Clear affine slots when dropping a `Module`

[alexcrichton]

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This commit implements a resource usage optimization for Wasmtime with the pooling instance allocator by ensuring that when a Module is dropped its backing virtual memory mappings are all removed. Currently when a Module is dropped it releases a strong reference to its internal memory image but the memory image may stick around in individual pooling instance allocator slots. When using the Random allocation strategy, for example, this means that the memory images could stick around for a long time.

While not a pressing issue this has resource usage implications for Wasmtime. Namely removing a Module does not guarantee the memfd, if in use for a memory image, is closed and deallocated within the kernel. Unfortunately simply closing the memfd is not sufficient as well as the mappings into the address space additionally all need to be removed for the kernel to release the resources for the memfd. This means that to release all kernel-level resources for a Module all slots which have the memory image mapped in must have the slot reset.

This problem isn't particularly present when using the NextAvailable allocation strategy since the number of lingering memfds is proportional to the maximum concurrent size of wasm instances. With the Random and ReuseAffinity strategies, however, it's much more prominent because the number of lingering memfds can reach the total number of slots available. This can appear as a leak of kernel-level memory which can cause other system instability.

To fix this issue this commit adds necessary instrumentation to Drop for Module to purge all references to the module in the pooling instance allocator. All index allocation strategies now maintain affinity tracking to ensure that regardless of the strategy in use a module that is dropped will remove all its memory mappings. A new allocation method was added to the index allocator for allocating an index without setting affinity and only allocating affine slots. This is used to iterate over all the affine slots without holding the global index lock for an unnecessarily long time while mappings are removed.

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[alexcrichton]

Thanks for the review! I also fixed an issue where alloc_affine_and_clear_affinity wasn't working for anything other than ReuseAffinity which was also one of the motivations for the refactoring here.

[cfallin]

Thanks for the review! I also fixed an issue where alloc_affine_and_clear_affinity wasn't working for anything other than ReuseAffinity which was also one of the motivations for the refactoring here.

Ah, good catch; updated diff LGTM.