[junon]

Thanks for the information!

> Passing buffers by virtual memory remapping is expensive, so we use memcpy everywhere.

Curious if you could expand on this a bit; memcpy still requires that two buffers are mapped in anyway. Do you mean that avoiding maps is more important than avoiding copies? Or is there something inherent about multiple linear addresses -> same physical address that is somehow slower on modern processors?

[tliltocatl]

Assume an (untrusted) application A wants to send a stream of somewhat long (several tens of KB/multiple pages each) messages to application B. A and B could establish a shared memory region for this, but that would possibly allow A to trigger a TOCTOU vulnerability in B by modifying the buffer after B started reading the message. If page capability reclamation would have been cheap, the OS could unmap the shared buffer from A before notifying B of incoming message. But nowadays unmapping requires synchronizing with all CPUs that might have TLBs with A's mapping, so memcpy is cheaper.