From a user perspective, it doesn't depend on swap. It's just slower
without swap because it does what MADV_DONTNEED does. The current
implementation can be dropped in where MADV_DONTNEED was previously used.

It just wouldn't replace existing layers of purging logic until that
edge case is fixed and it gains better THP integration.

It's already a very useful API with significant performance wins over
MADV_DONTNEED, so it will be useful. The only risk involved in landing
it is that a better feature might come along. Worst case scenario being
that the kernel ends up with a synonym for MADV_DONTNEED (but I think
there will still be a use case for this even if a pinning/unpinning API
existed, as this is more precise).