no syscall will be cheap to call in a fast path. You would need an hardware instruction that tells you if a load or store would fault.
Sometimes I feel modern technology is basically a delicately balanced house of cards that falls over when breathed upon or looked at incorrectly.
You have MADV_FREE pages/ranges. They get cleared when purged, so reading zeros tells you that the load would have faulted and needs to be populated from storage.
Memory pressure indicators exist, https://docs.kernel.org/accounting/psi.html
> have some way to have a shared data structure where you are told if it’s still resident and can lock it from being paged out.
What's more efficient than fetching data and comparing it with zero? Any write within the range will then cancel the MADV_FREE property on the written-to page thus "locking" it again, and this is also very efficient.
Hardware reference counting of memory allocations would be very interesting. It would be shockingly simple to implement compared to many other features hardware already has to tackle.
It's quite expensive to free pages under memory pressure (though it's not clear that there's any other choice to be made), but if the pages are never freed it should be cheap, AIUI.