RFR: 8343789: Move mutable nmethod data out of CodeCache [v9]

[Boris Ulasevich]

On Mon, 3 Feb 2025 12:43:35 GMT, Stefan Karlsson <stefank at openjdk.org> wrote:

>> Boris Ulasevich has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   Force the use of movk in combination with adrp and ldr instructions to address scenarios
>>   where os::malloc allocates buffers beyond the typical ±4GB range accessible with adrp
>
> src/hotspot/share/code/nmethod.cpp line 2162:
> 
>> 2160:     return nullptr;
>> 2161:   }
>> 2162:   return RawAccess<>::oop_load(oop_addr_at(index));
> 
> This change is removing the GC barriers and is likely the cause of the ZGC crash that Tobias listed.
> 
> However, the fix is not as simple as to just reinstate the NMethodAccess call. The ZGC code uses the `oop*` to find the associated `nmethod` in the code cache. We need another way to fetch the nmethod now. So, I'm experimenting with a small change to switch out the Access API call to a direct GC barrier set call and then I pass down the `this` pointer from this function. With that you should be able skip this change.
> 
> With that said, what was the motivation for changing this?

Right, I remember I encountered a "did not find nmethod" assertion in ZNMethod::load_oop. With my change the method fails to find nmethod by oop* which is supposed to be within the boundaries of nmethod. Based on naming semantics, I thought that if the oop wasn't in the nmethod, switching from NMethodAccess to RawAccess was acceptable. In reality, this bypasses the essential GC barrier.

Given that the nmethod is known in a call stack above, I think we should just pass it down instead of attempting a lookup.


call stack:
- ZNMethod::load_oop(oop*, unsigned long)
- AccessInternal::PostRuntimeDispatch<ZBarrierSet::AccessBarrier<., .>, ., .>::oop_access_barrier(void*)  
- AccessInternal::RuntimeDispatch<1122372ul, oop, (AccessInternal::BarrierType)2>::load_init(void*)  
- nmethod::oop_at_phantom(int) 

oop ZNMethod::load_oop(oop* p, DecoratorSet decorators) {
  nmethod* const nm = CodeCache::find_nmethod((void*)p);
  assert(nm != nullptr, "did not find nmethod");

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PR Review Comment: https://git.openjdk.org/jdk/pull/21276#discussion_r1944617160