Code review feedback.

* Improve comments.
* Drop old table element *after* updating the table.
* Extract out the same `cfg_if!` to a single constant.

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

@@ -45,7 +45,11 @@ fn decommit(addr: *mut u8, len: usize) -> Result<()> {
     }
 
     unsafe {
-        // On Linux, this is enough to cause the kernel to initialize the pages to 0 on next access
+        // On Linux, this tells the kernel to discard the backing of the pages in the range.
+        // If the discarded pages are part of a uffd region, then the next access will fault
+        // and the user fault handler will receive the event.
+        // If the pages are not monitored by uffd, the kernel will zero the page on next access,
+        // as if it were mmap'd for the first time.
         if libc::madvise(addr as _, len, libc::MADV_DONTNEED) != 0 {
             bail!(
                 "madvise failed to decommit: {}",
@@ -93,8 +97,6 @@ pub fn decommit_stack_pages(addr: *mut u8, len: usize) -> Result<()> {
 /// With uffd, however, the potentially accessible pages of the each linear memory are made `READ_WRITE` and
 /// the page fault handler will detect an out of bounds access and treat the page, temporarily,
 /// as a guard page.
-///
-/// This me
 pub(super) fn initialize_memory_pool(pool: &MemoryPool) -> Result<()> {
     if pool.memory_size == 0 || pool.max_wasm_pages == 0 {
         return Ok(());

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

@@ -5,6 +5,10 @@ fn decommit(addr: *mut u8, len: usize, protect: bool) -> Result<()> {
         return Ok(());
     }
 
+    // By creating a new mapping at the same location, this will discard the
+    // mapping for the pages in the given range.
+    // The new mapping will be to the CoW zero page, so this effectively
+    // zeroes the pages.
     if unsafe {
         libc::mmap(
             addr as _,

crates/runtime/src/table.rs

@@ -372,11 +372,13 @@ impl Table {
         }
     }
 
-    fn set_raw(ty: TableElementType, e: &mut *mut u8, val: TableElement) {
+    fn set_raw(ty: TableElementType, elem: &mut *mut u8, val: TableElement) {
         unsafe {
-            // Drop the existing element
+            let old = *elem;
-            let _ = TableElement::from_raw(ty, *e);
+            *elem = val.into_raw();
-            *e = val.into_raw();
+
+            // Drop the old element
+            let _ = TableElement::from_raw(ty, old);
         }
     }
 

crates/wasmtime/src/module.rs

@@ -307,6 +307,8 @@ impl Module {
     /// # }
     /// ```
     pub fn from_binary(engine: &Engine, binary: &[u8]) -> Result<Module> {
+        const USE_PAGED_MEM_INIT: bool = cfg!(all(feature = "uffd", target_os = "linux"));
+
         cfg_if::cfg_if! {
             if #[cfg(feature = "cache")] {
                 let (main_module, artifacts, types) = ModuleCacheEntry::new(
@@ -314,27 +316,11 @@ impl Module {
                     engine.cache_config(),
                 )
                 .get_data((engine.compiler(), binary), |(compiler, binary)| {
-                    cfg_if::cfg_if! {
+                    CompilationArtifacts::build(compiler, binary, USE_PAGED_MEM_INIT)
-                        if #[cfg(all(feature = "uffd", target_os = "linux"))] {
-                            let use_paged_mem_init = true;
-                        } else {
-                            let use_paged_mem_init = false;
-                        }
-                    };
-
-                    CompilationArtifacts::build(compiler, binary, use_paged_mem_init)
                 })?;
             } else {
-                cfg_if::cfg_if! {
-                    if #[cfg(all(feature = "uffd", target_os = "linux"))] {
-                        let use_paged_mem_init = true;
-                    } else {
-                        let use_paged_mem_init = false;
-                    }
-                };
-
                 let (main_module, artifacts, types) =
-                    CompilationArtifacts::build(engine.compiler(), binary, use_paged_mem_init)?;
+                    CompilationArtifacts::build(engine.compiler(), binary, USE_PAGED_MEM_INIT)?;
             }
         };