Function sparse_layout

vmm/src/memory_manager.rs:3374–3392

Build a file with a deterministic sparse layout: write each `(off, len, byte)` data extent, then punch every gap into a real hole. The resulting `SEEK_DATA`/`SEEK_HOLE` extents match `data` exactly, regardless of folio/THP allocation policy on the backing FS.

(f: &std::fs::File, total: u64, data: &[(u64, u64, u8)])

    /// The resulting `SEEK_DATA`/`SEEK_HOLE` extents match `data` exactly,
    /// regardless of folio/THP allocation policy on the backing FS.
    fn sparse_layout(f: &std::fs::File, total: u64, data: &[(u64, u64, u8)]) {
        f.set_len(total).unwrap();
        for &(off, len, byte) in data {
            f.write_all_at(&vec![byte; len as usize], off).unwrap();
        }
        let mut sorted: Vec<(u64, u64)> = data.iter().map(|&(o, l, _)| (o, l)).collect();
        sorted.sort_unstable();
        let mut cursor = 0u64;
        for (off, len) in sorted {
            assert!(off >= cursor, "overlapping data extents");
            if off > cursor {
                punch_hole(f, cursor, off - cursor);
            }
            cursor = off + len;
        }
        if cursor < total {
            punch_hole(f, cursor, total - cursor);
        }
    }

    #[test]
    fn empty_memfd_has_no_data_extents() {