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Add shared memories (#4187)

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* Add shared memories

This change adds the ability to use shared memories in Wasmtime when the
[threads proposal] is enabled. Shared memories are annotated as `shared`
in the WebAssembly syntax, e.g., `(memory 1 1 shared)`, and are
protected from concurrent access during `memory.size` and `memory.grow`.

[threads proposal]: https://github.com/WebAssembly/threads/blob/master/proposals/threads/Overview.md

In order to implement this in Wasmtime, there are two main cases to
cover:
    - a program may simply create a shared memory and possibly export it;
    this means that Wasmtime itself must be able to create shared
    memories
    - a user may create a shared memory externally and pass it in as an
    import during instantiation; this is the case when the program
    contains code like `(import "env" "memory" (memory 1 1
    shared))`--this case is handled by a new Wasmtime API
    type--`SharedMemory`

Because of the first case, this change allows any of the current
memory-creation mechanisms to work as-is. Wasmtime can still create
either static or dynamic memories in either on-demand or pooling modes,
and any of these memories can be considered shared. When shared, the
`Memory` runtime container will lock appropriately during `memory.size`
and `memory.grow` operations; since all memories use this container, it
is an ideal place for implementing the locking once and once only.

The second case is covered by the new `SharedMemory` structure. It uses
the same `Mmap` allocation under the hood as non-shared memories, but
allows the user to perform the allocation externally to Wasmtime and
share the memory across threads (via an `Arc`). The pointer address to
the actual memory is carefully wired through and owned by the
`SharedMemory` structure itself. This means that there are differing
views of where to access the pointer (i.e., `VMMemoryDefinition`): for
owned memories (the default), the `VMMemoryDefinition` is stored
directly by the `VMContext`; in the `SharedMemory` case, however, this
`VMContext` must point to this separate structure.

To ensure that the `VMContext` can always point to the correct
`VMMemoryDefinition`, this change alters the `VMContext` structure.
Since a `SharedMemory` owns its own `VMMemoryDefinition`, the
`defined_memories` table in the `VMContext` becomes a sequence of
pointers--in the shared memory case, they point to the
`VMMemoryDefinition` owned by the `SharedMemory` and in the owned memory
case (i.e., not shared) they point to `VMMemoryDefinition`s stored in a
new table, `owned_memories`.

This change adds an additional indirection (through the `*mut
VMMemoryDefinition` pointer) that could add overhead. Using an imported
memory as a proxy, we measured a 1-3% overhead of this approach on the
`pulldown-cmark` benchmark. To avoid this, Cranelift-generated code will
special-case the owned memory access (i.e., load a pointer directly to
the `owned_memories` entry) for `memory.size` so that only
shared memories (and imported memories, as before) incur the indirection
cost.

* review: remove thread feature check

* review: swap wasmtime-types dependency for existing wasmtime-environ use

* review: remove unused VMMemoryUnion

* review: reword cross-engine error message

* review: improve tests

* review: refactor to separate prevent Memory <-> SharedMemory conversion

* review: into_shared_memory -> as_shared_memory

* review: remove commented out code

* review: limit shared min/max to 32 bits

* review: skip imported memories

* review: imported memories are not owned

* review: remove TODO

* review: document unsafe send + sync

* review: add limiter assertion

* review: remove TODO

* review: improve tests

* review: fix doc test

* fix: fixes based on discussion with Alex

This changes several key parts:
 - adds memory indexes to imports and exports
 - makes `VMMemoryDefinition::current_length` an atomic usize

* review: add `Extern::SharedMemory`

* review: remove TODO

* review: atomically load from VMMemoryDescription in JIT-generated code

* review: add test probing the last available memory slot across threads

* fix: move assertion to new location due to rebase

* fix: doc link

* fix: add TODOs to c-api

* fix: broken doc link

* fix: modify pooling allocator messages in tests

* review: make owned_memory_index panic instead of returning an option

* review: clarify calculation of num_owned_memories

* review: move 'use' to top of file

* review: change '*const [u8]' to '*mut [u8]'

* review: remove TODO

* review: avoid hard-coding memory index

* review: remove 'preallocation' parameter from 'Memory::_new'

* fix: component model memory length

* review: check that shared memory plans are static

* review: ignore growth limits for shared memory

* review: improve atomic store comment

* review: add FIXME for memory growth failure

* review: add comment about absence of bounds-checked 'memory.size'

* review: make 'current_length()' doc comment more precise

* review: more comments related to memory.size non-determinism

* review: make 'vmmemory' unreachable for shared memory

* review: move code around

* review: thread plan through to 'wrap()'

* review: disallow shared memory allocation with the pooling allocator
This commit is contained in:
Andrew Brown
2022-06-08 10:13:40 -07:00
committed by GitHub
parent ed9db962de
commit 2b52f47b83
27 changed files with 1211 additions and 226 deletions
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52
crates/runtime/src/instance/allocator.rs
View File

@@ -10,7 +10,6 @@ use std::alloc;
use std::any::Any;
use std::convert::TryFrom;
use std::ptr;
use std::slice;
use std::sync::Arc;
use thiserror::Error;
use wasmtime_environ::{
@@ -315,7 +314,7 @@ fn check_memory_init_bounds(
.and_then(|start| start.checked_add(init.data.len()));
match end {
Some(end) if end <= memory.current_length => {
Some(end) if end <= memory.current_length() => {
// Initializer is in bounds
}
_ => {
@@ -331,7 +330,7 @@ fn check_memory_init_bounds(
fn initialize_memories(instance: &mut Instance, module: &Module) -> Result<(), InstantiationError> {
let memory_size_in_pages =
&|memory| (instance.get_memory(memory).current_length as u64) / u64::from(WASM_PAGE_SIZE);
&|memory| (instance.get_memory(memory).current_length() as u64) / u64::from(WASM_PAGE_SIZE);
// Loads the `global` value and returns it as a `u64`, but sign-extends
// 32-bit globals which can be used as the base for 32-bit memories.
@@ -372,10 +371,15 @@ fn initialize_memories(instance: &mut Instance, module: &Module) -> Result<(), I
}
}
let memory = instance.get_memory(memory_index);
let dst_slice =
unsafe { slice::from_raw_parts_mut(memory.base, memory.current_length) };
let dst = &mut dst_slice[usize::try_from(init.offset).unwrap()..][..init.data.len()];
dst.copy_from_slice(instance.wasm_data(init.data.clone()));
unsafe {
let src = instance.wasm_data(init.data.clone());
let dst = memory.base.add(usize::try_from(init.offset).unwrap());
// FIXME audit whether this is safe in the presence of shared
// memory
// (https://github.com/bytecodealliance/wasmtime/issues/4203).
ptr::copy_nonoverlapping(src.as_ptr(), dst, src.len())
}
true
},
);
@@ -513,6 +517,36 @@ impl Default for OnDemandInstanceAllocator {
}
}
/// Allocate an instance containing a single memory.
///
/// In order to import a [`Memory`] into a WebAssembly instance, Wasmtime
/// requires that memory to exist in its own instance. Here we bring to life
/// such a "Frankenstein" instance with the only purpose of exporting a
/// [`Memory`].
pub unsafe fn allocate_single_memory_instance(
req: InstanceAllocationRequest,
memory: Memory,
) -> Result<InstanceHandle, InstantiationError> {
let mut memories = PrimaryMap::default();
memories.push(memory);
let tables = PrimaryMap::default();
let module = req.runtime_info.module();
let offsets = VMOffsets::new(HostPtr, module);
let layout = Instance::alloc_layout(&offsets);
let instance = alloc::alloc(layout) as *mut Instance;
Instance::new_at(instance, layout.size(), offsets, req, memories, tables);
Ok(InstanceHandle { instance })
}
/// Internal implementation of [`InstanceHandle`] deallocation.
///
/// See [`InstanceAllocator::deallocate()`] for more details.
pub unsafe fn deallocate(handle: &InstanceHandle) {
let layout = Instance::alloc_layout(&handle.instance().offsets);
ptr::drop_in_place(handle.instance);
alloc::dealloc(handle.instance.cast(), layout);
}
unsafe impl InstanceAllocator for OnDemandInstanceAllocator {
unsafe fn allocate(
&self,
@@ -542,9 +576,7 @@ unsafe impl InstanceAllocator for OnDemandInstanceAllocator {
}
unsafe fn deallocate(&self, handle: &InstanceHandle) {
let layout = Instance::alloc_layout(&handle.instance().offsets);
ptr::drop_in_place(handle.instance);
alloc::dealloc(handle.instance.cast(), layout);
deallocate(handle)
}
#[cfg(feature = "async")]