path: root/crates/buddy_allocator/src/lib.rs

//! 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]

mod bitvec;
mod free_list;
mod tree;

// mod stripe;

use crate::{bitvec::BitVec, free_list::FreeList, tree::Tree};
use allocator_api2::alloc::{AllocError, Layout, LayoutError};
use contracts::requires;
use core::{mem, pin::Pin, ptr::NonNull};
use vernos_physmem_free_list::FreeListAllocator;
use vernos_utils::pin::pin_project_slice_mut_iter;

/// A buddy allocator.
#[derive(Debug)]
pub struct BuddyAllocator<
    'allocator,
    const PAGE_SIZE: usize,
    const PAGE_SIZE_BITS: usize,
    const SIZE_CLASS_COUNT: usize,
> {
    /// The free lists.
    free_lists: Pin<&'allocator mut [FreeList; SIZE_CLASS_COUNT]>,

    /// The metadata associated with each tree.
    trees: &'allocator mut [Tree<'allocator, PAGE_SIZE, PAGE_SIZE_BITS>],

    /// The shared bitset.
    bitset: &'allocator mut BitVec,
}

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: 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 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 prune 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 (_, mut len_pages) in free_list.iter() {
            while len_pages > 0 {
                let size_class = (len_pages.trailing_zeros() as usize).min(SIZE_CLASS_COUNT - 1);
                range_count += 1;
                size_class_counts[size_class] += 1;
                len_pages -= 1 << size_class;
            }
        }

        // Lay out the metadata. 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,
        )
        .map_err(|_| AllocError)?;

        // Allocate space for the metadata.
        let metadata = free_list.allocate_zeroed(metadata_layout.layout)?;

        // 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_lists: &'allocator mut [FreeList; SIZE_CLASS_COUNT] = unsafe {
            metadata
                .byte_add(metadata_layout.free_lists_offset)
                .cast()
                .as_mut()
        };
        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()
        };
        let bitset: &'allocator mut BitVec = unsafe {
            mem::transmute::<&mut [usize], &mut BitVec>(
                NonNull::slice_from_raw_parts(
                    metadata.byte_add(metadata_layout.bitset_offset).cast(),
                    metadata_layout.bitset_len,
                )
                .as_mut(),
            )
        };

        // Pin and 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.
        //
        // SAFETY: We never provide the ability to move out of a free list.
        let mut free_lists = unsafe { Pin::new_unchecked(free_lists) };
        for free_list in pin_project_slice_mut_iter(free_lists.as_mut()) {
            free_list.self_link();
        }

        // TODO: Actually initialize the trees with ranges from the allocator...

        // Make the buddy allocator and return.
        Ok(BuddyAllocator {
            free_lists,
            trees,
            bitset,
        })
    }

    /// Tries to allocate a subregion of a particular size class from the allocator.
    #[requires((0..SIZE_CLASS_COUNT).contains(&size_class))]
    pub fn alloc(&mut self, size_class: usize) -> Result<NonNull<u8>, AllocError> {
        Err(AllocError)
    }
}

#[derive(Debug)]
struct MetadataLayout<
    const PAGE_SIZE: usize,
    const PAGE_SIZE_BITS: usize,
    const SIZE_CLASS_COUNT: usize,
> {
    layout: Layout,
    free_lists_offset: usize,
    trees_offset: usize,
    trees_len: usize,
    bitset_offset: usize,
    bitset_len: 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,
        mut 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;
        // TODO: Is there a cleaner way to compute this?
        for i in (0..size_class_counts.len()).rev() {
            let count = size_class_counts[i];
            if count != 0 {
                bitset_len_bits += count;
                if i > 0 {
                    size_class_counts[i - 1] += count * 2;
                }
            }
        }
        let bitset_len = (bitset_len_bits + usize::BITS as usize - 1) / usize::BITS as usize;

        let free_lists_layout = Layout::new::<[FreeList; 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_lists_offset = 0;
        let (layout, trees_offset) = free_lists_layout.extend(trees_layout)?;
        let (layout, bitset_offset) = layout.extend(bitset_layout)?;

        Ok(MetadataLayout {
            layout,
            free_lists_offset,
            trees_offset,
            trees_len,
            bitset_offset,
            bitset_len,
        })
    }
}