Comment by chrisrohlf

Correct, this technique works on a page by page basis. It's the only way to do it with userfaultfd as the technique relies entirely on the first page fault being a read and not a write. Im working on a feature that returns an allocation that straddles 2 pages to catch reads of structure paddings but it will come at the cost of an additional 4k of memory and/or possibly returning unaligned memory.