Luna/kernel/src/memory/Heap.cpp

#include "memory/Heap.h"
#include "Log.h"
#include "arch/MMU.h"
#include "arch/Serial.h"
#include "memory/KernelVM.h"
#include "memory/MemoryManager.h"
#include <luna/Alignment.h>
#include <luna/LinkedList.h>
#include <luna/SafeArithmetic.h>
#include <luna/ScopeGuard.h>
#include <luna/String.h>
#include <luna/SystemError.h>

static constexpr int BLOCK_USED = 1 << 0;
static constexpr int BLOCK_START_MEM = 1 << 1;
static constexpr int BLOCK_END_MEM = 1 << 2;

static constexpr usize BLOCK_MAGIC = 0x6d616c6c6f63210a; // echo 'malloc!' | hexdump -C (includes a newline)
static constexpr usize BLOCK_DEAD = 0xdeaddeaddeaddead;

static constexpr usize MINIMUM_PAGES_PER_ALLOCATION = 4;

struct HeapBlock : DoublyLinkedListNode<HeapBlock>
{
    usize req_size;
    usize full_size;
    int status;
    usize magic;
};

static_assert(sizeof(HeapBlock) == 48UL);

static const isize HEAP_BLOCK_SIZE = 48;

static DoublyLinkedList<HeapBlock> heap;

static Result<HeapBlock*> allocate_pages(usize count)
{
    u64 virt = TRY(KernelVM::alloc_several_pages(count));
    auto vm_guard = make_scope_guard([&] { KernelVM::free_several_pages(virt, count).value(); });
    void* const ptr = (void*)TRY(MemoryManager::alloc_at(virt, count, MMU::ReadWrite | MMU::NoExecute));
    vm_guard.deactivate();
    return (HeapBlock*)ptr;
}

static Result<void> release_pages(void* ptr, usize count)
{
    TRY(KernelVM::free_several_pages((u64)ptr, count));
    return MemoryManager::unmap_owned((u64)ptr, count);
}

// If we're allocating a large amount of memory, map enough pages for it, but otherwise just use the default amount of
// pages.
static usize get_pages_for_allocation(usize bytes)
{
    usize pages = get_blocks_from_size(bytes, ARCH_PAGE_SIZE);
    if (pages < MINIMUM_PAGES_PER_ALLOCATION) pages = MINIMUM_PAGES_PER_ALLOCATION;
    return pages;
}

static bool is_block_free(HeapBlock* block)
{
    return !(block->status & BLOCK_USED);
}

static usize space_available(HeapBlock* block)
{
    expect(!is_block_free(block), "Attempting to split a free block");
    return block->full_size - block->req_size;
}

// The heap block is stored right behind a memory block.
static HeapBlock* get_heap_block_for_pointer(void* ptr)
{
    return (HeapBlock*)offset_ptr(ptr, -HEAP_BLOCK_SIZE);
}

static void* get_pointer_from_heap_block(HeapBlock* block)
{
    return (void*)offset_ptr(block, HEAP_BLOCK_SIZE);
}

static usize get_fair_offset_to_split_at(HeapBlock* block, usize min)
{
    usize available = space_available(block);

    available -= min; // reserve at least min size for the new block.

    available -= (available /
                  2); // reserve half of the rest for the new block, while still leaving another half for the old one.

    available = align_down<16>(available); // Everything has to be aligned on a 16-byte boundary

    return available + block->req_size;
}

static Option<HeapBlock*> split(HeapBlock* block, usize size)
{
    const usize available = space_available(block); // How much space can we steal from this block?
    const usize old_size =
        block->full_size; // Save the old value of this variable since we are going to use it after modifying it

    if (available < (size + sizeof(HeapBlock)))
        return {}; // This block hasn't got enough free space to hold the requested size.

    const usize offset = get_fair_offset_to_split_at(block, size + sizeof(HeapBlock));
    block->full_size = offset; // shrink the old block to fit this offset

    HeapBlock* new_block = offset_ptr(block, offset + sizeof(HeapBlock));

    new_block->magic = BLOCK_MAGIC;
    new_block->status = (block->status & BLOCK_END_MEM) ? BLOCK_END_MEM : 0;
    new_block->full_size = old_size - (offset + sizeof(HeapBlock));
    heap.append_after(block, new_block);

    block->status &= ~BLOCK_END_MEM; // this block is no longer the last block in its memory range

    return new_block;
}

static Result<void> combine_forward(HeapBlock* block)
{
    // The caller needs to ensure there is a next block.
    HeapBlock* const next = heap.next(block).value();
    heap.remove(next);
    next->magic = BLOCK_DEAD;

    if (next->status & BLOCK_END_MEM)
    {
        if (next->status & BLOCK_START_MEM)
        {
            TRY(release_pages(next, get_blocks_from_size(next->full_size + sizeof(HeapBlock), ARCH_PAGE_SIZE)));
            return {};
        }
        else
            block->status |= BLOCK_END_MEM;
    }

    block->full_size += next->full_size + sizeof(HeapBlock);

    return {};
}

static Result<HeapBlock*> combine_backward(HeapBlock* block)
{
    // The caller needs to ensure there is a last block.
    HeapBlock* const last = heap.previous(block).value();
    heap.remove(block);
    block->magic = BLOCK_DEAD;

    if (block->status & BLOCK_END_MEM)
    {
        if (block->status & BLOCK_START_MEM)
        {
            TRY(release_pages(block, get_blocks_from_size(block->full_size + sizeof(HeapBlock), ARCH_PAGE_SIZE)));
            return last;
        }
        else
            last->status |= BLOCK_END_MEM;
    }

    last->full_size += block->full_size + sizeof(HeapBlock);

    return last;
}

Result<void*> kmalloc(usize size)
{
    if (!size) return (void*)BLOCK_MAGIC;

    size = align_up<16>(size);

    if (!heap.first().has_value())
    {
        const usize pages = get_pages_for_allocation(size + sizeof(HeapBlock));
        HeapBlock* const block = TRY(allocate_pages(pages));

        block->full_size = (pages * ARCH_PAGE_SIZE) - sizeof(HeapBlock);
        block->magic = BLOCK_MAGIC;
        block->status = BLOCK_START_MEM | BLOCK_END_MEM;
        heap.append(block);
    }

    HeapBlock* block = heap.expect_first();
    while (block)
    {
        // Trying to find a free block...
        if (is_block_free(block))
        {
            if (block->full_size < size)
            {
                block = heap.next(block).value_or(nullptr);
                continue;
            }
            break; // We found a free block that's big enough!!
        }
        auto rc = split(block, size);
        if (rc.has_value())
        {
            block = rc.value(); // We managed to get a free block from a larger used block!!
            break;
        }
        block = heap.next(block).value_or(nullptr);
    }

    if (!block) // No free blocks, let's allocate a new one
    {
        usize pages = get_pages_for_allocation(size + sizeof(HeapBlock));
        block = TRY(allocate_pages(pages));

        block->full_size = (pages * ARCH_PAGE_SIZE) - sizeof(HeapBlock);
        block->magic = BLOCK_MAGIC;
        block->status = BLOCK_START_MEM | BLOCK_END_MEM;
        heap.append(block);
    }

    block->req_size = size;
    block->status |= BLOCK_USED;

    return get_pointer_from_heap_block(block);
}

Result<void> kfree(void* ptr)
{
    if (ptr == (void*)BLOCK_MAGIC) return {}; // This pointer was returned from a call to malloc(0)
    if (!ptr) return {};

    HeapBlock* block = get_heap_block_for_pointer(ptr);

    if (block->magic != BLOCK_MAGIC)
    {
        if (block->magic == BLOCK_DEAD)
        {
            kerrorln("ERROR: Attempt to free memory at %p, which was already freed", ptr);
        }
        else
            kerrorln("ERROR: Attempt to free memory at %p, which wasn't allocated with kmalloc", ptr);

        return err(EFAULT);
    }

    if (is_block_free(block))
    {
        kerrorln("ERROR: Attempt to free memory at %p, which was already freed", ptr);
        return err(EFAULT);
    }
    else
        block->status &= ~BLOCK_USED;

    auto maybe_next = heap.next(block);
    if (maybe_next.has_value() && is_block_free(maybe_next.value()))
    {
        // The next block is also free, thus we can merge!
        TRY(combine_forward(block));
    }

    auto maybe_last = heap.previous(block);
    if (maybe_last.has_value() && is_block_free(maybe_last.value()))
    {
        // The last block is also free, thus we can merge!
        block = TRY(combine_backward(block));
    }

    if ((block->status & BLOCK_START_MEM) && (block->status & BLOCK_END_MEM))
    {
        heap.remove(block);
        TRY(release_pages(block, get_blocks_from_size(block->full_size + sizeof(HeapBlock), ARCH_PAGE_SIZE)));
    }

    return {};
}

Result<void*> krealloc(void* ptr, usize size)
{
    if (!ptr) return kmalloc(size);
    if (ptr == (void*)BLOCK_MAGIC) return kmalloc(size);
    if (!size)
    {
        TRY(kfree(ptr));
        return (void*)BLOCK_MAGIC;
    }

    HeapBlock* const block = get_heap_block_for_pointer(ptr);

    if (block->magic != BLOCK_MAGIC)
    {
        if (block->magic == BLOCK_DEAD)
        {
            kerrorln("ERROR: Attempt to realloc memory at %p, which was already freed", ptr);
        }
        else
            kerrorln("ERROR: Attempt to realloc memory at %p, which wasn't allocated with kmalloc", ptr);

        return err(EFAULT);
    }

    size = align_up<16>(size);

    if (is_block_free(block))
    {
        kerrorln("ERROR: Attempt to realloc memory at %p, which was already freed", ptr);
        return err(EFAULT);
    }

    if (block->full_size >= size)
    {
        // This block is already large enough!
        block->req_size = size;
        return ptr;
    }

    void* const new_ptr = TRY(kmalloc(size));
    memcpy(new_ptr, ptr, block->req_size > size ? size : block->req_size);
    TRY(kfree(ptr));

    return new_ptr;
}

Result<void*> kcalloc(usize nmemb, usize size)
{
    const usize realsize = TRY(safe_mul(nmemb, size));
    void* const ptr = TRY(kmalloc(realsize));
    return memset(ptr, 0, realsize);
}

void dump_heap_usage()
{
    kdbgln("-- Dumping usage stats for kernel heap:");
    if (!heap.count())
    {
        kdbgln("- Heap is not currently being used");
        return;
    }
    usize alloc_total = 0;
    usize alloc_used = 0;
    HeapBlock* block = heap.expect_first();
    while (block)
    {
        if (is_block_free(block))
        {
            kdbgln("- Available block, of size %zu", block->full_size);
            alloc_total += block->full_size + sizeof(HeapBlock);
        }
        else
        {
            kdbgln("- Used block, of size %zu, of which %zu bytes are being used", block->full_size, block->req_size);
            alloc_total += block->full_size + sizeof(HeapBlock);
            alloc_used += block->req_size;
        }
        block = heap.next(block).value_or(nullptr);
    }

    kdbgln("-- Total memory allocated for heap: %zu bytes", alloc_total);
    kdbgln("-- Heap memory in use by the kernel: %zu bytes", alloc_used);
}

void* operator new(usize size) noexcept
{
    return kmalloc(size).value_or(nullptr);
}

void* operator new[](usize size) noexcept
{
    return kmalloc(size).value_or(nullptr);
}

void operator delete(void* p) noexcept
{
    kfree(p);
}

void operator delete[](void* p) noexcept
{
    kfree(p);
}

void operator delete(void* p, usize) noexcept
{
    kfree(p);
}

void operator delete[](void* p, usize) noexcept
{
    kfree(p);
}