Hardware memcpy instruction, except it just traps into the software memcpy implementation
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you can do this today if you define an interrupt vector specifically for memcpy
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Linux kernel moves pages for mremap so it ends up being ridiculously fast especially when transparent huge pages ended up being used. There's MREMAP_DONTUNMAP now to make the original mapping go back to the state of a fresh mapping where zeroed pages will be faulted in on access.
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It was designed to be used in combination with userfaultfd to catch the faults to the source mapping once the old pages are gone. It's an atomic operating and can be used in a compacting GC instead of needing to memory protect the source mapping and copy over the pages manually.