RFR: 8301988: VerifyLiveClosure::verify_liveness asserts on bad pointers outside heap [v2]

[David Holmes]

On Wed, 8 Feb 2023 10:24:00 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

>> src/hotspot/share/gc/g1/g1CollectedHeap.inline.hpp line 217:
>> 
>>> 215: 
>>> 216: inline bool G1CollectedHeap::is_obj_filler(const oop obj) {
>>> 217:   Klass* k = obj->klass_raw();
>> 
>> Not clear how you can get here from ` HeapRegion::is_obj_dead` with a bad oop, such that you need the raw variant. ??
>
> The object is in the heap, but the occupying memory has already been zapped (in debug mode); i.e. the call in `heapRegion.cpp:518` could read `badHeapWordVal` as (compressed) klass value in the header.
> 
> In that case the current code asserts in this call because in `oopDesc::klass()`, the call to `CompressedKlassPointers::decode_not_null` will assert in `compressedOops.inline.hpp:135` due to the `check_alignment` condition not satisfied.
> 
> This makes this verification code assert before printing out any useful information to diagnose the problem quickly (in my case this has been a change that wrongly managed remembered sets).
> 
> If I had had the remembered set verification printout, I would have found the problem immediately in this case (because the message would have told me that there is a problem with remembered sets). So it took a while to diagnose the issue, having to go into the debugger to painfully find the exact same information.
> 
> I.e. this makes the verification code more robust.
> 
> Imo the suggested solution to just continue execution is fine, because `is_obj_filler` will always return false (i.e. object is dead) for garbage objects and do the right thing here.
> There is the concern that now other non-verification code might not immediately trigger now, but most of it just fails the VM anyway if it finds a bad reference (after printing some information about it), for all other cases this is the right choice.

Thanks for the explanation.

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PR: https://git.openjdk.org/jdk/pull/12456