Update memfd support with a runtime toggle (#3778)

This commit updates the `memfd` support in Wasmtime to have a runtime
toggle as to whether it's used or not. The compile-time feature gating
`memfd` support is now also re-enabled by default, but the new runtime
switch is still disabled-by-default.

Additionally this commit updates our fuzz oracle to turn on/off the
memfd flag to re-enable fuzzing with memfd on oss-fuzz.

crates/fuzzing/src/generators.rs

@@ -61,6 +61,7 @@ pub struct WasmtimeConfig {
     pub(crate) consume_fuel: bool,
     memory_config: MemoryConfig,
     force_jump_veneers: bool,
+    memfd: bool,
 }
 
 #[derive(Arbitrary, Clone, Debug, Eq, Hash, PartialEq)]
@@ -99,7 +100,8 @@ impl Config {
             .cranelift_nan_canonicalization(self.wasmtime.canonicalize_nans)
             .cranelift_opt_level(self.wasmtime.opt_level.to_wasmtime())
             .interruptable(self.wasmtime.interruptable)
-            .consume_fuel(self.wasmtime.consume_fuel);
+            .consume_fuel(self.wasmtime.consume_fuel)
+            .memfd(self.wasmtime.memfd);
 
         // If the wasm-smith-generated module use nan canonicalization then we
         // don't need to enable it, but if it doesn't enable it already then we

crates/wasmtime/Cargo.toml

@@ -51,7 +51,16 @@ wasi-cap-std-sync = { path = "../wasi-common/cap-std-sync" }
 maintenance = { status = "actively-developed" }
 
 [features]
-default = ['async', 'cache', 'wat', 'jitdump', 'parallel-compilation', 'cranelift', 'pooling-allocator']
+default = [
+  'async',
+  'cache',
+  'wat',
+  'jitdump',
+  'parallel-compilation',
+  'cranelift',
+  'pooling-allocator',
+  'memfd',
+]
 
 # An on-by-default feature enabling runtime compilation of WebAssembly modules
 # with the Cranelift compiler. Cranelift is the default compilation backend of

crates/wasmtime/src/config.rs

@@ -104,6 +104,7 @@ pub struct Config {
     pub(crate) module_version: ModuleVersionStrategy,
     pub(crate) parallel_compilation: bool,
     pub(crate) paged_memory_initialization: bool,
+    pub(crate) memfd: bool,
 }
 
 impl Config {
@@ -129,6 +130,7 @@ impl Config {
             parallel_compilation: true,
             // Default to paged memory initialization when using uffd on linux
             paged_memory_initialization: cfg!(all(target_os = "linux", feature = "uffd")),
+            memfd: false,
         };
         #[cfg(compiler)]
         {
@@ -1170,6 +1172,33 @@ impl Config {
         self
     }
 
+    /// Configures whether `memfd`, if supported, will be used to initialize
+    /// applicable module memories.
+    ///
+    /// This is a Linux-specific feature since `memfd` is only supported on
+    /// Linux. Support for this is also enabled by default at compile time but
+    /// is otherwise disabled at runtime by default. This feature needs to be
+    /// enabled to `true` for support to be used.
+    ///
+    /// Also note that even if this feature is enabled it may not be applicable
+    /// to all memories in all wasm modules. At this time memories must meet
+    /// specific criteria to be memfd-initialized:
+    ///
+    /// * Only memories defined in the module can be initialized this way.
+    /// * Data segments for memory must use statically known offsets.
+    /// * Data segments for memory must all be in-bounds.
+    ///
+    /// If all of the above applies, this setting is enabled, and the current
+    /// platform is Linux the `memfd` will be used to efficiently initialize
+    /// linear memories with `mmap` to avoid copying data from initializers into
+    /// linear memory.
+    #[cfg(feature = "memfd")]
+    #[cfg_attr(nightlydoc, doc(cfg(feature = "memfd")))]
+    pub fn memfd(&mut self, memfd: bool) -> &mut Self {
+        self.memfd = memfd;
+        self
+    }
+
     pub(crate) fn build_allocator(&self) -> Result<Box<dyn InstanceAllocator>> {
         #[cfg(feature = "async")]
         let stack_size = self.async_stack_size;
@@ -1239,6 +1268,7 @@ impl Clone for Config {
             module_version: self.module_version.clone(),
             parallel_compilation: self.parallel_compilation,
             paged_memory_initialization: self.paged_memory_initialization,
+            memfd: self.memfd,
         }
     }
 }

crates/wasmtime/src/lib.rs

@@ -275,6 +275,21 @@
 //!   all architectures for both the JIT compiler and the `wasmtime compile` CLI
 //!   command.
 //!
+//! * `pooling-allocator` - Enabled by default, this feature adds support for
+//!   the pooling allocation strategy enabled via
+//!   [`Config::allocation_strategy`]. The pooling allocator can enable more
+//!   efficient reuse of resources for high-concurrency and
+//!   high-instantiation-count scenarios.
+//!
+//! * `memfd` - Enabled by default, this feature builds in support for a
+//!   Linux-specific feature of creating a `memfd` where applicable for a
+//!   [`Module`]'s initial memory. This makes instantiation much faster by
+//!   `mmap`-ing the initial memory image into place instead of copying memory
+//!   into place, allowing sharing pages that end up only getting read and
+//!   otherwise using copy-on-write for efficient initialization of memory. Note
+//!   that this is simply compile-time support and this must also be enabled at
+//!   run-time via [`Config::memfd`].
+//!
 //! ## Examples
 //!
 //! In addition to the examples below be sure to check out the [online embedding

crates/wasmtime/src/module.rs

@@ -724,6 +724,9 @@ impl Module {
     }
 
     pub(crate) fn memfds(&self) -> Result<Option<&Arc<ModuleMemFds>>> {
+        if !self.engine().config().memfd {
+            return Ok(None);
+        }
         Ok(self
             .inner
             .memfds