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Implementation of a segmented heap #84

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Implementation of a segmented heap #84

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18 changes: 11 additions & 7 deletions src/hole.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -388,17 +388,21 @@ impl HoleList {
// NOTE: We could probably replace this with an `Option` instead of a `Result` in a later
// release to remove this clippy warning
#[allow(clippy::result_unit_err)]
pub fn allocate_first_fit(&mut self, layout: Layout) -> Result<(NonNull<u8>, Layout), ()> {
let aligned_layout = Self::align_layout(layout).map_err(|_| ())?;
let mut cursor = self.cursor().ok_or(())?;
pub fn allocate_first_fit(&mut self, layout: Layout) -> Option<(NonNull<u8>, Layout)> {
let aligned_layout = Self::align_layout(layout).ok()?;
let mut cursor = self.cursor()?;

loop {
match cursor.split_current(aligned_layout) {
Ok((ptr, _len)) => {
return Ok((NonNull::new(ptr).ok_or(())?, aligned_layout));
return Some((
// SAFETY: This can not be null as it is derived from a NonNull pointer in `split_current`
unsafe { NonNull::new_unchecked(ptr) }
, aligned_layout
));
}
Err(curs) => {
cursor = curs.next().ok_or(())?;
cursor = curs.next()?;
}
}
}
Expand All @@ -419,7 +423,7 @@ impl HoleList {
/// The function performs exactly the same layout adjustments as [`allocate_first_fit`] and
/// returns the aligned layout.
pub unsafe fn deallocate(&mut self, ptr: NonNull<u8>, layout: Layout) -> Layout {
let aligned_layout = Self::align_layout(layout).unwrap();
let aligned_layout = Self::align_layout(layout).expect("This should never error, as the validity was checked during allocation.");
deallocate(self, ptr.as_ptr(), aligned_layout.size());
aligned_layout
}
Expand Down Expand Up @@ -689,7 +693,7 @@ pub mod test {
#[test]
fn cursor() {
let mut heap = new_heap();
let curs = heap.holes.cursor().unwrap();
let curs = heap.holes[0].cursor().unwrap();
// This is the "dummy" node
assert_eq!(curs.previous().size, 0);
// This is the "full" heap
Expand Down
230 changes: 160 additions & 70 deletions src/lib.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -28,36 +28,180 @@ pub mod hole;
mod test;

/// A fixed size heap backed by a linked list of free memory blocks.
pub struct Heap {
/*pub struct Heap {
used: usize,
holes: HoleList,
} */
pub struct SegmentedHeap<const SEGMENTS: usize> {
used: usize,
holes: [HoleList; SEGMENTS]
}

#[cfg(fuzzing)]
pub type Heap = SegmentedHeap<1>;

#[cfg(any(test, fuzzing))]
impl Heap {
pub fn debug(&mut self) {
println!(
"bottom: {:?}, top: {:?}, size: {}, pending: {}",
self.bottom(),
self.top(),
self.size(),
self.holes.first.size,
self.holes[0].first.size,
);
self.holes.debug();
self.holes[0].debug();
}
}

unsafe impl Send for Heap {}

impl Heap {

impl<const SEGMENTS: usize> SegmentedHeap<SEGMENTS> {
/// Creates an empty heap. All allocate calls will return `None`.
pub const fn empty() -> Heap {
Heap {
pub const fn empty() -> Self {
Self {
used: 0,
holes: HoleList::empty(),
holes: unsafe { core::mem::zeroed() },
}
}

/// Allocates a chunk of the given size with the given alignment. Returns a pointer to the
/// beginning of that chunk if it was successful. Else it returns `None`.
/// This function scans the list of free memory blocks and uses the first block that is big
/// enough. The runtime is in O(n) where n is the number of free blocks, but it should be
/// reasonably fast for small allocations.
pub fn allocate_first_fit(&mut self, layout: Layout) -> Option<NonNull<u8>> {
let allocation = self.holes.iter_mut()
// Applies the allocation function to each hole list and returns the first successful allocation
.find_map(|list| list.allocate_first_fit(layout));

if let Some((allocation, layout)) = allocation {
self.used += layout.size();
Some(allocation)
} else {
None
}
}

/// Frees the given allocation. `ptr` must be a pointer returned
/// by a call to the `allocate_first_fit` function with identical size and alignment.
///
/// This function walks the list of free memory blocks and inserts the freed block at the
/// correct place. If the freed block is adjacent to another free block, the blocks are merged
/// again. This operation is in `O(n)` since the list needs to be sorted by address.
///
/// # Safety
///
/// `ptr` must be a pointer returned by a call to the [`allocate_first_fit`] function with
/// identical layout. Undefined behavior may occur for invalid arguments. Panics when none of the hole lists contain the passed pointer.
pub unsafe fn deallocate(&mut self, ptr: NonNull<u8>, layout: Layout) {
for hole_list in self.holes.iter_mut() {
// If the pointer is in the hole list's memory range, deallocate from that list and exit
if ptr.as_ptr() >= hole_list.bottom && ptr.as_ptr() < hole_list.top {
self.used -= hole_list.deallocate(ptr, layout).size();
return;
}
}
panic!("The pointer for deallocation does not match any of the allocator's memory ranges.");
}

/// Returns the size of the heap.
///
/// This is the size the heap is using for allocations, not necessarily the
/// total amount of bytes given to the heap. To determine the exact memory
/// boundaries, use [`bottom`][Self::bottom] and [`top`][Self::top].
pub fn size(&self) -> usize {
unsafe {
self.holes
.iter()
.map(|hole_list| hole_list.top.offset_from(hole_list.bottom) as usize)
.sum()
}
}

/// Returns the size of the used part of the heap
pub fn used(&self) -> usize {
self.used
}

/// Returns the size of the free part of the heap
pub fn free(&self) -> usize {
self.size() - self.used
}

/// Initializes an empty heap
///
/// The `heap_bottom` pointer is automatically aligned, so the [`bottom()`][Self::bottom]
/// method might return a pointer that is larger than `heap_bottom` after construction.
///
/// The given `heap_size`s must be large enough to store the required
/// metadata, otherwise this function will panic. Depending on the
/// alignment of the `hole_addr` pointer, the minimum size is between
/// `2 * size_of::<usize>` and `3 * size_of::<usize>`.
///
/// The usable size for allocations will be truncated to the nearest
/// alignment of `align_of::<usize>`. Any extra bytes left at the end
/// will be reclaimed once sufficient additional space is given to
/// [`extend`][Heap::extend].
///
/// # Safety
///
/// This function must be called at most once and must only be used on an
/// empty heap.
///
/// The bottom addresses must be valid and the memory in the
/// `[heap_bottom, heap_bottom + heap_size)` range must not be used for anything else and must not overlap.
/// This function is unsafe because it can cause undefined behavior if the given addresses
/// are invalid.
///
/// The provided memory range must be valid for the `'static` lifetime.
pub unsafe fn init_from_pointers(&mut self, heaps: [(*mut u8, usize); SEGMENTS]) {
self.used = 0;
for (index, (heap_bottom, heap_size)) in heaps.iter().enumerate() {
self.holes[index] = HoleList::new(*heap_bottom, *heap_size)
}
}

/// Initialize an empty heap with provided memory.
///
/// The caller is responsible for procuring a region of raw memory that may be utilized by the
/// allocator. This might be done via any method such as (unsafely) taking a region from the
/// program's memory, from a mutable static, or by allocating and leaking such memory from
/// another allocator.
///
/// The latter approach may be especially useful if the underlying allocator does not perform
/// deallocation (e.g. a simple bump allocator). Then the overlaid linked-list-allocator can
/// provide memory reclamation.
///
/// The usable size for allocations will be truncated to the nearest
/// alignment of `align_of::<usize>`. Any extra bytes left at the end
/// will be reclaimed once sufficient additional space is given to
/// [`extend`][Heap::extend].
///
/// # Panics
///
/// This method panics if the heap is already initialized.
///
/// It also panics when the length of the given `mem` slice is not large enough to
/// store the required metadata. Depending on the alignment of the slice, the minimum
/// size is between `2 * size_of::<usize>` and `3 * size_of::<usize>`.
pub fn init_from_slices(&mut self, mut regions: [&'static mut [MaybeUninit<u8>]; SEGMENTS]) {
assert!(
self.holes.iter().all(|h| h.bottom.is_null()),
"The heap has already been initialized."
);
for (index, region) in regions.iter_mut().enumerate() {
// SAFETY: All initialization requires the bottom address to be valid, which implies it
// must not be 0. Initially the address is 0. The assertion above ensures that no
// initialization had been called before.
// The given address and size is valid according to the safety invariants of the mutable
// reference handed to us by the caller.
self.holes[index] = unsafe {HoleList::new((*region).as_mut_ptr().cast(), region.len())};
}
}
}

impl Heap {
/// Initializes an empty heap
///
/// The `heap_bottom` pointer is automatically aligned, so the [`bottom()`][Self::bottom]
Expand Down Expand Up @@ -86,7 +230,7 @@ impl Heap {
/// The provided memory range must be valid for the `'static` lifetime.
pub unsafe fn init(&mut self, heap_bottom: *mut u8, heap_size: usize) {
self.used = 0;
self.holes = HoleList::new(heap_bottom, heap_size);
self.holes[0] = HoleList::new(heap_bottom, heap_size);
}

/// Initialize an empty heap with provided memory.
Expand Down Expand Up @@ -153,7 +297,7 @@ impl Heap {
pub unsafe fn new(heap_bottom: *mut u8, heap_size: usize) -> Heap {
Heap {
used: 0,
holes: HoleList::new(heap_bottom, heap_size),
holes: [HoleList::new(heap_bottom, heap_size)],
}
}

Expand All @@ -170,55 +314,12 @@ impl Heap {
unsafe { Self::new(address, size) }
}

/// Allocates a chunk of the given size with the given alignment. Returns a pointer to the
/// beginning of that chunk if it was successful. Else it returns `None`.
/// This function scans the list of free memory blocks and uses the first block that is big
/// enough. The runtime is in O(n) where n is the number of free blocks, but it should be
/// reasonably fast for small allocations.
//
// NOTE: We could probably replace this with an `Option` instead of a `Result` in a later
// release to remove this clippy warning
#[allow(clippy::result_unit_err)]
pub fn allocate_first_fit(&mut self, layout: Layout) -> Result<NonNull<u8>, ()> {
match self.holes.allocate_first_fit(layout) {
Ok((ptr, aligned_layout)) => {
self.used += aligned_layout.size();
Ok(ptr)
}
Err(err) => Err(err),
}
}

/// Frees the given allocation. `ptr` must be a pointer returned
/// by a call to the `allocate_first_fit` function with identical size and alignment.
///
/// This function walks the list of free memory blocks and inserts the freed block at the
/// correct place. If the freed block is adjacent to another free block, the blocks are merged
/// again. This operation is in `O(n)` since the list needs to be sorted by address.
///
/// # Safety
///
/// `ptr` must be a pointer returned by a call to the [`allocate_first_fit`] function with
/// identical layout. Undefined behavior may occur for invalid arguments.
pub unsafe fn deallocate(&mut self, ptr: NonNull<u8>, layout: Layout) {
self.used -= self.holes.deallocate(ptr, layout).size();
}

/// Returns the bottom address of the heap.
///
/// The bottom pointer is automatically aligned, so the returned pointer
/// might be larger than the bottom pointer used for initialization.
pub fn bottom(&self) -> *mut u8 {
self.holes.bottom
}

/// Returns the size of the heap.
///
/// This is the size the heap is using for allocations, not necessarily the
/// total amount of bytes given to the heap. To determine the exact memory
/// boundaries, use [`bottom`][Self::bottom] and [`top`][Self::top].
pub fn size(&self) -> usize {
unsafe { self.holes.top.offset_from(self.holes.bottom) as usize }
self.holes[0].bottom
}

/// Return the top address of the heap.
Expand All @@ -227,17 +328,7 @@ impl Heap {
/// until there is enough room for metadata, but it still retains ownership
/// over memory from [`bottom`][Self::bottom] to the address returned.
pub fn top(&self) -> *mut u8 {
unsafe { self.holes.top.add(self.holes.pending_extend as usize) }
}

/// Returns the size of the used part of the heap
pub fn used(&self) -> usize {
self.used
}

/// Returns the size of the free part of the heap
pub fn free(&self) -> usize {
self.size() - self.used
unsafe { self.holes[0].top.add(self.holes[0].pending_extend as usize) }
}

/// Extends the size of the heap by creating a new hole at the end.
Expand All @@ -259,7 +350,7 @@ impl Heap {
/// by exactly `by` bytes, those bytes are still owned by the Heap for
/// later use.
pub unsafe fn extend(&mut self, by: usize) {
self.holes.extend(by);
self.holes[0].extend(by);
}
}

Expand All @@ -270,8 +361,8 @@ unsafe impl Allocator for LockedHeap {
return Ok(NonNull::slice_from_raw_parts(layout.dangling(), 0));
}
match self.0.lock().allocate_first_fit(layout) {
Ok(ptr) => Ok(NonNull::slice_from_raw_parts(ptr, layout.size())),
Err(()) => Err(AllocError),
Some(ptr) => Ok(NonNull::slice_from_raw_parts(ptr, layout.size())),
None => Err(AllocError),
}
}

Expand Down Expand Up @@ -312,7 +403,7 @@ impl LockedHeap {
pub unsafe fn new(heap_bottom: *mut u8, heap_size: usize) -> LockedHeap {
LockedHeap(Spinlock::new(Heap {
used: 0,
holes: HoleList::new(heap_bottom, heap_size),
holes: [HoleList::new(heap_bottom, heap_size)],
}))
}
}
Expand All @@ -332,7 +423,6 @@ unsafe impl GlobalAlloc for LockedHeap {
self.0
.lock()
.allocate_first_fit(layout)
.ok()
.map_or(core::ptr::null_mut(), |allocation| allocation.as_ptr())
}

Expand Down
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