refactor: Change GuestMemoryExtension::with_file into memfd_backed

The function was only ever called with a `File` object created from a
`memfd`. Thus instead of the call-site have to do two function calls
(one for the memfd and then another for creating the guest memory mmap,
which because of this had to do a `fstat` call to determine how big the
memfd is), have the `GuestMemoryMmap` factory directly create the
`memfd`.

This is mainly preparatory to prevent callers to pass mismatched
configurations to `memfd_create` and the subsequent `from_file` call
(e.g. different huge page configs).

Signed-off-by: Patrick Roy <roypat@amazon.co.uk>

Author: roypat

src/vmm/src/builder.rs

@@ -57,9 +57,7 @@ use crate::vmm_config::boot_source::BootConfig;
 use crate::vmm_config::drive::BlockDeviceType;
 use crate::vmm_config::instance_info::InstanceInfo;
 use crate::vmm_config::machine_config::{MachineConfigUpdate, VmConfig, VmConfigError};
-use crate::vstate::memory::{
-    create_memfd, GuestAddress, GuestMemory, GuestMemoryExtension, GuestMemoryMmap,
-};
+use crate::vstate::memory::{GuestAddress, GuestMemory, GuestMemoryExtension, GuestMemoryMmap};
 use crate::vstate::vcpu::{Vcpu, VcpuConfig, VcpuError};
 use crate::vstate::vm::Vm;
 use crate::{device_manager, EventManager, Vmm, VmmError};
@@ -239,10 +237,9 @@ pub fn build_microvm_for_boot(
         .ok_or(MissingKernelConfig)?;
 
     let track_dirty_pages = vm_resources.track_dirty_pages();
-    let memfd = create_memfd(vm_resources.vm_config.mem_size_mib)
-        .map_err(StartMicrovmError::GuestMemory)?;
-    let guest_memory = GuestMemoryMmap::with_file(memfd.as_file(), track_dirty_pages)
-        .map_err(StartMicrovmError::GuestMemory)?;
+    let guest_memory =
+        GuestMemoryMmap::memfd_backed(vm_resources.vm_config.mem_size_mib, track_dirty_pages)
+            .map_err(StartMicrovmError::GuestMemory)?;
     let entry_addr = load_kernel(boot_config, &guest_memory)?;
     let initrd = load_initrd_from_config(boot_config, &guest_memory)?;
     // Clone the command-line so that a failed boot doesn't pollute the original.

src/vmm/src/vstate/memory.rs

@@ -56,8 +56,8 @@ pub trait GuestMemoryExtension
 where
     Self: Sized,
 {
-    /// Creates a GuestMemoryMmap with `size` in MiB and guard pages backed by file.
-    fn with_file(file: &File, track_dirty_pages: bool) -> Result<Self, MemoryError>;
+    /// Creates a GuestMemoryMmap with `size` in MiB backed by a memfd.
+    fn memfd_backed(mem_size_mib: usize, track_dirty_pages: bool) -> Result<Self, MemoryError>;
 
     /// Creates a GuestMemoryMmap with `size` in MiB and guard pages.
     fn with_size(size: usize, track_dirty_pages: bool) -> Result<Self, MemoryError>;
@@ -121,16 +121,15 @@ pub struct GuestMemoryState {
 }
 
 impl GuestMemoryExtension for GuestMemoryMmap {
-    /// Creates a GuestMemoryMmap with `size` in MiB and guard pages backed by file.
-    fn with_file(file: &File, track_dirty_pages: bool) -> Result<Self, MemoryError> {
-        let metadata = file.metadata().map_err(MemoryError::FileError)?;
-        let mem_size = u64_to_usize(metadata.len());
+    /// Creates a GuestMemoryMmap with `size` in MiB backed by a memfd.
+    fn memfd_backed(mem_size_mib: usize, track_dirty_pages: bool) -> Result<Self, MemoryError> {
+        let memfd_file = create_memfd(mem_size_mib)?.into_file();
 
         let mut offset: u64 = 0;
-        let regions = crate::arch::arch_memory_regions(mem_size)
+        let regions = crate::arch::arch_memory_regions(mem_size_mib << 20)
             .iter()
             .map(|(guest_address, region_size)| {
-                let file_clone = file.try_clone().map_err(MemoryError::FileError)?;
+                let file_clone = memfd_file.try_clone().map_err(MemoryError::FileError)?;
                 let file_offset = FileOffset::new(file_clone, offset);
                 offset += *region_size as u64;
                 Ok((file_offset, *guest_address, *region_size))
@@ -337,7 +336,7 @@ impl GuestMemoryExtension for GuestMemoryMmap {
 }
 
 /// Creates a memfd file with the `size` in MiB.
-pub fn create_memfd(size: usize) -> Result<memfd::Memfd, MemoryError> {
+fn create_memfd(size: usize) -> Result<memfd::Memfd, MemoryError> {
     let mem_size = size << 20;
     // Create a memfd.
     let opts = memfd::MemfdOptions::default().allow_sealing(true);