Make build-config magic use memfd by default.

crates/runtime/Cargo.toml

@@ -25,6 +25,7 @@ backtrace = "0.3.61"
 lazy_static = "1.3.0"
 rand = "0.8.3"
 anyhow = "1.0.38"
+memfd = { version = "0.4.1", optional = true }
 
 [target.'cfg(target_os = "macos")'.dependencies]
 mach = "0.3.2"
@@ -37,7 +38,6 @@ winapi = { version = "0.3.7", features = ["winbase", "memoryapi", "errhandlingap
 
 [target.'cfg(target_os = "linux")'.dependencies]
 userfaultfd = { version = "0.4.1", optional = true }
-memfd = { version = "0.4.1", optional = true }
 
 [build-dependencies]
 cc = "1.0"
@@ -60,5 +60,3 @@ uffd = ["userfaultfd", "pooling-allocator"]
 # It is useful for applications that do not bind their own exception ports and
 # need portable signal handling.
 posix-signals-on-macos = []
-
-memfd-allocator = ["pooling-allocator", "memfd"]

crates/runtime/build.rs

@@ -10,4 +10,15 @@ fn main() {
         )
         .file("src/helpers.c")
         .compile("wasmtime-helpers");
+
+    // Check to see if we are on Linux and the `memfd` feature is
+    // active. If so, enable the `memfd` rustc cfg so `#[cfg(memfd)]`
+    // will work.
+    let os = env::var("CARGO_CFG_TARGET_OS").unwrap();
+    let is_memfd = env::var("CARGO_FEATURE_MEMFD").is_ok();
+    let is_pooling = env::var("CARGO_FEATURE_POOLING_ALLOCATOR").is_ok();
+    let is_uffd = env::var("CARGO_FEATURE_UFFD").is_ok();
+    if &os == "linux" && is_memfd && is_pooling && !is_uffd {
+        println!("cargo:rustc-cfg=memfd");
+    }
 }

crates/runtime/src/instance/allocator/pooling.rs

@@ -703,7 +703,7 @@ impl MemoryPool {
         let mapping = Mmap::accessible_reserved(0, allocation_size)
             .context("failed to create memory pool mapping")?;
 
-        let num_memfd_slots = if cfg!(feature = "memfd-allocator") {
+        let num_memfd_slots = if cfg!(memfd) {
             max_instances * max_memories
         } else {
             0

crates/runtime/src/lib.rs

@@ -19,7 +19,7 @@
         clippy::use_self
     )
 )]
-#![cfg_attr(feature = "memfd-allocator", allow(dead_code))]
+#![cfg_attr(memfd, allow(dead_code))]
 
 use std::sync::atomic::AtomicU64;
 
@@ -67,14 +67,14 @@ pub use crate::vmcontext::{
 mod module_id;
 pub use module_id::{CompiledModuleId, CompiledModuleIdAllocator};
 
-#[cfg(feature = "memfd-allocator")]
+#[cfg(memfd)]
 mod memfd;
-#[cfg(feature = "memfd-allocator")]
+#[cfg(memfd)]
 pub use crate::memfd::{MemFdSlot, MemoryMemFd, ModuleMemFds};
 
-#[cfg(not(feature = "memfd-allocator"))]
+#[cfg(not(memfd))]
 mod memfd_disabled;
-#[cfg(not(feature = "memfd-allocator"))]
+#[cfg(not(memfd))]
 pub use crate::memfd_disabled::{MemFdSlot, MemoryMemFd, ModuleMemFds};
 
 /// Version number of this crate.

crates/runtime/src/memfd_disabled.rs

@@ -37,11 +37,9 @@ impl ModuleMemFds {
 /// To allow MemFdSlot to be unconditionally passed around in various
 /// places (e.g. a `Memory`), we define a zero-sized type when memfd is
 /// not included in the build.
-#[cfg(not(feature = "memfd-allocator"))]
 #[derive(Debug)]
 pub struct MemFdSlot;
 
-#[cfg(not(feature = "memfd-allocator"))]
 #[allow(dead_code)]
 impl MemFdSlot {
     pub(crate) fn create(_: *mut libc::c_void, _: usize) -> Self {

crates/runtime/src/traphandlers/unix.rs

@@ -54,14 +54,7 @@ pub unsafe fn platform_init() {
     // Sometimes we need to handle SIGBUS too:
     // - On ARM, handle Unaligned Accesses.
     // - On Darwin, guard page accesses are raised as SIGBUS.
-    // - With the MemFD allocator, heap growth is controlled by
-    //   ftruncate'ing an mmap'd file, and so out-of-bounds accesses
-    //   are raised as SIGBUS.
-    if cfg!(target_arch = "arm")
-        || cfg!(target_os = "macos")
-        || cfg!(target_os = "freebsd")
-        || cfg!(feature = "memfd-allocator")
-    {
+    if cfg!(target_arch = "arm") || cfg!(target_os = "macos") || cfg!(target_os = "freebsd") {
         register(&mut PREV_SIGBUS, libc::SIGBUS);
     }
 }

crates/wasmtime/Cargo.toml

@@ -50,7 +50,7 @@ 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
@@ -90,4 +90,4 @@ all-arch = ["wasmtime-cranelift/all-arch"]
 # need portable signal handling.
 posix-signals-on-macos = ["wasmtime-runtime/posix-signals-on-macos"]
 
-memfd-allocator = ["wasmtime-runtime/memfd-allocator", "pooling-allocator"]
+memfd = ["wasmtime-runtime/memfd", "pooling-allocator"]