feat(virtio-pmem): add a Drop implementation

Event though during Firecracker normal run pmem devices
are never removed, so there is no need to unmap memory,
it is important for fuzzer runs, as otherwise it runs out
of virtual memory.
Additionally make `mmap_backing_file` private since it is
only called from within Pmem device.

Signed-off-by: Egor Lazarchuk <[email protected]>

Author: ShadowCurse

src/vmm/src/devices/virtio/pmem/device.rs

@@ -95,6 +95,17 @@ pub struct Pmem {
     pub config: PmemConfig,
 }
 
+impl Drop for Pmem {
+    fn drop(&mut self) {
+        let mmap_len = align_up(self.file_len, Self::ALIGNMENT);
+        // SAFETY: `mmap_ptr` is a valid pointer since Pmem can only be created with `new*` methods.
+        //         Mapping size calculation is same for original mmap call.
+        unsafe {
+            _ = libc::munmap(self.mmap_ptr as *mut libc::c_void, u64_to_usize(mmap_len));
+        }
+    }
+}
+
 impl Pmem {
     // Pmem devices need to have address and size to be
     // a multiple of 2MB
@@ -130,10 +141,7 @@ impl Pmem {
         })
     }
 
-    pub fn mmap_backing_file(
-        path: &str,
-        read_only: bool,
-    ) -> Result<(File, u64, u64, u64), PmemError> {
+    fn mmap_backing_file(path: &str, read_only: bool) -> Result<(File, u64, u64, u64), PmemError> {
         let file = OpenOptions::new()
             .read(true)
             .write(!read_only)