# Why mmap Page Residency Differs from Virtual Address Space Size

The virtual address space size of an mmap is just a *reservation* — it says nothing about how much physical memory is actually backing it. Several mechanisms cause the gap:

## 1. Lazy allocation (demand paging)
`mmap` doesn't allocate physical pages at call time. It only creates virtual mappings. Physical frames are assigned when you first *touch* each page, triggering a page fault. A 1 GB anonymous mapping that's never written may use ~0 bytes of RAM.

## 2. Copy-on-write (COW)
A `MAP