I've created a pull request in LuaJIT upstream for fixing lightud for 48 bit VA: 

  https://www.freelists.org/post/luajit/fix-lightud-type-for-48bit-virtual-address

Please help take a look if you have time, thanks!

Because this issue introduces changes in generic code base and may also affect other platform with 48 bit virtual address configuration, I feel discuss it in upstream may be more appropriate.
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