That doesn't speed up uerspace<>kernel memcopy, it just reduces cache churn. Despite its name it still goes through the page cache, it just triggers writeback and drops the pages once that's done. For example when copying to a tmpfs it makes zero difference since that lives entirely in memory.

So you're less dependent on the page replacement algorithm being scan-resistant, since you can use this flag for scan/loop workloads, right?

I would initially add it for WAL writes and reads. There should never be another read in normal operation.