Add paged_memory_initialization to Config.

This commit adds a `paged_memory_initialization` setting to `Config`.

The setting controls whether or not an attempt is made to organize data
segments into Wasm pages during compilation.

When used in conjunction with the `uffd` feature on Linux, Wasmtime can
completely skip initializing linear memories and instead initialize any pages
that are accessed for the first time during Wasm execution.

crates/wasmtime/src/config.rs

@@ -352,6 +352,7 @@ pub struct Config {
     pub(crate) async_support: bool,
     pub(crate) deserialize_check_wasmtime_version: bool,
     pub(crate) parallel_compilation: bool,
+    pub(crate) paged_memory_initialization: bool,
 }
 
 impl Config {
@@ -375,6 +376,8 @@ impl Config {
             async_support: false,
             deserialize_check_wasmtime_version: true,
             parallel_compilation: true,
+            // Default to paged memory initialization when using uffd on linux
+            paged_memory_initialization: cfg!(all(target_os = "linux", feature = "uffd")),
         };
         #[cfg(compiler)]
         {
@@ -984,6 +987,27 @@ impl Config {
         self
     }
 
+    /// Sets whether or not an attempt is made to initialize linear memories by page.
+    ///
+    /// This setting is `false` by default and Wasmtime initializes linear memories
+    /// by copying individual data segments from the compiled module.
+    ///
+    /// Setting this to `true` will cause compilation to attempt to organize the
+    /// data segments into WebAssembly pages and linear memories are initialized by
+    /// copying each page rather than individual data segments.
+    ///
+    /// Modules that import a memory or have data segments that use a global base
+    /// will continue to be initialized by copying each data segment individually.
+    ///
+    /// When combined with the `uffd` feature on Linux, this will allow Wasmtime
+    /// to delay initialization of a linear memory page until it is accessed
+    /// for the first time during WebAssembly execution; this may improve
+    /// instantiation performance as a result.
+    pub fn paged_memory_initialization(&mut self, value: bool) -> &mut Self {
+        self.paged_memory_initialization = value;
+        self
+    }
+
     /// Configures the maximum size, in bytes, where a linear memory is
     /// considered static, above which it'll be considered dynamic.
     ///
@@ -1329,6 +1353,7 @@ impl Clone for Config {
             async_stack_size: self.async_stack_size,
             deserialize_check_wasmtime_version: self.deserialize_check_wasmtime_version,
             parallel_compilation: self.parallel_compilation,
+            paged_memory_initialization: self.paged_memory_initialization,
         }
     }
 }

crates/wasmtime/src/module.rs

@@ -380,7 +380,7 @@ impl Module {
 
             // If configured, attempt to use paged memory initialization
             // instead of the default mode of memory initialization
-            if cfg!(all(feature = "uffd", target_os = "linux")) {
+            if engine.config().paged_memory_initialization {
                 translation.try_paged_init();
             }
 

tests/all/instance.rs

@@ -32,6 +32,29 @@ fn initializes_linear_memory() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn initializes_linear_memory_paged() -> Result<()> {
+    let wat = r#"
+        (module
+            (memory (export "memory") 2)
+            (data (i32.const 0) "Hello World!")
+        )"#;
+
+    let mut config = Config::new();
+    config.paged_memory_initialization(true);
+
+    let module = Module::new(&Engine::new(&config)?, wat)?;
+
+    let mut store = Store::new(module.engine(), ());
+    let instance = Instance::new(&mut store, &module, &[])?;
+    let memory = instance.get_memory(&mut store, "memory").unwrap();
+
+    let mut bytes = [0; 12];
+    memory.read(&store, 0, &mut bytes)?;
+    assert_eq!(bytes, "Hello World!".as_bytes());
+    Ok(())
+}
+
 #[test]
 fn linear_memory_limits() -> Result<()> {
     // this test will allocate 4GB of virtual memory space, and may not work in