On 4/16/19 12:47 AM, Michal Hocko wrote: > You definitely have to follow policy. You cannot demote to a node which > is outside of the cpuset/mempolicy because you are breaking contract > expected by the userspace. That implies doing a rmap walk. What *is* the contract with userspace, anyway? :) Obviously, the preferred policy doesn't have any strict contract. The strict binding has a bit more of a contract, but it doesn't prevent swapping. Strict binding also doesn't keep another app from moving the memory. We have a reasonable argument that demotion is better than swapping. So, we could say that even if a VMA has a strict NUMA policy, demoting pages mapped there pages still beats swapping them or tossing the page cache. It's doing them a favor to demote them. Or, maybe we just need a swap hybrid where demotion moves the page but keeps it unmapped and in the swap cache. That way an access gets a fault and we can promote the page back to where it should be. That would be faster than I/O-based swap for sure. Anyway, I agree that the kernel probably shouldn't be moving pages around willy-nilly with no consideration for memory policies, but users might give us some wiggle room too.
