From d1ce52f3a8cffe8c474a08ac552b3edc291ff805 Mon Sep 17 00:00:00 2001
From: Sebastien Boeuf <sebastien.boeuf@intel.com>
Date: Fri, 21 Aug 2020 13:13:24 +0200
Subject: [PATCH] vmm: memory_manager: Make backing file from snapshot optional

Let's not assume that a backing file is going to be the result from
a snapshot for each memory region. These regions might be backed by
a file on the host filesystem (not a temporary file in host RAM), which
means they don't need to be copied and stored into dedicated files.

That's why this commit prepares for further changes by introducing an
optional PathBuf associated with the snapshot of each memory region.

Signed-off-by: Sebastien Boeuf <sebastien.boeuf@intel.com>
---
 vmm/src/memory_manager.rs | 44 ++++++++++++++++++++++-----------------
 1 file changed, 25 insertions(+), 19 deletions(-)

diff --git a/vmm/src/memory_manager.rs b/vmm/src/memory_manager.rs
index 745422cb7..57c7f1d30 100644
--- a/vmm/src/memory_manager.rs
+++ b/vmm/src/memory_manager.rs
@@ -380,7 +380,7 @@ impl MemoryManager {
 
                 for region in ext_regions.iter() {
                     mem_regions.push(MemoryManager::create_ram_region(
-                        &Some(region.backing_file.clone()),
+                        &region.backing_file,
                         0,
                         region.start_addr,
                         region.size as usize,
@@ -600,9 +600,11 @@ impl MemoryManager {
 
             let mut ext_regions = mem_snapshot.memory_regions;
             for region in ext_regions.iter_mut() {
-                let mut memory_region_path = vm_snapshot_path.clone();
-                memory_region_path.push(region.backing_file.clone());
-                region.backing_file = memory_region_path;
+                if let Some(backing_file) = &mut region.backing_file {
+                    let mut memory_region_path = vm_snapshot_path.clone();
+                    memory_region_path.push(backing_file.clone());
+                    *backing_file = memory_region_path;
+                }
             }
 
             // In case there was no backing file, we can safely use CoW by
@@ -621,20 +623,24 @@ impl MemoryManager {
             // new regions so that we can still use MAP_SHARED when restoring
             // the VM.
             guest_memory.memory().with_regions(|index, region| {
-                // Open (read only) the snapshot file for the given region.
-                let mut memory_region_file = OpenOptions::new()
-                    .read(true)
-                    .open(&ext_regions[index].backing_file)
-                    .map_err(|e| Error::Restore(MigratableError::MigrateReceive(e.into())))?;
+                if let Some(backing_file) = &ext_regions[index].backing_file {
+                    // Open (read only) the snapshot file for the given region.
+                    let mut memory_region_file = OpenOptions::new()
+                        .read(true)
+                        .open(backing_file)
+                        .map_err(|e| {
+                        Error::Restore(MigratableError::MigrateReceive(e.into()))
+                    })?;
 
-                // Fill the region with the file content.
-                region
-                    .read_from(
-                        MemoryRegionAddress(0),
-                        &mut memory_region_file,
-                        region.len().try_into().unwrap(),
-                    )
-                    .map_err(|e| Error::Restore(MigratableError::MigrateReceive(e.into())))?;
+                    // Fill the region with the file content.
+                    region
+                        .read_from(
+                            MemoryRegionAddress(0),
+                            &mut memory_region_file,
+                            region.len().try_into().unwrap(),
+                        )
+                        .map_err(|e| Error::Restore(MigratableError::MigrateReceive(e.into())))?;
+                }
 
                 Ok(())
             })?;
@@ -1510,7 +1516,7 @@ pub struct GuestAddressDef(pub u64);
 
 #[derive(Serialize, Deserialize)]
 pub struct MemoryRegion {
-    backing_file: PathBuf,
+    backing_file: Option<PathBuf>,
     #[serde(with = "GuestAddressDef")]
     start_addr: GuestAddress,
     size: GuestUsize,
@@ -1545,7 +1551,7 @@ impl Snapshottable for MemoryManager {
             }
 
             memory_regions.push(MemoryRegion {
-                backing_file: PathBuf::from(format!("memory-region-{}", index)),
+                backing_file: Some(PathBuf::from(format!("memory-region-{}", index))),
                 start_addr: region.start_addr(),
                 size: region.len(),
             });