In V8, JITed JavaScript code is never writable and executable at the same time; the JIT writes into a buffer, then quiesces JS execution, then copies/flips perm

In V8, JITed JavaScript code is never writable and executable at the same time; the JIT writes into a buffer, then quiesces JS execution, then copies/flips permissions, then starts running JS code again. In the Wasm engine inside of V8, the code is writable and executable at the same time because the JIT uses multiple concurrent threads in the background during execution and incrementally commits new JITed code as it is finished. (And, funnily enough, this performance optimization is mostly for asm.js code, which is verified and internally translated to Wasm, to be compiled and executed by the Wasm engine). The long-term holy grail is to move the JIT compiler entirely to another process, so only that process has write permissions, and the renderer process (where Wasm and JS execute) has only R and execute permissions, and they used shared memory underneath.