Testing so far with recent Wasmtime has not been able to show the need
for avoiding the process-wide mmap lock in real-world use-cases. As
such, the technique of using an anonymous file and ftruncate() to extend
it seems unnecessary; instead, memfd can always use anonymous zeroed
memory for heap backing where the CoW image is not present, and
mprotect() to extend the heap limit by changing page protections.
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