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-//! A buddy allocator, used to allocate pages.
-//!
-//! The allocator can be split into three abstractions: stripes, trees, and the allocator.
-//!
-//! TODO: See if there's standard terminology for these.
-//!
-//! ## Stripes
-//!
-//! The buddy allocator works in terms of size classes, which are power-of-two sized, starting at a
-//! single page and going up from there. Each stripe corresponds to a single size class and a
-//! particular region of memory.
-//!
-//! A stripe contains a circular doubly-linked free-list for subregions of that size class, and a
-//! bitset marking whether a particular region has been allocated or not. Being a circular
-//! doubly-linked list makes it cheap to remove an element whose address we know, as well as cheap
-//! to push and pop elements.
-//!
-//! It's efficient to go from the address of a subregion to the index of its corresponding bit, so
-//! when we hand out a subregion from the free-list or put one back, it's cheap to read or write
-//! its bit.
-//!
-//! ## Trees
-//!
-//! A tree is logically a collection of stripes, one per size class. To pack the structures more
-//! efficiently, they are stored interleaved with each other, and the tree manages this.
-//!
-//! The important logic at the level of trees happens when we allocate a subregion from a size
-//! class whose stripe's free-list is empty, and when we free subregions.
-//!
-//! When a stripe's free-list is empty, the tree instead allocates a subregion of a larger size
-//! from the next stripe. It can then store the unused portion in the current size class.
-//!
-//! The really important bit is the ability to merge subregions when they're freed. When we free a
-//! subregion of a certain size class, we can check whether its neighbor (its buddy) is unallocated
-//! as well. If so, we can remove it from the free-list by its address. We can combine the two
-//! subregions into one of the next larger size class, and then return that subregion to the next
-//! stripe.
-//!
-//! ## The buddy allocator
-//!
-//! Finally, the overall buddy allocator needs to be able to handle multiple memory regions. To
-//! facilitate this, the trees are stored in an array, which forms the overall allocator.
-#![no_std]
-
-extern crate std; // TODO: Remove me
-
-mod bitvec;
-mod free_list;
-mod tree;
-
-use crate::{
- bitvec::{Bitset, SubregionStatus},
- free_list::{FreeList, FreeListNode},
- tree::Tree,
-};
-use allocator_api2::alloc::{AllocError, Layout, LayoutError};
-use contracts::{ensures, requires};
-use core::{fmt, mem, ptr::NonNull};
-use vernos_physmem_free_list::FreeListAllocator;
-
-/// A buddy allocator.
-pub struct BuddyAllocator<
- 'allocator,
- const PAGE_SIZE: usize,
- const PAGE_SIZE_BITS: usize,
- const SIZE_CLASS_COUNT: usize,
-> {
- /// The free list sentinels (`SIZE_CLASS_COUNT` of them).
- free_list_sentinels: NonNull<FreeListNode>,
-
- /// The metadata associated with each tree.
- trees: &'allocator mut [Tree<'allocator, PAGE_SIZE, PAGE_SIZE_BITS>],
-
- /// The shared bitset.
- bitset: &'allocator mut Bitset,
-}
-
-impl<
- 'allocator,
- const PAGE_SIZE: usize,
- const PAGE_SIZE_BITS: usize,
- const SIZE_CLASS_COUNT: usize,
- > BuddyAllocator<'allocator, PAGE_SIZE, PAGE_SIZE_BITS, SIZE_CLASS_COUNT>
-{
- /// Returns a buddy allocator backed by the page ranges in the free list.
- pub fn new(
- mut free_list_alloc: FreeListAllocator<'allocator, PAGE_SIZE>,
- ) -> Result<BuddyAllocator<'allocator, PAGE_SIZE, PAGE_SIZE_BITS, SIZE_CLASS_COUNT>, AllocError>
- {
- assert_eq!(PAGE_SIZE, 1 << PAGE_SIZE_BITS);
- assert_ne!(SIZE_CLASS_COUNT, 0);
-
- // We split up all the nodes into size-class-sized chunks here. After this, it's important
- // for safety that we never split a node again -- we'll see why later.
- free_list_alloc.split_to_powers_of_two_no_larger_than(1 << (SIZE_CLASS_COUNT - 1));
-
- // We use one contiguous region to store the free lists and all the information about each
- // tree (the base, the maximum order, and the bitset). We do one pass through the free list
- // to find out how much storage we need, then we try to find a convenient spot to steal the
- // storage we need from a page range in the list.
- let mut range_count = 0;
- let mut size_class_counts = [0; SIZE_CLASS_COUNT];
- for (_, len_pages) in free_list_alloc.iter() {
- assert!(len_pages.is_power_of_two());
- let size_class = len_pages.trailing_zeros() as usize;
- assert!((0..SIZE_CLASS_COUNT).contains(&size_class));
- range_count += 1;
- size_class_counts[size_class] += 1;
- }
-
- // Lay out the metadata.
- //
- // UNWRAP: The error here really ought to be impossible, because the pages in the free list
- // needed to fit in there in the first place.
- let metadata_layout = MetadataLayout::<PAGE_SIZE, PAGE_SIZE_BITS, SIZE_CLASS_COUNT>::new(
- range_count,
- size_class_counts,
- )
- .unwrap();
-
- // Allocate space for the metadata.
- //
- // SAFETY: We need to make _absolutely_ sure nothing in the buddy allocator uses this
- // memory before we have a chance to remove it from the allocator. This function guarantees
- // that the returned memory won't overlap with the first page of a live node, so as long as
- // we don't split a node or write past the first page of one, we should be OK.
- let metadata = unsafe {
- free_list_alloc.allocate_without_always_removing_the_node(metadata_layout.layout)?
- };
- // SAFETY: The pointer must be valid to have been returned here.
- unsafe {
- metadata.cast::<u8>().write_bytes(0, metadata.len());
- }
-
- // Break out each component of the metadata.
- //
- // SAFETY: The layout is computed to make all of this sound if the data is valid. All of
- // these types are valid when zero-initialized.
- let free_list_sentinels: NonNull<FreeListNode> = unsafe {
- metadata
- .byte_add(metadata_layout.free_list_sentinels_offset)
- .cast()
- };
- let trees: &'allocator mut [Tree<PAGE_SIZE, PAGE_SIZE_BITS>] = unsafe {
- NonNull::slice_from_raw_parts(
- metadata.byte_add(metadata_layout.trees_offset).cast(),
- metadata_layout.trees_len,
- )
- .as_mut()
- };
- // SAFETY: This type is valid when zero-initialized, and is a newtype for `[usize]` so has
- // the same layout.
- let bitset: &'allocator mut Bitset = unsafe {
- mem::transmute::<&mut [usize], &mut Bitset>(
- NonNull::slice_from_raw_parts(
- metadata.byte_add(metadata_layout.bitset_offset).cast(),
- metadata_layout.bitset_len,
- )
- .as_mut(),
- )
- };
-
- // Self-link each free list. Although they're valid in a UB sense when zeroed, they're not
- // yet ready to use until this is done.
- for size_class in 0..SIZE_CLASS_COUNT {
- // SAFETY: This is the number of nodes we allocate, so we'll stay in-bounds.
- unsafe { FreeListNode::init_sentinel(free_list_sentinels.add(size_class)) };
- }
-
- // Initialize the trees, adding subregions into the allocator as we do. We don't actually
- // assign bitset offsets yet, because we're going to sort the trees by address before we
- // do. (This isn't algorithmically necessary, but it makes debugging a bit easier.)
- let mut i = 0;
- while let Some((ptr, mut len_pages)) = free_list_alloc.pop() {
- while len_pages > 0 {
- let size_class = (len_pages.trailing_zeros() as usize).min(SIZE_CLASS_COUNT - 1);
- trees[i].base_ptr = Some(ptr.cast());
- trees[i].size_class = size_class;
- i += 1;
- len_pages -= 1 << size_class;
- }
- }
- assert!(
- i == trees.len() || i + 1 == trees.len(),
- "found {} trees but allocated {} slots",
- i,
- trees.len()
- );
-
- // Slice the trees slice to fit how many trees actually remained, then sort them by
- // address.
- let trees = &mut trees[..i];
- trees.sort_by_key(|tree| tree.base_addr());
-
- // Compute the number of bits that belong to each tree and assign them.
- let mut bitset_offset = 0;
- for tree in &mut *trees {
- tree.bitset_offset = bitset_offset;
- bitset_offset += (1 << (tree.size_class + 1)) - 1;
- }
- assert!(bitset_offset <= metadata_layout.bitset_len_bits);
-
- // Add regions the trees should own to the free lists, excluding the subregions that
- // metadata is allocated in.
-
- // Since we know that the metadata was allocated at the end of the page range it was
- // allocated in, we can just test the end addresses.
- //
- // SAFETY: This is the one-past-the-end address.
- let metadata_addr_hi =
- unsafe { metadata.cast::<u8>().add(metadata.len()).as_ptr() } as usize;
- for tree in &mut *trees {
- // SAFETY: This is the one-past-the-end address.
- let tree_addr_hi = tree.base_addr() + (PAGE_SIZE << tree.size_class);
- if metadata_addr_hi == tree_addr_hi {
- // If the metadata was present inside the tree... TODO.
- std::eprintln!("TODO");
- } else {
- // Otherwise, we can take the whole range of the tree and add it to the free list.
-
- // SAFETY: There are as many sentinels as size classes, so this will be in-bounds.
- let sentinel = unsafe { free_list_sentinels.add(tree.size_class) };
-
- // SAFETY: The free list is kept valid.
- let mut free_list = unsafe { FreeList::from_sentinel(sentinel) };
-
- // SAFETY: This region is not owned or borrowed by anything else (the only thing it
- // could be would be the metadata, but that's not in this tree), is of the correct
- // size, and is composed of untyped memory.
- unsafe { free_list.push(tree.base_ptr.unwrap().cast()) };
-
- // Then, set a bit in the bitset to say that this region is present.
- tree.bitset_mark_as_present(bitset, tree.size_class, 0);
- }
- }
-
- // Make the buddy allocator's struct and return it.
- Ok(BuddyAllocator {
- free_list_sentinels,
- trees,
- bitset,
- })
- }
-
- /// Tries to allocate a subregion of a particular size class from the allocator.
- #[requires(size_class < SIZE_CLASS_COUNT)]
- pub fn alloc(&mut self, size_class: usize) -> Result<NonNull<u8>, AllocError> {
- if let Some(ptr) = self.free_list(size_class).pop() {
- // Fast-path: try allocating from the right size class's free list.
-
- // Find the corresponding tree and the offset of the subregion in the tree.
- let (tree_index, offset) = self.tree_index_and_offset(ptr);
- let tree = &mut self.trees[tree_index];
-
- // Mark the pointer as no longer being in the tree.
- tree.bitset_mark_as_absent(&mut self.bitset, size_class, offset);
-
- // Return the pointer.
- Ok(ptr)
- } else {
- // Try finding a larger chunk of memory in a larger size class.
- Err(AllocError)
- }
- }
-
- /// Returns a subregion of a particular size class to the allocator.
- ///
- /// # Safety
- ///
- /// - `ptr` must have been returned by a call to `alloc` with the same `size_class`.
- /// - `ptr` must convey ownership over the entire region.
- #[requires(size_class < SIZE_CLASS_COUNT)]
- pub unsafe fn dealloc(&mut self, ptr: NonNull<u8>, size_class: usize) {
- // Find the corresponding tree and the offset of the subregion in the tree.
- let (tree_index, offset) = self.tree_index_and_offset(ptr);
- let tree = &mut self.trees[tree_index];
-
- // Ensure that the memory is marked as not being in the free list.
- assert_eq!(
- tree.bitset_get(&mut self.bitset, size_class, offset),
- SubregionStatus::NotInFreeList
- );
-
- // Start combining size classes.
- assert!(size_class <= tree.size_class);
- while size_class < tree.size_class {
- let buddy_offset = offset ^ (PAGE_SIZE << size_class);
- todo!("{buddy_offset:#x} {offset:#x}");
- }
-
- // Mark the pointer as being in the tree.
- tree.bitset_mark_as_present(&mut self.bitset, size_class, offset);
-
- // Return the pointer to the appropriate free list.
- self.free_list(size_class).push(ptr);
- }
-
- /// Returns the free list with the given size class.
- #[requires(size_class < SIZE_CLASS_COUNT)]
- fn free_list(&mut self, size_class: usize) -> FreeList {
- // SAFETY: The bounds-check is a precondition.
- let sentinel = unsafe { self.free_list_sentinels.add(size_class) };
-
- // SAFETY: The free list is kept valid.
- unsafe { FreeList::from_sentinel(sentinel) }
- }
-
- /// Returns the index of the tree this pointer was allocated in.
- #[ensures(self.trees[ret.0].contains(ptr.as_ptr() as usize))]
- #[ensures(ret.1 < (1 << self.trees[ret.0].size_class))]
- fn tree_index_and_offset(&self, ptr: NonNull<u8>) -> (usize, usize) {
- let addr = ptr.as_ptr() as usize;
- let tree_index = self
- .trees
- .binary_search_by_key(&addr, |tree| tree.base_addr());
- let tree_index = match tree_index {
- Ok(i) => i,
- Err(i) => {
- assert_ne!(i, 0);
- i - 1
- }
- };
- let offset = addr - self.trees[tree_index].base_addr();
- (tree_index, offset)
- }
-}
-
-impl<
- 'allocator,
- const PAGE_SIZE: usize,
- const PAGE_SIZE_BITS: usize,
- const SIZE_CLASS_COUNT: usize,
- > fmt::Debug for BuddyAllocator<'allocator, PAGE_SIZE, PAGE_SIZE_BITS, SIZE_CLASS_COUNT>
-{
- fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
- struct FreeLists<const SIZE_CLASS_COUNT: usize>(NonNull<FreeListNode>);
-
- impl<const SIZE_CLASS_COUNT: usize> fmt::Debug for FreeLists<SIZE_CLASS_COUNT> {
- fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
- fmt.debug_list()
- .entries((0..SIZE_CLASS_COUNT).map(|size_class| {
- // SAFETY: The free lists are kept valid, and the range of size classes is
- // necessarily in-bounds.
- unsafe { FreeList::from_sentinel(self.0.add(size_class)) }
- }))
- .finish()
- }
- }
-
- fmt.debug_struct("BuddyAllocator")
- .field(
- "free_lists",
- &FreeLists::<SIZE_CLASS_COUNT>(self.free_list_sentinels),
- )
- .field("trees", &self.trees)
- .field("bitset", &self.bitset)
- .finish()
- }
-}
-
-#[derive(Debug)]
-struct MetadataLayout<
- const PAGE_SIZE: usize,
- const PAGE_SIZE_BITS: usize,
- const SIZE_CLASS_COUNT: usize,
-> {
- layout: Layout,
- free_list_sentinels_offset: usize,
- trees_offset: usize,
- trees_len: usize,
- bitset_offset: usize,
- bitset_len: usize,
- bitset_len_bits: usize,
-}
-
-impl<const PAGE_SIZE: usize, const PAGE_SIZE_BITS: usize, const SIZE_CLASS_COUNT: usize>
- MetadataLayout<PAGE_SIZE, PAGE_SIZE_BITS, SIZE_CLASS_COUNT>
-{
- fn new(
- range_count: usize,
- size_class_counts: [usize; SIZE_CLASS_COUNT],
- ) -> Result<MetadataLayout<PAGE_SIZE, PAGE_SIZE_BITS, SIZE_CLASS_COUNT>, LayoutError> {
- let trees_len = range_count;
- let mut bitset_len_bits = 0;
- for (size_class, &count) in size_class_counts.iter().enumerate() {
- let bits_for_size_class = (1 << (size_class + 1)) - 1;
- bitset_len_bits += bits_for_size_class * count;
- }
- let bitset_len = (bitset_len_bits + usize::BITS as usize - 1) / usize::BITS as usize;
-
- let free_list_sentinels_layout = Layout::new::<[FreeListNode; SIZE_CLASS_COUNT]>();
- let trees_layout = Layout::array::<Tree<PAGE_SIZE, PAGE_SIZE_BITS>>(trees_len)?;
- let bitset_layout = Layout::array::<usize>(bitset_len)?;
-
- let free_list_sentinels_offset = 0;
- let (layout, trees_offset) = free_list_sentinels_layout.extend(trees_layout)?;
- let (layout, bitset_offset) = layout.extend(bitset_layout)?;
-
- Ok(MetadataLayout {
- layout,
- free_list_sentinels_offset,
- trees_offset,
- trees_len,
- bitset_offset,
- bitset_len,
- bitset_len_bits,
- })
- }
-}
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