Function punch_hole

vmm/src/memory_manager.rs:3351–3368

Punch an explicit hole into `f`. Tests use this instead of relying on "didn't write here" to mean "is a hole": modern shmem/tmpfs may allocate a multi-page folio on the first write and report the whole folio as data, so per-page hole tracking after a partial write is not portable. `fallocate(PUNCH_HOLE)` is the explicit "deallocate these pages" syscall and is honored on every Linux filesystem we r

(f: &std::fs::File, off: u64, len: u64)

    /// these pages" syscall and is honored on every Linux filesystem we
    /// run tests on (tmpfs, ext4, xfs, btrfs).
    fn punch_hole(f: &std::fs::File, off: u64, len: u64) {
        // SAFETY: FFI call; f is a valid open fd for the duration of the
        // call.
        let r = unsafe {
            libc::fallocate(
                f.as_raw_fd(),
                libc::FALLOC_FL_PUNCH_HOLE | libc::FALLOC_FL_KEEP_SIZE,
                off as libc::off_t,
                len as libc::off_t,
            )
        };
        assert_eq!(
            r,
            0,
            "fallocate PUNCH_HOLE off={off} len={len}: {}",
            std::io::Error::last_os_error(),
        );
    }

    /// Build a file with a deterministic sparse layout: write each `(off,
    /// len, byte)` data extent, then punch every gap into a real hole.