Luna/kernel/src/memory/Heap.cpp

#include "memory/Heap.h"
#include "Log.h"
#include "arch/MMU.h"
#include "arch/Serial.h"
#include "memory/MemoryManager.h"
#include <luna/Alignment.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
{
    usize req_size;
    usize full_size;
    int status;
    HeapBlock* next;
    HeapBlock* last;
    usize magic;
};

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

static HeapBlock* heap_start = nullptr;
static HeapBlock* heap_end = nullptr;

static usize start_addr = 0xffffffff80000000;

static Result<HeapBlock*> allocate_pages(
    usize count) // FIXME: Keep track of virtual address space usage. For now, since the address
                 // space is so huge, we can just start at a fairly large address and assume
                 // we'll never run into anything, but this will probably bite us in the future.
{
    void* ptr = (void*)TRY(MemoryManager::alloc_at(start_addr, count, MMU::ReadWrite | MMU::NoExecute));
    if (ptr) start_addr += (count * ARCH_PAGE_SIZE);
    return (HeapBlock*)ptr;
}

static Result<void> release_pages(void* ptr, usize 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;
}

static HeapBlock* get_heap_block_for_pointer(void* ptr)
{
    return (HeapBlock*)offset_ptr(ptr, -48);
}

static void* get_pointer_from_heap_block(HeapBlock* block)
{
    return (void*)offset_ptr(block, 48);
}

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.

    check(is_aligned(available,
                     16UL)); // If necessary, we can just align it. This is more of a sanity check than a requirement.

    return available + block->req_size;
}

static Result<HeapBlock*> split(HeapBlock* block, usize size)
{
    usize available = space_available(block); // How much space can we steal from this block?
    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 err(0); // This error is not propagated.

    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));
    new_block->next = block->next;
    new_block->last = block;

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

    return new_block;
}

static Result<void> combine_forward(HeapBlock* block)
{
    HeapBlock* next = block->next;
    if (next == heap_end) heap_end = block;
    next->magic = BLOCK_DEAD;

    block->next = block->next->next;
    if (block->next) block->next->last = block;

    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)
{
    HeapBlock* last = block->last;
    if (block == heap_end) heap_end = last;
    block->magic = BLOCK_DEAD;

    last->next = block->next;
    if (last->next) last->next->last = last;

    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(size, 16UL);

    if (!heap_start)
    {
        usize pages = get_pages_for_allocation(size);
        auto* 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;
        block->next = block->last = nullptr;
        heap_start = block;

        check(!heap_end);

        heap_end = heap_start;
    }

    HeapBlock* block = heap_start;
    while (block)
    {
        // Trying to find a free block...
        if (is_block_free(block))
        {
            if (block->full_size < size)
            {
                block = block->next; // Let's not try to split this block, it's not big enough
                continue;
            }
            break; // We found a free block that's big enough!!
        }
        auto rc = split(block, size);
        if (rc.has_value())
        {
            block = rc.release_value(); // We managed to get a free block from a larger used block!!
            break;
        }
        block = block->next;
    }

    if (!block) // No free blocks, let's allocate a new one
    {
        usize pages = get_pages_for_allocation(size);
        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;
        block->next = nullptr;
        block->last = heap_end;

        heap_end = 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;

    if (block->next && is_block_free(block->next))
    {
        // The next block is also free, thus we can merge!
        TRY(combine_forward(block));
    }

    if (block->last && is_block_free(block->last))
    {
        // 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))
    {
        if (block == heap_start) heap_start = block->next;
        if (block == heap_end) heap_end = block->last;
        if (block->last) block->last->next = block->next;
        if (block->next) block->next->last = block->last;
        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* 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(size, 16UL);

    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* 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)
{
    // FIXME: Check for overflows.
    usize realsize = nmemb * size;
    void* ptr = TRY(kmalloc(realsize));
    return memset(ptr, 0, realsize);
}

void dump_heap_usage()
{
    kdbgln("-- Dumping usage stats for kernel heap:");
    if (!heap_start)
    {
        kdbgln("- Heap is not currently being used");
        return;
    }
    usize alloc_total = 0;
    usize alloc_used = 0;
    HeapBlock* block = heap_start;
    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 = block->next;
    }

    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)
{
    auto rc = kmalloc(size);
    if (rc.has_error()) { expect(false, rc.error_string()); }
    return rc.release_value();
}

void* operator new[](usize size)
{
    auto rc = kmalloc(size);
    if (rc.has_error()) { expect(false, rc.error_string()); }
    return rc.release_value();
}

void operator delete(void* p)
{
    auto rc = kfree(p);
    if (rc.has_error()) { expect(false, rc.error_string()); }
    return rc.release_value();
}

void operator delete[](void* p)
{
    auto rc = kfree(p);
    if (rc.has_error()) { expect(false, rc.error_string()); }
    return rc.release_value();
}

void operator delete(void* p, usize)
{
    auto rc = kfree(p);
    if (rc.has_error()) { expect(false, rc.error_string()); }
    return rc.release_value();
}

void operator delete[](void* p, usize)
{
    auto rc = kfree(p);
    if (rc.has_error()) { expect(false, rc.error_string()); }
    return rc.release_value();
}