8372513: Shenandoah: ShenandoahMaxRegionSize can produce an unaligned heap alignment

[stefank]

While rewriting some of the heap size initialization code we hit a corner-case where the setting of ShenandoahMaxRegionSize to something that isn't a power-of-2 will hit an assert in max_heap_for_compressed_oops.

When running with:

java -XX:+UseShenandoahGC -XX:+UnlockExperimentalVMOptions -XX:ShenandoahMaxRegionSize=33m -version

The following code:

  size_t displacement_due_to_null_page = align_up(os::vm_page_size(),
                                                  _conservative_max_heap_alignment)

triggers:

# assert(is_power_of_2(alignment)) failed: must be a power of 2: 34603008

because _conservative_max_heap_alignment is not a power-of-2.

This happens because Shenandoah's conservative_max_heap_alignment()
returns a potentially unaligned ShenandoahMaxRegionSize value.

size_t ShenandoahArguments::conservative_max_heap_alignment() {
  size_t align = ShenandoahMaxRegionSize;
  if (UseLargePages) {
    align = MAX2(align, os::large_page_size());
  }
  return align;
}

I propose a small fix to adjust align to be a power-of-2. I've also added an earlier assert about this in set_conservative_max_heap_alignment and added an additional test-case in TestRegionSizeArgs.java

WDYT, is this an OK fix for this corner-case?

---

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Issue

• JDK-8372513: Shenandoah: ShenandoahMaxRegionSize can produce an unaligned heap alignment (Bug - P4)

Reviewers

• Joel Sikström (@jsikstro - Reviewer)
• Erik Österlund (@fisk - Reviewer)
• Aleksey Shipilev (@shipilev - Reviewer)

Reviewing

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8372513: Shenandoah: ShenandoahMaxRegionSize can produce an unaligned heap alignment

Reviewed-by: jsikstro, eosterlund, shade

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At the time when this comment was updated there had been 4 new commits pushed to the master branch:

• b36b694: 8371259: ML-DSA AVX2 and AVX512 intrinsics and improvements
• c0abecd: 8372441: JFR: Improve logging of TestBackToBackSensitive
• c123006: 8363943: ARM32: Represent Registers as values
• ... and 1 more: https://git.openjdk.org/jdk/compare/d94c52ccf2fed3fc66d25a34254c9b581c175fa1...master

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[jsikstro]

I think this looks good.

[stefank]

I realized that having an unconstrained flag like this can cause overflows and asserts because of that. If we want to fix that we could do something like the following:

stefank/jdk@8372513_shenandoah_max_region_size...stefank:jdk:8372513_shenandoah_max_region_size_constraints

Then running

 java -XX:+UseShenandoahGC -XX:+UnlockExperimentalVMOptions -XX:ShenandoahMaxRegionSize=9223372036854775808 -version

Will give this error message instead of an assert:

$ java -XX:+UseShenandoahGC -XX:+UnlockExperimentalVMOptions -XX:ShenandoahMaxRegionSize=9223372036854775808 -version
ShenandoahMaxRegionSize 8589934592G should be lower than (8589934592G).
Improperly specified VM option 'ShenandoahMaxRegionSize=9223372036854775808'
Error: Could not create the Java Virtual Machine.
Error: A fatal exception has occurred. Program will exit.

[shipilev]

Yes, this is the correct fix. Thanks for handling this!

[stefank]

Thanks, Aleksey!

What do you think about the issue with an extreme user-specified value as described in:
#28492 (comment)

Do you want that change or not?

[shipilev]

What do you think about the issue with an extreme user-specified value as described in: #28492 (comment)
Do you want that change or not?

Feel free to submit a bug and let Shenandoah folks handle it.

This PR stands on its own, and I think it unblocks some of the pending work, so ship it.

[stefank]

Sounds good. Thanks again!

[stefank]

/integrate

[openjdk]

Going to push as commit 5291e1c.
Since your change was applied there have been 15 commits pushed to the master branch:

• 5fe731d: 8349835: C2: Simplify IGV property printing
• 275cb9f: 8360510: C2: Template Assertion Predicates are not cloned to the inner counted loop with -XX:+StressDuplicateBackedge
• 46ee8d5: 8372547: New test sun/security/provider/acvp/ML_DSA_Intrinsic_Test.java should be restricted to x86_64
• ... and 12 more: https://git.openjdk.org/jdk/compare/d94c52ccf2fed3fc66d25a34254c9b581c175fa1...master

Your commit was automatically rebased without conflicts.

[openjdk]

@stefank Pushed as commit 5291e1c.

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