wasmtime_runtime/

memory.rs

//! Memory management for linear memories.
//!
//! `RuntimeLinearMemory` is to WebAssembly linear memories what `Table` is to WebAssembly tables.

use crate::mmap::Mmap;
use crate::parking_spot::ParkingSpot;
use crate::vmcontext::VMMemoryDefinition;
use crate::{MemoryImage, MemoryImageSlot, Store, WaitResult};
use anyhow::Error;
use anyhow::{bail, format_err, Result};
use std::convert::TryFrom;
use std::ops::Range;
use std::sync::atomic::{AtomicU32, AtomicU64, Ordering};
use std::sync::{Arc, RwLock};
use std::time::Instant;
use wasmtime_environ::{MemoryPlan, MemoryStyle, Trap, WASM32_MAX_PAGES, WASM64_MAX_PAGES};

const WASM_PAGE_SIZE: usize = wasmtime_environ::WASM_PAGE_SIZE as usize;
const WASM_PAGE_SIZE_U64: u64 = wasmtime_environ::WASM_PAGE_SIZE as u64;

/// A memory allocator
pub trait RuntimeMemoryCreator: Send + Sync {
    /// Create new RuntimeLinearMemory
    fn new_memory(
        &self,
        plan: &MemoryPlan,
        minimum: usize,
        maximum: Option<usize>,
        // Optionally, a memory image for CoW backing.
        memory_image: Option<&Arc<MemoryImage>>,
    ) -> Result<Box<dyn RuntimeLinearMemory>>;
}

/// A default memory allocator used by Wasmtime
pub struct DefaultMemoryCreator;

impl RuntimeMemoryCreator for DefaultMemoryCreator {
    /// Create new MmapMemory
    fn new_memory(
        &self,
        plan: &MemoryPlan,
        minimum: usize,
        maximum: Option<usize>,
        memory_image: Option<&Arc<MemoryImage>>,
    ) -> Result<Box<dyn RuntimeLinearMemory>> {
        Ok(Box::new(MmapMemory::new(
            plan,
            minimum,
            maximum,
            memory_image,
        )?))
    }
}

/// A linear memory
pub trait RuntimeLinearMemory: Send + Sync {
    /// Returns the number of allocated bytes.
    fn byte_size(&self) -> usize;

    /// Returns the maximum number of bytes the memory can grow to.
    /// Returns `None` if the memory is unbounded.
    fn maximum_byte_size(&self) -> Option<usize>;

    /// Grows a memory by `delta_pages`.
    ///
    /// This performs the necessary checks on the growth before delegating to
    /// the underlying `grow_to` implementation. A default implementation of
    /// this memory is provided here since this is assumed to be the same for
    /// most kinds of memory; one exception is shared memory, which must perform
    /// all the steps of the default implementation *plus* the required locking.
    ///
    /// The `store` is used only for error reporting.
    fn grow(
        &mut self,
        delta_pages: u64,
        mut store: Option<&mut dyn Store>,
    ) -> Result<Option<(usize, usize)>, Error> {
        let old_byte_size = self.byte_size();

        // Wasm spec: when growing by 0 pages, always return the current size.
        if delta_pages == 0 {
            return Ok(Some((old_byte_size, old_byte_size)));
        }

        // The largest wasm-page-aligned region of memory is possible to
        // represent in a `usize`. This will be impossible for the system to
        // actually allocate.
        let absolute_max = 0usize.wrapping_sub(WASM_PAGE_SIZE);

        // Calculate the byte size of the new allocation. Let it overflow up to
        // `usize::MAX`, then clamp it down to `absolute_max`.
        let new_byte_size = usize::try_from(delta_pages)
            .unwrap_or(usize::MAX)
            .saturating_mul(WASM_PAGE_SIZE)
            .saturating_add(old_byte_size);
        let new_byte_size = if new_byte_size > absolute_max {
            absolute_max
        } else {
            new_byte_size
        };

        let maximum = self.maximum_byte_size();
        // Store limiter gets first chance to reject memory_growing.
        if let Some(store) = &mut store {
            if !store.memory_growing(old_byte_size, new_byte_size, maximum)? {
                return Ok(None);
            }
        }

        // Never exceed maximum, even if limiter permitted it.
        if let Some(max) = maximum {
            if new_byte_size > max {
                if let Some(store) = store {
                    // FIXME: shared memories may not have an associated store
                    // to report the growth failure to but the error should not
                    // be dropped
                    // (https://github.com/bytecodealliance/wasmtime/issues/4240).
                    store.memory_grow_failed(&format_err!("Memory maximum size exceeded"));
                }
                return Ok(None);
            }
        }

        match self.grow_to(new_byte_size) {
            Ok(_) => Ok(Some((old_byte_size, new_byte_size))),
            Err(e) => {
                // FIXME: shared memories may not have an associated store to
                // report the growth failure to but the error should not be
                // dropped
                // (https://github.com/bytecodealliance/wasmtime/issues/4240).
                if let Some(store) = store {
                    store.memory_grow_failed(&e);
                }
                Ok(None)
            }
        }
    }

    /// Grow memory to the specified amount of bytes.
    ///
    /// Returns an error if memory can't be grown by the specified amount
    /// of bytes.
    fn grow_to(&mut self, size: usize) -> Result<()>;

    /// Return a `VMMemoryDefinition` for exposing the memory to compiled wasm
    /// code.
    fn vmmemory(&mut self) -> VMMemoryDefinition;

    /// Does this memory need initialization? It may not if it already
    /// has initial contents courtesy of the `MemoryImage` passed to
    /// `RuntimeMemoryCreator::new_memory()`.
    fn needs_init(&self) -> bool;

    /// Used for optional dynamic downcasting.
    fn as_any_mut(&mut self) -> &mut dyn std::any::Any;

    /// Returns the range of addresses that may be reached by WebAssembly.
    ///
    /// This starts at the base of linear memory and ends at the end of the
    /// guard pages, if any.
    fn wasm_accessible(&self) -> Range<usize>;
}

/// A linear memory instance.
#[derive(Debug)]
pub struct MmapMemory {
    // The underlying allocation.
    mmap: Mmap,

    // The number of bytes that are accessible in `mmap` and available for
    // reading and writing.
    //
    // This region starts at `pre_guard_size` offset from the base of `mmap`.
    accessible: usize,

    // The optional maximum accessible size, in bytes, for this linear memory.
    //
    // Note that this maximum does not factor in guard pages, so this isn't the
    // maximum size of the linear address space reservation for this memory.
    maximum: Option<usize>,

    // The amount of extra bytes to reserve whenever memory grows. This is
    // specified so that the cost of repeated growth is amortized.
    extra_to_reserve_on_growth: usize,

    // Size in bytes of extra guard pages before the start and after the end to
    // optimize loads and stores with constant offsets.
    pre_guard_size: usize,
    offset_guard_size: usize,

    // An optional CoW mapping that provides the initial content of this
    // MmapMemory, if mapped.
    memory_image: Option<MemoryImageSlot>,
}

impl MmapMemory {
    /// Create a new linear memory instance with specified minimum and maximum
    /// number of wasm pages.
    pub fn new(
        plan: &MemoryPlan,
        minimum: usize,
        mut maximum: Option<usize>,
        memory_image: Option<&Arc<MemoryImage>>,
    ) -> Result<Self> {
        // It's a programmer error for these two configuration values to exceed
        // the host available address space, so panic if such a configuration is
        // found (mostly an issue for hypothetical 32-bit hosts).
        let offset_guard_bytes = usize::try_from(plan.offset_guard_size).unwrap();
        let pre_guard_bytes = usize::try_from(plan.pre_guard_size).unwrap();

        let (alloc_bytes, extra_to_reserve_on_growth) = match plan.style {
            // Dynamic memories start with the minimum size plus the `reserve`
            // amount specified to grow into.
            MemoryStyle::Dynamic { reserve } => (minimum, usize::try_from(reserve).unwrap()),

            // Static memories will never move in memory and consequently get
            // their entire allocation up-front with no extra room to grow into.
            // Note that the `maximum` is adjusted here to whatever the smaller
            // of the two is, the `maximum` given or the `bound` specified for
            // this memory.
            MemoryStyle::Static { bound } => {
                assert!(bound >= plan.memory.minimum);
                let bound_bytes =
                    usize::try_from(bound.checked_mul(WASM_PAGE_SIZE_U64).unwrap()).unwrap();
                maximum = Some(bound_bytes.min(maximum.unwrap_or(usize::MAX)));
                (bound_bytes, 0)
            }
        };

        let request_bytes = pre_guard_bytes
            .checked_add(alloc_bytes)
            .and_then(|i| i.checked_add(extra_to_reserve_on_growth))
            .and_then(|i| i.checked_add(offset_guard_bytes))
            .ok_or_else(|| format_err!("cannot allocate {} with guard regions", minimum))?;
        let mut mmap = Mmap::accessible_reserved(0, request_bytes)?;

        if minimum > 0 {
            mmap.make_accessible(pre_guard_bytes, minimum)?;
        }

        // If a memory image was specified, try to create the MemoryImageSlot on
        // top of our mmap.
        let memory_image = match memory_image {
            Some(image) => {
                let base = unsafe { mmap.as_mut_ptr().add(pre_guard_bytes) };
                let mut slot = MemoryImageSlot::create(
                    base.cast(),
                    minimum,
                    alloc_bytes + extra_to_reserve_on_growth,
                );
                slot.instantiate(minimum, Some(image), &plan)?;
                // On drop, we will unmap our mmap'd range that this slot was
                // mapped on top of, so there is no need for the slot to wipe
                // it with an anonymous mapping first.
                slot.no_clear_on_drop();
                Some(slot)
            }
            None => None,
        };

        Ok(Self {
            mmap,
            accessible: minimum,
            maximum,
            pre_guard_size: pre_guard_bytes,
            offset_guard_size: offset_guard_bytes,
            extra_to_reserve_on_growth,
            memory_image,
        })
    }
}

impl RuntimeLinearMemory for MmapMemory {
    fn byte_size(&self) -> usize {
        self.accessible
    }

    fn maximum_byte_size(&self) -> Option<usize> {
        self.maximum
    }

    fn grow_to(&mut self, new_size: usize) -> Result<()> {
        if new_size > self.mmap.len() - self.offset_guard_size - self.pre_guard_size {
            // If the new size of this heap exceeds the current size of the
            // allocation we have, then this must be a dynamic heap. Use
            // `new_size` to calculate a new size of an allocation, allocate it,
            // and then copy over the memory from before.
            let request_bytes = self
                .pre_guard_size
                .checked_add(new_size)
                .and_then(|s| s.checked_add(self.extra_to_reserve_on_growth))
                .and_then(|s| s.checked_add(self.offset_guard_size))
                .ok_or_else(|| format_err!("overflow calculating size of memory allocation"))?;

            let mut new_mmap = Mmap::accessible_reserved(0, request_bytes)?;
            new_mmap.make_accessible(self.pre_guard_size, new_size)?;

            new_mmap.as_mut_slice()[self.pre_guard_size..][..self.accessible]
                .copy_from_slice(&self.mmap.as_slice()[self.pre_guard_size..][..self.accessible]);

            // Now drop the MemoryImageSlot, if any. We've lost the CoW
            // advantages by explicitly copying all data, but we have
            // preserved all of its content; so we no longer need the
            // mapping. We need to do this before we (implicitly) drop the
            // `mmap` field by overwriting it below.
            drop(self.memory_image.take());

            self.mmap = new_mmap;
        } else if let Some(image) = self.memory_image.as_mut() {
            // MemoryImageSlot has its own growth mechanisms; defer to its
            // implementation.
            image.set_heap_limit(new_size)?;
        } else {
            // If the new size of this heap fits within the existing allocation
            // then all we need to do is to make the new pages accessible. This
            // can happen either for "static" heaps which always hit this case,
            // or "dynamic" heaps which have some space reserved after the
            // initial allocation to grow into before the heap is moved in
            // memory.
            assert!(new_size > self.accessible);
            self.mmap.make_accessible(
                self.pre_guard_size + self.accessible,
                new_size - self.accessible,
            )?;
        }

        self.accessible = new_size;

        Ok(())
    }

    fn vmmemory(&mut self) -> VMMemoryDefinition {
        VMMemoryDefinition {
            base: unsafe { self.mmap.as_mut_ptr().add(self.pre_guard_size) },
            current_length: self.accessible.into(),
        }
    }

    fn needs_init(&self) -> bool {
        // If we're using a CoW mapping, then no initialization
        // is needed.
        self.memory_image.is_none()
    }

    fn as_any_mut(&mut self) -> &mut dyn std::any::Any {
        self
    }

    fn wasm_accessible(&self) -> Range<usize> {
        let base = self.mmap.as_mut_ptr() as usize + self.pre_guard_size;
        let end = base + (self.mmap.len() - self.pre_guard_size);
        base..end
    }
}

/// A "static" memory where the lifetime of the backing memory is managed
/// elsewhere. Currently used with the pooling allocator.
struct StaticMemory {
    /// The memory in the host for this wasm memory. The length of this
    /// slice is the maximum size of the memory that can be grown to.
    base: &'static mut [u8],

    /// The current size, in bytes, of this memory.
    size: usize,

    /// The size, in bytes, of the virtual address allocation starting at `base`
    /// and going to the end of the guard pages at the end of the linear memory.
    memory_and_guard_size: usize,

    /// The image management, if any, for this memory. Owned here and
    /// returned to the pooling allocator when termination occurs.
    memory_image: MemoryImageSlot,
}

impl StaticMemory {
    fn new(
        base: &'static mut [u8],
        initial_size: usize,
        maximum_size: Option<usize>,
        memory_image: MemoryImageSlot,
        memory_and_guard_size: usize,
    ) -> Result<Self> {
        if base.len() < initial_size {
            bail!(
                "initial memory size of {} exceeds the pooling allocator's \
                 configured maximum memory size of {} bytes",
                initial_size,
                base.len(),
            );
        }

        // Only use the part of the slice that is necessary.
        let base = match maximum_size {
            Some(max) if max < base.len() => &mut base[..max],
            _ => base,
        };

        Ok(Self {
            base,
            size: initial_size,
            memory_image,
            memory_and_guard_size,
        })
    }
}

impl RuntimeLinearMemory for StaticMemory {
    fn byte_size(&self) -> usize {
        self.size
    }

    fn maximum_byte_size(&self) -> Option<usize> {
        Some(self.base.len())
    }

    fn grow_to(&mut self, new_byte_size: usize) -> Result<()> {
        // Never exceed the static memory size; this check should have been made
        // prior to arriving here.
        assert!(new_byte_size <= self.base.len());

        self.memory_image.set_heap_limit(new_byte_size)?;

        // Update our accounting of the available size.
        self.size = new_byte_size;
        Ok(())
    }

    fn vmmemory(&mut self) -> VMMemoryDefinition {
        VMMemoryDefinition {
            base: self.base.as_mut_ptr().cast(),
            current_length: self.size.into(),
        }
    }

    fn needs_init(&self) -> bool {
        !self.memory_image.has_image()
    }

    fn as_any_mut(&mut self) -> &mut dyn std::any::Any {
        self
    }

    fn wasm_accessible(&self) -> Range<usize> {
        let base = self.base.as_ptr() as usize;
        let end = base + self.memory_and_guard_size;
        base..end
    }
}

/// For shared memory (and only for shared memory), this lock-version restricts
/// access when growing the memory or checking its size. This is to conform with
/// the [thread proposal]: "When `IsSharedArrayBuffer(...)` is true, the return
/// value should be the result of an atomic read-modify-write of the new size to
/// the internal `length` slot."
///
/// [thread proposal]:
///     https://github.com/WebAssembly/threads/blob/master/proposals/threads/Overview.md#webassemblymemoryprototypegrow
#[derive(Clone)]
pub struct SharedMemory(Arc<SharedMemoryInner>);

struct SharedMemoryInner {
    memory: RwLock<Box<dyn RuntimeLinearMemory>>,
    spot: ParkingSpot,
    ty: wasmtime_environ::Memory,
    def: LongTermVMMemoryDefinition,
}

impl SharedMemory {
    /// Construct a new [`SharedMemory`].
    pub fn new(plan: MemoryPlan) -> Result<Self> {
        let (minimum_bytes, maximum_bytes) = Memory::limit_new(&plan, None)?;
        let mmap_memory = MmapMemory::new(&plan, minimum_bytes, maximum_bytes, None)?;
        Self::wrap(&plan, Box::new(mmap_memory), plan.memory)
    }

    /// Wrap an existing [Memory] with the locking provided by a [SharedMemory].
    pub fn wrap(
        plan: &MemoryPlan,
        mut memory: Box<dyn RuntimeLinearMemory>,
        ty: wasmtime_environ::Memory,
    ) -> Result<Self> {
        if !ty.shared {
            bail!("shared memory must have a `shared` memory type");
        }
        if !matches!(plan.style, MemoryStyle::Static { .. }) {
            bail!("shared memory can only be built from a static memory allocation")
        }
        assert!(
            memory.as_any_mut().type_id() != std::any::TypeId::of::<SharedMemory>(),
            "cannot re-wrap a shared memory"
        );
        Ok(Self(Arc::new(SharedMemoryInner {
            ty,
            spot: ParkingSpot::default(),
            def: LongTermVMMemoryDefinition(memory.vmmemory()),
            memory: RwLock::new(memory),
        })))
    }

    /// Return the memory type for this [`SharedMemory`].
    pub fn ty(&self) -> wasmtime_environ::Memory {
        self.0.ty
    }

    /// Convert this shared memory into a [`Memory`].
    pub fn as_memory(self) -> Memory {
        Memory(Box::new(self))
    }

    /// Return a pointer to the shared memory's [VMMemoryDefinition].
    pub fn vmmemory_ptr(&self) -> *const VMMemoryDefinition {
        &self.0.def.0
    }

    /// Same as `RuntimeLinearMemory::grow`, except with `&self`.
    pub fn grow(
        &self,
        delta_pages: u64,
        store: Option<&mut dyn Store>,
    ) -> Result<Option<(usize, usize)>, Error> {
        let mut memory = self.0.memory.write().unwrap();
        let result = memory.grow(delta_pages, store)?;
        if let Some((_old_size_in_bytes, new_size_in_bytes)) = result {
            // Store the new size to the `VMMemoryDefinition` for JIT-generated
            // code (and runtime functions) to access. No other code can be
            // growing this memory due to the write lock, but code in other
            // threads could have access to this shared memory and we want them
            // to see the most consistent version of the `current_length`; a
            // weaker consistency is possible if we accept them seeing an older,
            // smaller memory size (assumption: memory only grows) but presently
            // we are aiming for accuracy.
            //
            // Note that it could be possible to access a memory address that is
            // now-valid due to changes to the page flags in `grow` above but
            // beyond the `memory.size` that we are about to assign to. In these
            // and similar cases, discussion in the thread proposal concluded
            // that: "multiple accesses in one thread racing with another
            // thread's `memory.grow` that are in-bounds only after the grow
            // commits may independently succeed or trap" (see
            // https://github.com/WebAssembly/threads/issues/26#issuecomment-433930711).
            // In other words, some non-determinism is acceptable when using
            // `memory.size` on work being done by `memory.grow`.
            self.0
                .def
                .0
                .current_length
                .store(new_size_in_bytes, Ordering::SeqCst);
        }
        Ok(result)
    }

    /// Implementation of `memory.atomic.notify` for this shared memory.
    pub fn atomic_notify(&self, addr_index: u64, count: u32) -> Result<u32, Trap> {
        validate_atomic_addr(&self.0.def.0, addr_index, 4, 4)?;
        Ok(self.0.spot.unpark(addr_index, count))
    }

    /// Implementation of `memory.atomic.wait32` for this shared memory.
    pub fn atomic_wait32(
        &self,
        addr_index: u64,
        expected: u32,
        timeout: Option<Instant>,
    ) -> Result<WaitResult, Trap> {
        let addr = validate_atomic_addr(&self.0.def.0, addr_index, 4, 4)?;
        // SAFETY: `addr_index` was validated by `validate_atomic_addr` above.
        assert!(std::mem::size_of::<AtomicU32>() == 4);
        assert!(std::mem::align_of::<AtomicU32>() <= 4);
        let atomic = unsafe { &*(addr as *const AtomicU32) };

        // We want the sequential consistency of `SeqCst` to ensure that the `load` sees the value that the `notify` will/would see.
        // All WASM atomic operations are also `SeqCst`.
        let validate = || atomic.load(Ordering::SeqCst) == expected;

        Ok(self.0.spot.park(addr_index, validate, timeout))
    }

    /// Implementation of `memory.atomic.wait64` for this shared memory.
    pub fn atomic_wait64(
        &self,
        addr_index: u64,
        expected: u64,
        timeout: Option<Instant>,
    ) -> Result<WaitResult, Trap> {
        let addr = validate_atomic_addr(&self.0.def.0, addr_index, 8, 8)?;
        // SAFETY: `addr_index` was validated by `validate_atomic_addr` above.
        assert!(std::mem::size_of::<AtomicU64>() == 8);
        assert!(std::mem::align_of::<AtomicU64>() <= 8);
        let atomic = unsafe { &*(addr as *const AtomicU64) };

        // We want the sequential consistency of `SeqCst` to ensure that the `load` sees the value that the `notify` will/would see.
        // All WASM atomic operations are also `SeqCst`.
        let validate = || atomic.load(Ordering::SeqCst) == expected;

        Ok(self.0.spot.park(addr_index, validate, timeout))
    }
}

/// Shared memory needs some representation of a `VMMemoryDefinition` for
/// JIT-generated code to access. This structure owns the base pointer and
/// length to the actual memory and we share this definition across threads by:
/// - never changing the base pointer; according to the specification, shared
///   memory must be created with a known maximum size so it can be allocated
///   once and never moved
/// - carefully changing the length, using atomic accesses in both the runtime
///   and JIT-generated code.
struct LongTermVMMemoryDefinition(VMMemoryDefinition);
unsafe impl Send for LongTermVMMemoryDefinition {}
unsafe impl Sync for LongTermVMMemoryDefinition {}

/// Proxy all calls through the [`RwLock`].
impl RuntimeLinearMemory for SharedMemory {
    fn byte_size(&self) -> usize {
        self.0.memory.read().unwrap().byte_size()
    }

    fn maximum_byte_size(&self) -> Option<usize> {
        self.0.memory.read().unwrap().maximum_byte_size()
    }

    fn grow(
        &mut self,
        delta_pages: u64,
        store: Option<&mut dyn Store>,
    ) -> Result<Option<(usize, usize)>, Error> {
        SharedMemory::grow(self, delta_pages, store)
    }

    fn grow_to(&mut self, size: usize) -> Result<()> {
        self.0.memory.write().unwrap().grow_to(size)
    }

    fn vmmemory(&mut self) -> VMMemoryDefinition {
        // `vmmemory()` is used for writing the `VMMemoryDefinition` of a memory
        // into its `VMContext`; this should never be possible for a shared
        // memory because the only `VMMemoryDefinition` for it should be stored
        // in its own `def` field.
        unreachable!()
    }

    fn needs_init(&self) -> bool {
        self.0.memory.read().unwrap().needs_init()
    }

    fn as_any_mut(&mut self) -> &mut dyn std::any::Any {
        self
    }

    fn wasm_accessible(&self) -> Range<usize> {
        self.0.memory.read().unwrap().wasm_accessible()
    }
}

/// Representation of a runtime wasm linear memory.
pub struct Memory(Box<dyn RuntimeLinearMemory>);

impl Memory {
    /// Create a new dynamic (movable) memory instance for the specified plan.
    pub fn new_dynamic(
        plan: &MemoryPlan,
        creator: &dyn RuntimeMemoryCreator,
        store: &mut dyn Store,
        memory_image: Option<&Arc<MemoryImage>>,
    ) -> Result<Self> {
        let (minimum, maximum) = Self::limit_new(plan, Some(store))?;
        let allocation = creator.new_memory(plan, minimum, maximum, memory_image)?;
        let allocation = if plan.memory.shared {
            Box::new(SharedMemory::wrap(plan, allocation, plan.memory)?)
        } else {
            allocation
        };
        Ok(Memory(allocation))
    }

    /// Create a new static (immovable) memory instance for the specified plan.
    pub fn new_static(
        plan: &MemoryPlan,
        base: &'static mut [u8],
        memory_image: MemoryImageSlot,
        memory_and_guard_size: usize,
        store: &mut dyn Store,
    ) -> Result<Self> {
        let (minimum, maximum) = Self::limit_new(plan, Some(store))?;
        let pooled_memory =
            StaticMemory::new(base, minimum, maximum, memory_image, memory_and_guard_size)?;
        let allocation = Box::new(pooled_memory);
        let allocation: Box<dyn RuntimeLinearMemory> = if plan.memory.shared {
            // FIXME: since the pooling allocator owns the memory allocation
            // (which is torn down with the instance), the current shared memory
            // implementation will cause problems; see
            // https://github.com/bytecodealliance/wasmtime/issues/4244.
            todo!("using shared memory with the pooling allocator is a work in progress");
        } else {
            allocation
        };
        Ok(Memory(allocation))
    }

    /// Calls the `store`'s limiter to optionally prevent a memory from being allocated.
    ///
    /// Returns the minimum size and optional maximum size of the memory, in
    /// bytes.
    fn limit_new(
        plan: &MemoryPlan,
        store: Option<&mut dyn Store>,
    ) -> Result<(usize, Option<usize>)> {
        // Sanity-check what should already be true from wasm module validation.
        let absolute_max = if plan.memory.memory64 {
            WASM64_MAX_PAGES
        } else {
            WASM32_MAX_PAGES
        };
        assert!(plan.memory.minimum <= absolute_max);
        assert!(plan.memory.maximum.is_none() || plan.memory.maximum.unwrap() <= absolute_max);

        // This is the absolute possible maximum that the module can try to
        // allocate, which is our entire address space minus a wasm page. That
        // shouldn't ever actually work in terms of an allocation because
        // presumably the kernel wants *something* for itself, but this is used
        // to pass to the `store`'s limiter for a requested size
        // to approximate the scale of the request that the wasm module is
        // making. This is necessary because the limiter works on `usize` bytes
        // whereas we're working with possibly-overflowing `u64` calculations
        // here. To actually faithfully represent the byte requests of modules
        // we'd have to represent things as `u128`, but that's kinda
        // overkill for this purpose.
        let absolute_max = 0usize.wrapping_sub(WASM_PAGE_SIZE);

        // If the minimum memory size overflows the size of our own address
        // space, then we can't satisfy this request, but defer the error to
        // later so the `store` can be informed that an effective oom is
        // happening.
        let minimum = plan
            .memory
            .minimum
            .checked_mul(WASM_PAGE_SIZE_U64)
            .and_then(|m| usize::try_from(m).ok());

        // The plan stores the maximum size in units of wasm pages, but we
        // use units of bytes. Unlike for the `minimum` size we silently clamp
        // the effective maximum size to `absolute_max` above if the maximum is
        // too large. This should be ok since as a wasm runtime we get to
        // arbitrarily decide the actual maximum size of memory, regardless of
        // what's actually listed on the memory itself.
        let mut maximum = plan.memory.maximum.map(|max| {
            usize::try_from(max)
                .ok()
                .and_then(|m| m.checked_mul(WASM_PAGE_SIZE))
                .unwrap_or(absolute_max)
        });

        // If this is a 32-bit memory and no maximum is otherwise listed then we
        // need to still specify a maximum size of 4GB. If the host platform is
        // 32-bit then there's no need to limit the maximum this way since no
        // allocation of 4GB can succeed, but for 64-bit platforms this is
        // required to limit memories to 4GB.
        if !plan.memory.memory64 && maximum.is_none() {
            maximum = usize::try_from(1u64 << 32).ok();
        }

        // Inform the store's limiter what's about to happen. This will let the
        // limiter reject anything if necessary, and this also guarantees that
        // we should call the limiter for all requested memories, even if our
        // `minimum` calculation overflowed. This means that the `minimum` we're
        // informing the limiter is lossy and may not be 100% accurate, but for
        // now the expected uses of limiter means that's ok.
        if let Some(store) = store {
            // We ignore the store limits for shared memories since they are
            // technically not created within a store (though, trickily, they
            // may be associated with one in order to get a `vmctx`).
            if !plan.memory.shared {
                if !store.memory_growing(0, minimum.unwrap_or(absolute_max), maximum)? {
                    bail!(
                        "memory minimum size of {} pages exceeds memory limits",
                        plan.memory.minimum
                    );
                }
            }
        }

        // At this point we need to actually handle overflows, so bail out with
        // an error if we made it this far.
        let minimum = minimum.ok_or_else(|| {
            format_err!(
                "memory minimum size of {} pages exceeds memory limits",
                plan.memory.minimum
            )
        })?;
        Ok((minimum, maximum))
    }

    /// Returns the number of allocated wasm pages.
    pub fn byte_size(&self) -> usize {
        self.0.byte_size()
    }

    /// Returns the maximum number of pages the memory can grow to at runtime.
    ///
    /// Returns `None` if the memory is unbounded.
    ///
    /// The runtime maximum may not be equal to the maximum from the linear memory's
    /// Wasm type when it is being constrained by an instance allocator.
    pub fn maximum_byte_size(&self) -> Option<usize> {
        self.0.maximum_byte_size()
    }

    /// Returns whether or not this memory needs initialization. It
    /// may not if it already has initial content thanks to a CoW
    /// mechanism.
    pub(crate) fn needs_init(&self) -> bool {
        self.0.needs_init()
    }

    /// Grow memory by the specified amount of wasm pages.
    ///
    /// Returns `None` if memory can't be grown by the specified amount
    /// of wasm pages. Returns `Some` with the old size of memory, in bytes, on
    /// successful growth.
    ///
    /// # Safety
    ///
    /// Resizing the memory can reallocate the memory buffer for dynamic memories.
    /// An instance's `VMContext` may have pointers to the memory's base and will
    /// need to be fixed up after growing the memory.
    ///
    /// Generally, prefer using `InstanceHandle::memory_grow`, which encapsulates
    /// this unsafety.
    ///
    /// Ensure that the provided Store is not used to get access any Memory
    /// which lives inside it.
    pub unsafe fn grow(
        &mut self,
        delta_pages: u64,
        store: Option<&mut dyn Store>,
    ) -> Result<Option<usize>, Error> {
        self.0
            .grow(delta_pages, store)
            .map(|opt| opt.map(|(old, _new)| old))
    }

    /// Return a `VMMemoryDefinition` for exposing the memory to compiled wasm code.
    pub fn vmmemory(&mut self) -> VMMemoryDefinition {
        self.0.vmmemory()
    }

    /// Consume the memory, returning its [`MemoryImageSlot`] if any is present.
    /// The image should only be present for a subset of memories created with
    /// [`Memory::new_static()`].
    #[cfg(feature = "pooling-allocator")]
    pub fn unwrap_static_image(mut self) -> MemoryImageSlot {
        let mem = self.0.as_any_mut().downcast_mut::<StaticMemory>().unwrap();
        std::mem::replace(&mut mem.memory_image, MemoryImageSlot::dummy())
    }

    /// If the [Memory] is a [SharedMemory], unwrap it and return a clone to
    /// that shared memory.
    pub fn as_shared_memory(&mut self) -> Option<&mut SharedMemory> {
        let as_any = self.0.as_any_mut();
        if let Some(m) = as_any.downcast_mut::<SharedMemory>() {
            Some(m)
        } else {
            None
        }
    }

    /// Implementation of `memory.atomic.notify` for all memories.
    pub fn atomic_notify(&mut self, addr: u64, count: u32) -> Result<u32, Trap> {
        match self.0.as_any_mut().downcast_mut::<SharedMemory>() {
            Some(m) => m.atomic_notify(addr, count),
            None => {
                validate_atomic_addr(&self.vmmemory(), addr, 4, 4)?;
                Ok(0)
            }
        }
    }

    /// Implementation of `memory.atomic.wait32` for all memories.
    pub fn atomic_wait32(
        &mut self,
        addr: u64,
        expected: u32,
        deadline: Option<Instant>,
    ) -> Result<WaitResult, Trap> {
        match self.0.as_any_mut().downcast_mut::<SharedMemory>() {
            Some(m) => m.atomic_wait32(addr, expected, deadline),
            None => {
                validate_atomic_addr(&self.vmmemory(), addr, 4, 4)?;
                Err(Trap::AtomicWaitNonSharedMemory)
            }
        }
    }

    /// Implementation of `memory.atomic.wait64` for all memories.
    pub fn atomic_wait64(
        &mut self,
        addr: u64,
        expected: u64,
        deadline: Option<Instant>,
    ) -> Result<WaitResult, Trap> {
        match self.0.as_any_mut().downcast_mut::<SharedMemory>() {
            Some(m) => m.atomic_wait64(addr, expected, deadline),
            None => {
                validate_atomic_addr(&self.vmmemory(), addr, 8, 8)?;
                Err(Trap::AtomicWaitNonSharedMemory)
            }
        }
    }

    /// Returns the range of bytes that WebAssembly should be able to address in
    /// this linear memory. Note that this includes guard pages which wasm can
    /// hit.
    pub fn wasm_accessible(&self) -> Range<usize> {
        self.0.wasm_accessible()
    }
}

/// In the configurations where bounds checks were elided in JIT code (because
/// we are using static memories with virtual memory guard pages) this manual
/// check is here so we don't segfault from Rust. For other configurations,
/// these checks are required anyways.
fn validate_atomic_addr(
    def: &VMMemoryDefinition,
    addr: u64,
    access_size: u64,
    access_alignment: u64,
) -> Result<*mut u8, Trap> {
    debug_assert!(access_alignment.is_power_of_two());
    if !(addr % access_alignment == 0) {
        return Err(Trap::HeapMisaligned);
    }

    let length = u64::try_from(def.current_length()).unwrap();
    if !(addr.saturating_add(access_size) < length) {
        return Err(Trap::MemoryOutOfBounds);
    }

    Ok(def.base.wrapping_add(addr as usize))
}