Decommit instance memory after a runtime call on Linux (#8998)

* Decommit instance memory after a runtime call on Linux

* Update documentation for the test

* Remove unfinished comment

* Use saturating_sub.

Also update the doc comment.

* Precise RSS tracking in the test

Instead of tracking RSS for the whole process we just look at the particular mapping that is associated with the linear memory of the runtime instance

* Remove unused import

* Fix unused imports

* Fix the unused imports error for good

* Rollback an accidental change to benches

* Fix the test

* Remove now unneeded code

substrate/client/executor/runtime-test/src/lib.rs

@@ -69,6 +69,36 @@ sp_core::wasm_export_functions! {
 
 	fn test_empty_return() {}
 
+	fn test_dirty_plenty_memory(heap_base: u32, heap_pages: u32) {
+		// This piece of code will dirty multiple pages of memory. The number of pages is given by
+		// the `heap_pages`. It's unit is a wasm page (64KiB). The first page to be cleared
+		// is a wasm page that that follows the one that holds the `heap_base` address.
+		//
+		// This function dirties the **host** pages. I.e. we dirty 4KiB at a time and it will take
+		// 16 writes to process a single wasm page.
+
+		let mut heap_ptr = heap_base as usize;
+
+		// Find the next wasm page boundary.
+		let heap_ptr = round_up_to(heap_ptr, 65536);
+
+		// Make it an actual pointer
+		let heap_ptr = heap_ptr as *mut u8;
+
+		// Traverse the host pages and make each one dirty
+		let host_pages = heap_pages as usize * 16;
+		for i in 0..host_pages {
+			unsafe {
+				// technically this is an UB, but there is no way Rust can find this out.
+				heap_ptr.add(i * 4096).write(0);
+			}
+		}
+
+		fn round_up_to(n: usize, divisor: usize) -> usize {
+			(n + divisor - 1) / divisor
+		}
+	}
+
 	fn test_exhaust_heap() -> Vec<u8> { Vec::with_capacity(16777216) }
 
 	fn test_panic() { panic!("test panic") }