in the spirit of "do not touch if it is not broken", I am fine if we restrict the change to just aarch64.

@tstuefe @stefank please let me know if you agree with just doing the point-fix to aarch64.

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PR Comment: https://git.openjdk.org/jdk/pull/18941#issuecomment-2079493432