xref: /wasmtime-44.0.1/crates/wasmtime/src/runtime/vm/memory.rs (revision dfc4358d)

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

use crate::prelude::*;
use crate::runtime::vm::mmap::Mmap;
use crate::runtime::vm::vmcontext::VMMemoryDefinition;
use crate::runtime::vm::{
    round_usize_up_to_host_pages, usize_is_multiple_of_host_page_size, MemoryImage,
    MemoryImageSlot, SendSyncPtr, SharedMemory, Store, WaitResult,
};
use alloc::sync::Arc;
use core::ops::Range;
use core::ptr::NonNull;
use core::time::Duration;
use wasmtime_environ::{MemoryPlan, MemoryStyle, Trap};

/// 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's backing storage.
///
/// This does not a full Wasm linear memory, as it may
pub trait RuntimeLinearMemory: Send + Sync {
    /// Returns the log2 of this memory's page size, in bytes.
    fn page_size_log2(&self) -> u8;

    /// Returns this memory's page size, in bytes.
    fn page_size(&self) -> u64 {
        let log2 = self.page_size_log2();
        debug_assert!(log2 == 16 || log2 == 0);
        1 << self.page_size_log2()
    }

    /// 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)));
        }

        let page_size = usize::try_from(self.page_size()).unwrap();

        // 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(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(page_size)
            .saturating_add(old_byte_size)
            .min(absolute_max);

        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 core::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 current length of this Wasm memory, in bytes.
    //
    // This region starts at `pre_guard_size` offset from the base of `mmap`. It
    // is always accessible, which means that if the Wasm page size is smaller
    // than the host page size, there may be some trailing region in the `mmap`
    // that is accessible but should not be accessed. (We rely on explicit
    // bounds checks in the compiled code to protect this region.)
    len: 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.
    //
    // This is *not* always a multiple of the host page size, and
    // `self.accessible()` may go past `self.maximum` when Wasm is using a small
    // custom page size due to `self.accessible()`'s rounding up to the host
    // page size.
    maximum: Option<usize>,

    // The log2 of this Wasm memory's page size, in bytes.
    page_size_log2: u8,

    // 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();

        // Ensure that our guard regions are multiples of the host page size.
        let offset_guard_bytes = round_usize_up_to_host_pages(offset_guard_bytes)?;
        let pre_guard_bytes = round_usize_up_to_host_pages(pre_guard_bytes)?;

        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 } => (
                round_usize_up_to_host_pages(minimum)?,
                round_usize_up_to_host_pages(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 { byte_reservation } => {
                assert!(byte_reservation >= plan.memory.minimum_byte_size().unwrap());
                let bound_bytes = usize::try_from(byte_reservation).unwrap();
                let bound_bytes = round_usize_up_to_host_pages(bound_bytes)?;
                maximum = Some(bound_bytes.min(maximum.unwrap_or(usize::MAX)));
                (bound_bytes, 0)
            }
        };
        assert!(usize_is_multiple_of_host_page_size(alloc_bytes));

        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))?;
        assert!(usize_is_multiple_of_host_page_size(request_bytes));

        let mut mmap = Mmap::accessible_reserved(0, request_bytes)?;

        if minimum > 0 {
            let accessible = round_usize_up_to_host_pages(minimum)?;
            mmap.make_accessible(pre_guard_bytes, accessible)?;
        }

        // 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,
            len: minimum,
            maximum,
            page_size_log2: plan.memory.page_size_log2,
            pre_guard_size: pre_guard_bytes,
            offset_guard_size: offset_guard_bytes,
            extra_to_reserve_on_growth,
            memory_image,
        })
    }

    /// Get the length of the accessible portion of the underlying `mmap`. This
    /// is the same region as `self.len` but rounded up to a multiple of the
    /// host page size.
    fn accessible(&self) -> usize {
        let accessible =
            round_usize_up_to_host_pages(self.len).expect("accessible region always fits in usize");
        debug_assert!(accessible <= self.mmap.len() - self.offset_guard_size - self.pre_guard_size);
        accessible
    }
}

impl RuntimeLinearMemory for MmapMemory {
    fn page_size_log2(&self) -> u8 {
        self.page_size_log2
    }

    fn byte_size(&self) -> usize {
        self.len
    }

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

    fn grow_to(&mut self, new_size: usize) -> Result<()> {
        assert!(usize_is_multiple_of_host_page_size(self.offset_guard_size));
        assert!(usize_is_multiple_of_host_page_size(self.pre_guard_size));
        assert!(usize_is_multiple_of_host_page_size(self.mmap.len()));

        let new_accessible = round_usize_up_to_host_pages(new_size)?;
        if new_accessible > 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_accessible)
                .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"))?;
            assert!(usize_is_multiple_of_host_page_size(request_bytes));

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

            // This method has an exclusive reference to `self.mmap` and just
            // created `new_mmap` so it should be safe to acquire references
            // into both of them and copy between them.
            unsafe {
                let range = self.pre_guard_size..self.pre_guard_size + self.len;
                let src = self.mmap.slice(range.clone());
                let dst = new_mmap.slice_mut(range);
                dst.copy_from_slice(src);
            }

            // 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.len);
            assert!(self.maximum.map_or(true, |max| new_size <= max));
            assert!(new_size <= self.mmap.len() - self.offset_guard_size - self.pre_guard_size);

            let new_accessible = round_usize_up_to_host_pages(new_size)?;
            assert!(
                new_accessible <= self.mmap.len() - self.offset_guard_size - self.pre_guard_size,
            );

            // If the Wasm memory's page size is smaller than the host's page
            // size, then we might not need to actually change permissions,
            // since we are forced to round our accessible range up to the
            // host's page size.
            if new_accessible > self.accessible() {
                self.mmap.make_accessible(
                    self.pre_guard_size + self.accessible(),
                    new_accessible - self.accessible(),
                )?;
            }
        }

        self.len = new_size;

        Ok(())
    }

    fn vmmemory(&mut self) -> VMMemoryDefinition {
        VMMemoryDefinition {
            base: unsafe { self.mmap.as_mut_ptr().add(self.pre_guard_size) },
            current_length: self.len.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 core::any::Any {
        self
    }

    fn wasm_accessible(&self) -> Range<usize> {
        let base = self.mmap.as_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 base pointer of this static memory, wrapped up in a send/sync
    /// wrapper.
    base: SendSyncPtr<u8>,

    /// The byte capacity of the `base` pointer.
    capacity: usize,

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

    /// The log2 of this memory's page size.
    page_size_log2: u8,

    /// 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_ptr: *mut u8,
        base_capacity: usize,
        initial_size: usize,
        maximum_size: Option<usize>,
        page_size_log2: u8,
        memory_image: MemoryImageSlot,
        memory_and_guard_size: usize,
    ) -> Result<Self> {
        if base_capacity < initial_size {
            bail!(
                "initial memory size of {} exceeds the pooling allocator's \
                 configured maximum memory size of {} bytes",
                initial_size,
                base_capacity,
            );
        }

        // Only use the part of the slice that is necessary.
        let base_capacity = match maximum_size {
            Some(max) if max < base_capacity => max,
            _ => base_capacity,
        };

        Ok(Self {
            base: SendSyncPtr::new(NonNull::new(base_ptr).unwrap()),
            capacity: base_capacity,
            size: initial_size,
            page_size_log2,
            memory_image,
            memory_and_guard_size,
        })
    }
}

impl RuntimeLinearMemory for StaticMemory {
    fn page_size_log2(&self) -> u8 {
        self.page_size_log2
    }

    fn byte_size(&self) -> usize {
        self.size
    }

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

    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.capacity);

        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_ptr(),
            current_length: self.size.into(),
        }
    }

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

    fn as_any_mut(&mut self) -> &mut dyn core::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
    }
}

/// Representation of a runtime wasm linear memory.
pub struct Memory(pub(crate) 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_ptr: *mut u8,
        base_capacity: usize,
        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_ptr,
            base_capacity,
            minimum,
            maximum,
            plan.memory.page_size_log2,
            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 a tuple of the minimum size, optional maximum size, and log(page
    /// size) of the memory, all in bytes.
    pub(crate) fn limit_new(
        plan: &MemoryPlan,
        store: Option<&mut dyn Store>,
    ) -> Result<(usize, Option<usize>)> {
        let page_size = usize::try_from(plan.memory.page_size()).unwrap();

        // 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(page_size);

        // Sanity-check what should already be true from wasm module validation.
        if let Ok(size) = plan.memory.minimum_byte_size() {
            assert!(size <= u64::try_from(absolute_max).unwrap());
        }
        if let Ok(max) = plan.memory.maximum_byte_size() {
            assert!(max <= u64::try_from(absolute_max).unwrap());
        }

        // 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_byte_size()
            .ok()
            .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 the limits of what we can track. If the
        // maximum size exceeds `usize` or `u64` then there's no need to further
        // keep track of it as some sort of runtime limit will kick in long
        // before we reach the statically declared maximum size.
        let maximum = plan
            .memory
            .maximum_byte_size()
            .ok()
            .and_then(|m| usize::try_from(m).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 this memory's page size, in bytes.
    pub fn page_size(&self) -> u64 {
        self.0.page_size()
    }

    /// Returns the number of allocated wasm pages.
    pub fn byte_size(&self) -> usize {
        self.0.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();
        core::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,
        timeout: Option<Duration>,
    ) -> Result<WaitResult, Trap> {
        match self.0.as_any_mut().downcast_mut::<SharedMemory>() {
            Some(m) => m.atomic_wait32(addr, expected, timeout),
            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,
        timeout: Option<Duration>,
    ) -> Result<WaitResult, Trap> {
        match self.0.as_any_mut().downcast_mut::<SharedMemory>() {
            Some(m) => m.atomic_wait64(addr, expected, timeout),
            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.
pub 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))
}