Decommit instance memory after a runtime call on Linux

client/executor/Cargo.toml

@@ -50,6 +50,7 @@ sc-tracing = { version = "3.0.0", path = "../tracing" }
 tracing = "0.1.25"
 tracing-subscriber = "0.2.18"
 paste = "1.0"
+regex = "1"
 
 [features]
 default = [ "std" ]

client/executor/common/src/wasm_runtime.rs

@@ -93,4 +93,13 @@ pub trait WasmInstance: Send {
 	///
 	/// This method is only suitable for getting immutable globals.
 	fn get_global_const(&self, name: &str) -> Result<Option<Value>, Error>;
+
+	/// **Testing Only**. This function returns the base address of the linear memory.
+	///
+	/// This is meant to be the starting address of the memory mapped area for the linear memory.
+	///
+	/// This function is intended only for a specific test that measures physical memory consumption.
+	fn linear_memory_base_ptr(&self) -> Option<*const u8> {
+		None
+	}
 }

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") }

client/executor/src/integration_tests/linux.rs

@@ -0,0 +1,73 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2021 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: GPL-3.0-or-later WITH Classpath-exception-2.0
+
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+//! Tests that are only relevant for Linux.
+
+// Constrain this only to wasmtime for the time being. Without this rustc will complain on unused
+// imports and items. The alternative is to plop `cfg(feature = wasmtime)` everywhere which seems
+// borthersome.
+#![cfg(feature = "wasmtime")]
+
+use crate::WasmExecutionMethod;
+use super::mk_test_runtime;
+use codec::Encode as _;
+
+mod smaps;
+
+use self::smaps::Smaps;
+
+#[test]
+fn memory_consumption_compiled() {
+	// This aims to see if linear memory stays backed by the physical memory after a runtime call.
+	//
+	// For that we make a series of runtime calls, probing the RSS for the VMA matching the linear
+	// memory. After the call we expect RSS to be equal to 0.
+
+	let runtime = mk_test_runtime(WasmExecutionMethod::Compiled, 1024);
+
+	let instance = runtime.new_instance().unwrap();
+	let heap_base = instance
+		.get_global_const("__heap_base")
+		.expect("`__heap_base` is valid")
+		.expect("`__heap_base` exists")
+		.as_i32()
+		.expect("`__heap_base` is an `i32`");
+
+	fn probe_rss(instance: &dyn sc_executor_common::wasm_runtime::WasmInstance) -> usize {
+		let base_addr = instance.linear_memory_base_ptr().unwrap() as usize;
+		Smaps::new().get_rss(base_addr).expect("failed to get rss")
+	}
+
+	instance
+		.call_export(
+			"test_dirty_plenty_memory",
+			&(heap_base as u32, 1u32).encode(),
+		)
+		.unwrap();
+	let probe_1 = probe_rss(&*instance);
+	instance
+		.call_export(
+			"test_dirty_plenty_memory",
+			&(heap_base as u32, 1024u32).encode(),
+		)
+		.unwrap();
+	let probe_2 = probe_rss(&*instance);
+
+	assert_eq!(probe_1, 0);
+	assert_eq!(probe_2, 0);
+}

client/executor/src/integration_tests/linux/smaps.rs

@@ -0,0 +1,82 @@
+// This file is part of Substrate.
+
+// Copyright (C) 2017-2021 Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: GPL-3.0-or-later WITH Classpath-exception-2.0
+
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+//! A tool for extracting information about the memory consumption of the current process from
+//! the procfs.
+
+use std::ops::Range;
+use std::collections::BTreeMap;
+
+/// An interface to the /proc/self/smaps
+///
+/// See docs about [procfs on kernel.org][procfs]
+///
+/// [procfs]: https://www.kernel.org/doc/html/latest/filesystems/proc.html
+pub struct Smaps(Vec<(Range<usize>, BTreeMap<String, usize>)>);
+
+impl Smaps {
+	pub fn new() -> Self {
+		let regex_start = regex::RegexBuilder::new("^([0-9a-f]+)-([0-9a-f]+)")
+			.multi_line(true)
+			.build()
+			.unwrap();
+		let regex_kv = regex::RegexBuilder::new(r#"^([^:]+):\s*(\d+) kB"#)
+			.multi_line(true)
+			.build()
+			.unwrap();
+		let smaps = std::fs::read_to_string("/proc/self/smaps").unwrap();
+		let boundaries: Vec<_> = regex_start
+			.find_iter(&smaps)
+			.map(|matched| matched.start())
+			.chain(std::iter::once(smaps.len()))
+			.collect();
+
+		let mut output = Vec::new();
+		for window in boundaries.windows(2) {
+			let chunk = &smaps[window[0]..window[1]];
+			let caps = regex_start.captures(chunk).unwrap();
+			let start = usize::from_str_radix(caps.get(1).unwrap().as_str(), 16).unwrap();
+			let end = usize::from_str_radix(caps.get(2).unwrap().as_str(), 16).unwrap();
+
+			let values = regex_kv
+				.captures_iter(chunk)
+				.map(|cap| {
+					let key = cap.get(1).unwrap().as_str().to_owned();
+					let value = cap.get(2).unwrap().as_str().parse().unwrap();
+					(key, value)
+				})
+				.collect();
+
+			output.push((start..end, values));
+		}
+
+		Self(output)
+	}
+
+	fn get_map(&self, addr: usize) -> &BTreeMap<String, usize> {
+		&self.0
+			.iter()
+			.find(|(range, _)| addr >= range.start && addr < range.end)
+			.unwrap()
+			.1
+	}
+
+	pub fn get_rss(&self, addr: usize) -> Option<usize> {
+		self.get_map(addr).get("Rss").cloned()
+	}
+}

client/executor/src/integration_tests/mod.rs

@@ -15,6 +15,9 @@
 
 // You should have received a copy of the GNU General Public License
 // along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+#[cfg(target_os = "linux")]
+mod linux;
 mod sandbox;
 
 use std::sync::Arc;

client/executor/wasmtime/Cargo.toml

@@ -13,6 +13,8 @@ readme = "README.md"
 targets = ["x86_64-unknown-linux-gnu"]
 
 [dependencies]
+libc = "0.2.90"
+cfg-if = "1.0"
 log = "0.4.8"
 scoped-tls = "1.0"
 parity-wasm = "0.42.0"

client/executor/wasmtime/src/instance_wrapper.rs

@@ -415,6 +415,43 @@ impl InstanceWrapper {
 			slice::from_raw_parts_mut(ptr, len)
 		}
 	}
+
+	/// Returns the pointer to the first byte of the linear memory for this instance.
+	pub fn base_ptr(&self) -> *const u8 {
+		self.memory.data_ptr()
+	}
+
+	/// Removes physical backing from the allocated linear memory. This leads to returning the memory
+	/// back to the system. While the memory is zeroed this is considered as a side-effect and is not
+	/// relied upon. Thus this function acts as a hint.
+	pub fn decommit(&self) {
+		if self.memory.data_size() == 0 {
+			return;
+		}
+
+		cfg_if::cfg_if! {
+			if #[cfg(target_os = "linux")] {
+				use std::sync::Once;
+
+				unsafe {
+					let ptr = self.memory.data_ptr();
+					let len = self.memory.data_size();
+
+					// Linux handles MADV_DONTNEED reliably. The result is that the given area
+					// is unmapped and will be zeroed on the next pagefault.
+					if libc::madvise(ptr as _, len, libc::MADV_DONTNEED) != 0 {
+						static LOGGED: Once = Once::new();
+						LOGGED.call_once(|| {
+							log::warn!(
+								"madvise(MADV_DONTNEED) failed: {}",
+								std::io::Error::last_os_error(),
+							);
+						});
+					}
+				}
+			}
+		}
+	}
 }
 
 impl runtime_blob::InstanceGlobals for InstanceWrapper {

client/executor/wasmtime/src/runtime.rs

@@ -150,7 +150,13 @@ impl WasmInstance for WasmtimeInstance {
 				globals_snapshot.apply(&**instance_wrapper);
 				let allocator = FreeingBumpHeapAllocator::new(*heap_base);
 
-				perform_call(data, Rc::clone(&instance_wrapper), entrypoint, allocator)
+				let result = perform_call(data, Rc::clone(&instance_wrapper), entrypoint, allocator);
+
+				// Signal to the OS that we are done with the linear memory and that it can be
+				// reclaimed.
+				instance_wrapper.decommit();
+
+				result
 			}
 			Strategy::RecreateInstance(instance_creator) => {
 				let instance_wrapper = instance_creator.instantiate()?;
@@ -173,6 +179,19 @@ impl WasmInstance for WasmtimeInstance {
 			}
 		}
 	}
+
+	fn linear_memory_base_ptr(&self) -> Option<*const u8> {
+		match &self.strategy {
+			Strategy::RecreateInstance(_) => {
+				// We do not keep the wasm instance around, therefore there is no linear memory
+				// associated with it.
+				None
+			}
+			Strategy::FastInstanceReuse {
+				instance_wrapper, ..
+			} => Some(instance_wrapper.base_ptr()),
+		}
+	}
 }
 
 /// Prepare a directory structure and a config file to enable wasmtime caching.