Round hashglobe allocations up to the nearest page size.

MozReview-Commit-ID: 34KFtcwCkBB

components/hashglobe/src/lib.rs

@@ -52,3 +52,6 @@ impl fmt::Display for FailedAllocationError {
         self.reason.fmt(f)
     }
 }
+
+// The size of memory pages on this system. Set when initializing geckolib.
+pub static SYSTEM_PAGE_SIZE: ::std::sync::atomic::AtomicUsize = ::std::sync::atomic::ATOMIC_USIZE_INIT;

components/hashglobe/src/table.rs

@@ -777,7 +777,7 @@ impl<K, V> RawTable<K, V> {
 
 
         // FORK NOTE: Uses alloc shim instead of Heap.alloc
-        let buffer = alloc(size, alignment);
+        let buffer = alloc(round_up_to_page_size(size), alignment);
         
         if buffer.is_null() {
             
@@ -1201,3 +1201,19 @@ impl<K, V> Drop for RawTable<K, V> {
         }
     }
 }
+
+// Force all allocations to fill their pages for the duration of the mprotect
+// experiment.
+#[inline]
+fn round_up_to_page_size(size: usize) -> usize {
+    let page_size = ::SYSTEM_PAGE_SIZE.load(::std::sync::atomic::Ordering::Relaxed);
+    debug_assert!(page_size != 0);
+    let mut result = size;
+    let remainder = size % page_size;
+    if remainder != 0 {
+        result += page_size - remainder;
+    }
+    debug_assert!(result % page_size == 0);
+    debug_assert!(result - size < page_size);
+    result
+}