Merge branch 'feat_inital_context_switching' into 'develop_ba'

Implement Context Switching

See merge request teachos/kernel!6

12 files changed, 337 insertions, 170 deletions

diff --git a/arch/x86_64/src/memory/allocator/area_frame_allocator.cpp b/arch/x86_64/src/memory/allocator/area_frame_allocator.cpp
index cb4fefa..a5a1b49 100644
--- a/arch/x86_64/src/memory/allocator/area_frame_allocator.cpp
+++ b/arch/x86_64/src/memory/allocator/area_frame_allocator.cpp
@@ -9,7 +9,7 @@
 namespace teachos::arch::memory::allocator
 {
   area_frame_allocator::area_frame_allocator(multiboot::memory_information const & mem_info)
-      : next_free_frame(0U)
+      : next_free_frame()
       , current_area(std::nullopt)
       , memory_areas(mem_info.areas)
       , kernel_start(physical_frame::containing_address(mem_info.kernel_start))
diff --git a/arch/x86_64/src/memory/cpu/control_register.cpp b/arch/x86_64/src/memory/cpu/control_register.cpp
deleted file mode 100644
index 7ee88b5..0000000
--- a/arch/x86_64/src/memory/cpu/control_register.cpp
+++ /dev/null
@@ -1,72 +0,0 @@
-#include "arch/memory/cpu/control_register.hpp"
-
-#include "arch/exception_handling/panic.hpp"
-
-#include <type_traits>
-
-namespace teachos::arch::memory::cpu
-{
-  auto read_control_register(control_register cr) -> uint64_t
-  {
-    uint64_t current_value;
-    switch (cr)
-    {
-      case control_register::CR0:
-        asm volatile("mov %%cr0, %[output]" : [output] "=r"(current_value));
-        break;
-      case control_register::CR2:
-        asm volatile("mov %%cr2, %[output]" : [output] "=r"(current_value));
-        break;
-      case control_register::CR3:
-        asm volatile("mov %%cr3, %[output]" : [output] "=r"(current_value));
-        break;
-      case control_register::CR4:
-        asm volatile("mov %%cr4, %[output]" : [output] "=r"(current_value));
-        break;
-      default:
-        exception_handling::panic("[Control Register] Attempted to read non-existent or reserved control register");
-        break;
-    }
-    return current_value;
-  }
-
-  auto write_control_register(control_register cr, uint64_t new_value) -> void
-  {
-    switch (cr)
-    {
-      case control_register::CR0:
-        asm volatile("mov %[input], %%cr0"
-                     : /* no output from call */
-                     : [input] "r"(new_value)
-                     : "memory");
-        break;
-      case control_register::CR2:
-        asm volatile("mov %[input], %%cr2"
-                     : /* no output from call */
-                     : [input] "r"(new_value)
-                     : "memory");
-        break;
-      case control_register::CR3:
-        asm volatile("mov %[input], %%cr3"
-                     : /* no output from call */
-                     : [input] "r"(new_value)
-                     : "memory");
-        break;
-      case control_register::CR4:
-        asm volatile("mov %[input], %%cr4"
-                     : /* no output from call */
-                     : [input] "r"(new_value)
-                     : "memory");
-        break;
-      default:
-        exception_handling::panic("[Control Register] Attempted to write non-existent or reserved control register");
-        break;
-    }
-  }
-
-  auto set_cr0_bit(cr0_flags flag) -> void
-  {
-    auto const cr0 = read_control_register(control_register::CR0);
-    write_control_register(control_register::CR0, static_cast<std::underlying_type<cr0_flags>::type>(flag) | cr0);
-  }
-}  // namespace teachos::arch::memory::cpu
diff --git a/arch/x86_64/src/memory/cpu/msr.cpp b/arch/x86_64/src/memory/cpu/msr.cpp
deleted file mode 100644
index b83f902..0000000
--- a/arch/x86_64/src/memory/cpu/msr.cpp
+++ /dev/null
@@ -1,31 +0,0 @@
-#include "arch/memory/cpu/msr.hpp"
-
-namespace teachos::arch::memory::cpu
-{
-  namespace
-  {
-    auto constexpr IA32_EFER_ADDRESS = 0xC0000080;
-  }
-
-  auto read_msr(uint32_t msr) -> uint64_t
-  {
-    uint32_t low, high;
-    asm volatile("rdmsr" : "=a"(low), "=d"(high) : "c"(msr));
-    return (static_cast<uint64_t>(high) << 32) | low;
-  }
-
-  auto write_msr(uint32_t msr, uint64_t value) -> void
-  {
-    uint32_t low = value & 0xFFFFFFFF;
-    uint32_t high = value >> 32;
-    asm volatile("wrmsr"
-                 : /* no output from call */
-                 : "c"(msr), "a"(low), "d"(high));
-  }
-
-  auto set_efer_bit(efer_flags flag) -> void
-  {
-    auto const efer = read_msr(IA32_EFER_ADDRESS);
-    write_msr(IA32_EFER_ADDRESS, static_cast<std::underlying_type<efer_flags>::type>(flag) | efer);
-  }
-}  // namespace teachos::arch::memory::cpu
diff --git a/arch/x86_64/src/memory/cpu/tlb.cpp b/arch/x86_64/src/memory/cpu/tlb.cpp
deleted file mode 100644
index 591d9fc..0000000
--- a/arch/x86_64/src/memory/cpu/tlb.cpp
+++ /dev/null
@@ -1,16 +0,0 @@
-#include "arch/memory/cpu/tlb.hpp"
-
-#include "arch/memory/cpu/control_register.hpp"
-
-namespace teachos::arch::memory::cpu
-{
-  auto tlb_flush(paging::virtual_address address) -> void
-  {
-    asm volatile("invlpg (%[input])" : /* no output from call */ : [input] "r"(address) : "memory");
-  }
-
-  auto tlb_flush_all() -> void
-  {
-    write_control_register(cpu::control_register::CR3, read_control_register(cpu::control_register::CR3));
-  }
-}  // namespace teachos::arch::memory::cpu
diff --git a/arch/x86_64/src/memory/heap/bump_allocator.cpp b/arch/x86_64/src/memory/heap/bump_allocator.cpp
index df95346..525f45c 100644
--- a/arch/x86_64/src/memory/heap/bump_allocator.cpp
+++ b/arch/x86_64/src/memory/heap/bump_allocator.cpp
@@ -24,11 +24,13 @@ namespace teachos::arch::memory::heap
 
   auto bump_allocator::allocate(std::size_t size) -> void *
   {
+    // Reading the value only has to be done once, because compare_exchange_weak updates the value as well if the
+    // exchange failed, becuase the value was not the expected one.
+    auto alloc_start = next.load(std::memory_order::relaxed);
     // Repeat allocation until it succeeds, has to be done, because another allocator could overtake it at any time
     // causing the value to differ and the calculation to have to be redone.
     for (;;)
     {
-      auto alloc_start = next.load(std::memory_order::relaxed);
       auto const alloc_end = saturating_add(alloc_start, size);
       arch::exception_handling::assert(alloc_end <= heap_end, "[Heap Allocator] Out of memory");
       // Check if the atomic value is still the one initally loaded, if it isn't we have been overtaken by another
diff --git a/arch/x86_64/src/memory/heap/global_heap_allocator.cpp b/arch/x86_64/src/memory/heap/global_heap_allocator.cpp
index c1ca160..35cd623 100644
--- a/arch/x86_64/src/memory/heap/global_heap_allocator.cpp
+++ b/arch/x86_64/src/memory/heap/global_heap_allocator.cpp
@@ -1,20 +1,43 @@
 #include "arch/memory/heap/global_heap_allocator.hpp"
 
+#include "arch/context_switching/syscall/main.hpp"
 #include "arch/exception_handling/assert.hpp"
+#include "arch/kernel/cpu/segment_register.hpp"
 #include "arch/memory/heap/bump_allocator.hpp"
 #include "arch/memory/heap/linked_list_allocator.hpp"
+#include "arch/memory/heap/user_heap_allocator.hpp"
 
 namespace teachos::arch::memory::heap
 {
-  heap_allocator * global_heap_allocator::allocator_instance = nullptr;
+  namespace
+  {
+    constexpr char NOT_REGISTRED_ERROR_MESSAGE[] =
+        "Attempted to allocate or deallocate using the global_heap_allocator before "
+        "register_heap_allocation_type was called.";
+    constexpr uint16_t KERNEL_CODE_INDEX = 1U;
+
+    [[gnu::section(".user_text")]]
+    auto os_in_kernel_mode() -> bool
+    {
+      auto const cs = teachos::arch::kernel::cpu::read_code_segment_register();
+      return cs.get_index() == KERNEL_CODE_INDEX;
+    }
+  }  // namespace
+
+  heap_allocator * global_heap_allocator::kernel_allocator_instance = nullptr;
+  user_heap_allocator * global_heap_allocator::user_allocator_instance = nullptr;
+
+  auto global_heap_allocator::kmalloc(std::size_t size) -> void * { return kernel().allocate(size); }
 
-  auto global_heap_allocator::allocate(std::size_t size) -> void * { return get().allocate(size); }
+  auto global_heap_allocator::kfree(void * pointer) noexcept -> void { kernel().deallocate(pointer); }
 
-  auto global_heap_allocator::deallocate(void * pointer) noexcept -> void { get().deallocate(pointer); }
+  auto global_heap_allocator::malloc(std::size_t size) -> void * { return user().allocate(size); }
+
+  auto global_heap_allocator::free(void * pointer) noexcept -> void { user().deallocate(pointer); }
 
   auto global_heap_allocator::register_heap_allocator(heap_allocator_type new_type) -> void
   {
-    exception_handling::assert(allocator_instance == nullptr,
+    exception_handling::assert(kernel_allocator_instance == nullptr,
                                "Calling register_heap_allocator_type can only be done once.");
 
     switch (new_type)
@@ -23,56 +46,90 @@ namespace teachos::arch::memory::heap
         // Nothing to do
         break;
       case heap_allocator_type::BUMP: {
-        static bump_allocator allocator{HEAP_START, HEAP_START + HEAP_SIZE};
-        allocator_instance = &allocator;
+        static bump_allocator kernel_allocator{KERNEL_HEAP_START, KERNEL_HEAP_START + KERNEL_HEAP_SIZE};
+        kernel_allocator_instance = &kernel_allocator;
         break;
       }
       case heap_allocator_type::LINKED_LIST: {
-        static linked_list_allocator allocator{HEAP_START, HEAP_START + HEAP_SIZE};
-        allocator_instance = &allocator;
+        static linked_list_allocator kernel_allocator{KERNEL_HEAP_START, KERNEL_HEAP_START + KERNEL_HEAP_SIZE};
+        kernel_allocator_instance = &kernel_allocator;
         break;
       }
     }
+
+    [[gnu::section(".user_data")]]
+    static user_heap_allocator user_allocator{};
+    user_allocator_instance = &user_allocator;
   }
 
-  auto global_heap_allocator::get() -> heap_allocator &
+  auto global_heap_allocator::kernel() -> heap_allocator &
   {
-    exception_handling::assert(allocator_instance != nullptr,
-                               "Attempted to allocate or deallocate using the global_heap_allocator before "
-                               "register_heap_allocation_type was called.");
+    exception_handling::assert(kernel_allocator_instance != nullptr, NOT_REGISTRED_ERROR_MESSAGE);
+
+    return *kernel_allocator_instance;
+  }
 
-    return *allocator_instance;
+  auto global_heap_allocator::user() -> user_heap_allocator &
+  {
+    context_switching::syscall::syscall(
+        context_switching::syscall::type::ASSERT,
+        {user_allocator_instance != nullptr, reinterpret_cast<uint64_t>(&NOT_REGISTRED_ERROR_MESSAGE)});
+    return *user_allocator_instance;
   }
 }  // namespace teachos::arch::memory::heap
 
 auto operator new(std::size_t size) -> void *
 {
-  return teachos::arch::memory::heap::global_heap_allocator::allocate(size);
+  if (teachos::arch::memory::heap::os_in_kernel_mode())
+  {
+    return teachos::arch::memory::heap::global_heap_allocator::kmalloc(size);
+  }
+  return teachos::arch::memory::heap::global_heap_allocator::malloc(size);
 }
 
 auto operator delete(void * pointer) noexcept -> void
 {
-  teachos::arch::memory::heap::global_heap_allocator::deallocate(pointer);
+  if (teachos::arch::memory::heap::os_in_kernel_mode())
+  {
+    teachos::arch::memory::heap::global_heap_allocator::kfree(pointer);
+  }
+  teachos::arch::memory::heap::global_heap_allocator::free(pointer);
 }
 
 auto operator delete(void * pointer, std::size_t size) noexcept -> void
 {
   (void)size;
-  teachos::arch::memory::heap::global_heap_allocator::deallocate(pointer);
+  if (teachos::arch::memory::heap::os_in_kernel_mode())
+  {
+    teachos::arch::memory::heap::global_heap_allocator::kfree(pointer);
+  }
+  teachos::arch::memory::heap::global_heap_allocator::free(pointer);
 }
 
 auto operator new[](std::size_t size) -> void *
 {
-  return teachos::arch::memory::heap::global_heap_allocator::allocate(size);
+  if (teachos::arch::memory::heap::os_in_kernel_mode())
+  {
+    return teachos::arch::memory::heap::global_heap_allocator::kmalloc(size);
+  }
+  return teachos::arch::memory::heap::global_heap_allocator::malloc(size);
 }
 
 auto operator delete[](void * pointer) noexcept -> void
 {
-  teachos::arch::memory::heap::global_heap_allocator::deallocate(pointer);
+  if (teachos::arch::memory::heap::os_in_kernel_mode())
+  {
+    teachos::arch::memory::heap::global_heap_allocator::kfree(pointer);
+  }
+  teachos::arch::memory::heap::global_heap_allocator::free(pointer);
 }
 
 auto operator delete[](void * pointer, std::size_t size) noexcept -> void
 {
   (void)size;
-  teachos::arch::memory::heap::global_heap_allocator::deallocate(pointer);
+  if (teachos::arch::memory::heap::os_in_kernel_mode())
+  {
+    teachos::arch::memory::heap::global_heap_allocator::kfree(pointer);
+  }
+  teachos::arch::memory::heap::global_heap_allocator::free(pointer);
 }
diff --git a/arch/x86_64/src/memory/heap/linked_list_allocator.cpp b/arch/x86_64/src/memory/heap/linked_list_allocator.cpp
index a824c8a..63a6111 100644
--- a/arch/x86_64/src/memory/heap/linked_list_allocator.cpp
+++ b/arch/x86_64/src/memory/heap/linked_list_allocator.cpp
@@ -8,10 +8,8 @@
 namespace teachos::arch::memory::heap
 {
   linked_list_allocator::linked_list_allocator(std::size_t heap_start, std::size_t heap_end)
-      : heap_start(heap_start)
-      , heap_end(heap_end)
-      , first(nullptr)
-      , mutex{shared::mutex{}}
+      : first(nullptr)
+      , mutex{stl::mutex{}}
   {
     auto const heap_size = heap_end - heap_start;
     exception_handling::assert(
diff --git a/arch/x86_64/src/memory/heap/memory_block.cpp b/arch/x86_64/src/memory/heap/memory_block.cpp
index 446cd96..bc97bd6 100644
--- a/arch/x86_64/src/memory/heap/memory_block.cpp
+++ b/arch/x86_64/src/memory/heap/memory_block.cpp
@@ -6,10 +6,10 @@ namespace teachos::arch::memory::heap
 {
   memory_block::memory_block(std::size_t size, memory_block * next)
   {
-    memset(static_cast<void *>(this), 0, size);
+    memset(static_cast<void *>(this), 0U, size);
     this->size = size;
     this->next = next;
   }
 
-  memory_block::~memory_block() { memset(static_cast<void *>(this), 0, sizeof(memory_block)); }
+  memory_block::~memory_block() { memset(static_cast<void *>(this), 0U, sizeof(memory_block)); }
 }  // namespace teachos::arch::memory::heap
diff --git a/arch/x86_64/src/memory/heap/user_heap_allocator.cpp b/arch/x86_64/src/memory/heap/user_heap_allocator.cpp
new file mode 100644
index 0000000..427a68a
--- /dev/null
+++ b/arch/x86_64/src/memory/heap/user_heap_allocator.cpp
@@ -0,0 +1,200 @@
+#include "arch/memory/heap/user_heap_allocator.hpp"
+
+#include "arch/context_switching/syscall/main.hpp"
+
+#include <algorithm>
+
+namespace teachos::arch::memory::heap
+{
+  auto user_heap_allocator::allocate(std::size_t size) -> void *
+  {
+    // Add size of size_t to the total allocated size, because we add a header that includes the size of the allocated
+    // block, to allow for deallocation without the need to call with the corresponding size
+    auto const total_size = size + sizeof(std::size_t);
+    mutex.lock();
+
+    memory_block * previous = nullptr;
+    auto current = first;
+
+    while (current != nullptr)
+    {
+      auto memory = allocate_into_memory_block_if_big_enough(current, previous, total_size);
+      if (memory.has_value())
+      {
+        return memory.value();
+      }
+
+      previous = current;
+      current = current->next;
+    }
+
+    current = expand_heap_if_full();
+
+    if (current != nullptr)
+    {
+      auto memory = allocate_into_memory_block_if_big_enough(current, previous, total_size);
+      if (memory.has_value())
+      {
+        return memory.value();
+      }
+    }
+
+    char constexpr OUT_OF_MEMORY_ERROR_MESSAGE[] = "[Linked List Allocator] Out of memory";
+    context_switching::syscall::syscall(context_switching::syscall::type::ASSERT,
+                                        {false, reinterpret_cast<uint64_t>(&OUT_OF_MEMORY_ERROR_MESSAGE)});
+    return nullptr;
+  }
+
+  auto user_heap_allocator::deallocate(void * pointer) noexcept -> void
+  {
+    mutex.lock();
+
+    // Read configured header size of the complete allocated block
+    auto const header_pointer = reinterpret_cast<void *>(reinterpret_cast<std::size_t>(pointer) - sizeof(std::size_t));
+    auto const total_size = *reinterpret_cast<std::size_t *>(header_pointer);
+
+    auto const start_address = reinterpret_cast<std::size_t>(header_pointer);
+    auto const end_address = start_address + total_size;
+
+    memory_block * previous = nullptr;
+    auto current = first;
+
+    while (current != nullptr)
+    {
+      // Current address of the free memory block now points to an address that is after our block to deallocate in heap
+      // memory space.
+      if (reinterpret_cast<std::size_t>(current) >= end_address)
+      {
+        break;
+      }
+
+      previous = current;
+      current = current->next;
+    }
+
+    coalesce_free_memory_block(previous, current, header_pointer, total_size);
+    mutex.unlock();
+  }
+
+  auto user_heap_allocator::allocate_into_memory_block_if_big_enough(memory_block * current, memory_block * previous,
+                                                                     std::size_t total_size) -> std::optional<void *>
+  {
+    if (current->size == total_size)
+    {
+      auto const memory_address = remove_free_memory_block(previous, current);
+      new (memory_address) std::size_t(total_size);
+      mutex.unlock();
+      return reinterpret_cast<void *>(reinterpret_cast<std::size_t>(memory_address) + sizeof(std::size_t));
+    }
+    else if (current->size >= total_size + min_allocatable_size())
+    {
+      // Ensure that the allocated size block is atleast 16 bytes (required because if we free the hole afterwards
+      // there needs to be enough space for a memory block). Therefore we allocate more than is actually required if
+      // the total size was less and simply deallocate it as well
+      auto const max_size = std::max(total_size, min_allocatable_size());
+      auto const memory_address = split_free_memory_block(previous, current, max_size);
+      new (memory_address) std::size_t(max_size);
+      mutex.unlock();
+      return reinterpret_cast<void *>(reinterpret_cast<std::size_t>(memory_address) + sizeof(std::size_t));
+    }
+    return std::nullopt;
+  }
+
+  auto user_heap_allocator::expand_heap_if_full() -> memory_block *
+  {
+    auto const result = context_switching::syscall::syscall(context_switching::syscall::type::EXPAND_HEAP);
+
+    uint64_t const heap_start = result.values.arg_0;
+    uint64_t const heap_size = result.values.arg_1;
+    return !result.error_code ? new (reinterpret_cast<void *>(heap_start)) memory_block(heap_size, nullptr) : nullptr;
+  }
+
+  auto user_heap_allocator::remove_free_memory_block(memory_block * previous_block, memory_block * current_block)
+      -> void *
+  {
+    return replace_free_memory_block(previous_block, current_block, current_block->next);
+  }
+
+  auto user_heap_allocator::split_free_memory_block(memory_block * previous_block, memory_block * current_block,
+                                                    std::size_t size) -> void *
+  {
+    auto const end_address = reinterpret_cast<std::size_t>(current_block) + size;
+    auto const new_block =
+        new (reinterpret_cast<void *>(end_address)) memory_block(current_block->size - size, current_block->next);
+    return replace_free_memory_block(previous_block, current_block, new_block);
+  }
+
+  auto user_heap_allocator::replace_free_memory_block(memory_block * previous_block, memory_block * current_block,
+                                                      memory_block * new_block) -> void *
+  {
+    auto const start_address = reinterpret_cast<std::size_t>(current_block);
+    // If we want to allocate into the first block that is before any other free block, then there exists no previous
+    // free block (nullptr). Therefore we have to overwrite the first block instead of overwriting its next value.
+    if (previous_block == nullptr)
+    {
+      first = new_block;
+    }
+    else
+    {
+      previous_block->next = new_block;
+    }
+    current_block->~memory_block();
+    return reinterpret_cast<void *>(start_address);
+  }
+
+  auto user_heap_allocator::coalesce_free_memory_block(memory_block * previous_block, memory_block * current_block,
+                                                       void * pointer, std::size_t size) -> void
+  {
+    auto const start_address = reinterpret_cast<std::size_t>(pointer);
+    auto const end_address = start_address + size;
+
+    // Inital values if there are no adjacent blocks either before or after, meaning we have to simply create a free
+    // memory block that is placed in between the previous and next block.
+    auto block_size = size;
+    auto next_block = current_block;
+
+    // If the block we want to deallocate is before another free block and we can therefore combine both into one.
+    // This is done by deleting the current free block and creating a new block at the start address of the block to
+    // deallocate with both the size of the block to deallcoate and the free block next to it.
+    if (end_address == reinterpret_cast<std::size_t>(current_block))
+    {
+      block_size += current_block->size;
+      next_block = current_block->next;
+      current_block->~memory_block();
+    }
+
+    // If the block we want to deallocate is behind another free block and we can therefore combine both into one.
+    // This is done by simply changin the size of the previous block to include the size of the block to deallocate.
+    // This is done, because the previous block might still be referencered by the next field of other memory blocks.
+    if (previous_block != nullptr &&
+        start_address == (reinterpret_cast<std::size_t>(previous_block) + previous_block->size))
+    {
+      block_size += previous_block->size;
+
+      previous_block->size = block_size;
+      previous_block->next = next_block;
+      return;
+    }
+
+    // Check if the block we want to deallocate is contained in the previous block, because if it is it can only mean
+    // that the block has already been deallocated and we therefore attempted a double free.
+    char constexpr DOUBLE_FREE_ERROR_MESSAGE[] = "[Linked List Allocator] Attempted double free detected";
+    context_switching::syscall::syscall(
+        context_switching::syscall::type::ASSERT,
+        {previous_block == nullptr ||
+             start_address >= (reinterpret_cast<std::size_t>(previous_block) + previous_block->size),
+         reinterpret_cast<uint64_t>(&DOUBLE_FREE_ERROR_MESSAGE)});
+
+    auto const new_block = new (pointer) memory_block(block_size, next_block);
+    // If we want to deallocate the first block that is before any other free block, then there exists no previous free
+    // block (nullptr). Therefore we have to overwrite the first block instead of overwriting its
+    // next value.
+    if (previous_block == nullptr)
+    {
+      first = new_block;
+      return;
+    }
+    previous_block->next = new_block;
+  }
+
+}  // namespace teachos::arch::memory::heap
diff --git a/arch/x86_64/src/memory/main.cpp b/arch/x86_64/src/memory/main.cpp
index a6f91d9..2746a71 100644
--- a/arch/x86_64/src/memory/main.cpp
+++ b/arch/x86_64/src/memory/main.cpp
@@ -1,33 +1,57 @@
 #include "arch/memory/main.hpp"
 
 #include "arch/exception_handling/assert.hpp"
+#include "arch/kernel/cpu/control_register.hpp"
+#include "arch/kernel/cpu/msr.hpp"
 #include "arch/memory/allocator/area_frame_allocator.hpp"
-#include "arch/memory/cpu/control_register.hpp"
-#include "arch/memory/cpu/msr.hpp"
-#include "arch/memory/heap/heap_allocator.hpp"
+#include "arch/memory/allocator/concept.hpp"
+#include "arch/memory/heap/global_heap_allocator.hpp"
 #include "arch/memory/paging/active_page_table.hpp"
 #include "arch/memory/paging/kernel_mapper.hpp"
 
+#include <optional>
+
 namespace teachos::arch::memory
 {
   namespace
   {
-    auto remap_heap(allocator::area_frame_allocator allocator, paging::active_page_table & active_table) -> void
+    static std::optional<allocator::area_frame_allocator> frame_allocator;
+
+    auto create_frame_allocator(multiboot::memory_information const & memory_information)
+        -> allocator::area_frame_allocator &
     {
-      auto const start_page = paging::virtual_page::containing_address(memory::heap::HEAP_START);
-      auto const end_page =
-          ++(paging::virtual_page::containing_address(memory::heap::HEAP_START + memory::heap::HEAP_SIZE - 1));
-      paging::page_container::iterator const begin{start_page};
-      paging::page_container::iterator const end{end_page};
-      paging::page_container const pages{begin, end};
-
-      for (auto const & page : pages)
-      {
-        active_table.map_page_to_next_free_frame(allocator, page, paging::entry::WRITABLE);
-      }
+      frame_allocator.emplace(memory_information);
+      return frame_allocator.value();
+    }
+
+    auto get_frame_allocator() -> allocator::area_frame_allocator &
+    {
+      exception_handling::assert(frame_allocator.has_value(),
+                                 "[Initialization] Frame allocator has not been created yet");
+      return frame_allocator.value();
     }
   }  // namespace
 
+  auto remap_heap(std::size_t heap_start, std::size_t heap_size, paging::entry::bitset additional_flags = {}) -> void
+  {
+    decltype(auto) allocator = get_frame_allocator();
+    decltype(auto) active_table = paging::active_page_table::create_or_get();
+    auto const start_page = paging::virtual_page::containing_address(heap_start);
+    auto const end_page = ++(paging::virtual_page::containing_address(heap_start + heap_size - 1));
+
+    paging::page_container::iterator const begin{start_page};
+    paging::page_container::iterator const end{end_page};
+    paging::page_container const pages{begin, end};
+
+    constexpr auto base_flags = paging::entry::WRITABLE;
+    auto const flags = base_flags | additional_flags;
+
+    for (auto const & page : pages)
+    {
+      active_table.map_page_to_next_free_frame(allocator, page, flags);
+    }
+  }
+
   auto initialize_memory_management() -> void
   {
     static bool has_been_called = false;
@@ -36,17 +60,17 @@ namespace teachos::arch::memory
     has_been_called = true;
 
     auto const memory_information = multiboot::read_multiboot2();
-    allocator::area_frame_allocator allocator(memory_information);
+    decltype(auto) allocator = create_frame_allocator(memory_information);
 
-    cpu::set_cr0_bit(memory::cpu::cr0_flags::WRITE_PROTECT);
-    cpu::set_efer_bit(memory::cpu::efer_flags::NXE);
+    kernel::cpu::set_cr0_bit(kernel::cpu::cr0_flags::WRITE_PROTECT);
+    kernel::cpu::set_efer_bit(kernel::cpu::efer_flags::NXE);
 
     paging::kernel_mapper kernel(allocator, memory_information);
-    auto & active_table = kernel.remap_kernel();
+    kernel.remap_kernel();
     video::vga::text::write("Kernel remapping successful", video::vga::text::common_attributes::green_on_black);
     video::vga::text::newline();
 
-    remap_heap(allocator, active_table);
+    remap_heap(heap::KERNEL_HEAP_START, heap::KERNEL_HEAP_SIZE);
     video::vga::text::write("Heap remapping successful", video::vga::text::common_attributes::green_on_black);
     video::vga::text::newline();
   }
diff --git a/arch/x86_64/src/memory/paging/page_entry.cpp b/arch/x86_64/src/memory/paging/page_entry.cpp
index 5aa0982..57045ca 100644
--- a/arch/x86_64/src/memory/paging/page_entry.cpp
+++ b/arch/x86_64/src/memory/paging/page_entry.cpp
@@ -21,10 +21,12 @@ namespace teachos::arch::memory::paging
     {
       flags |= entry::PRESENT;
     }
+
     if (elf_flags.contains_flags(multiboot::elf_section_flags::WRITABLE))
     {
       flags |= entry::WRITABLE;
     }
+
     if (!elf_flags.contains_flags(multiboot::elf_section_flags::EXECUTABLE_CODE))
     {
       flags |= entry::EXECUTING_CODE_FORBIDDEN;
@@ -35,6 +37,8 @@ namespace teachos::arch::memory::paging
 
   auto entry::set_unused() -> void { flags = 0U; }
 
+  auto entry::set_user_accessible() -> void { flags |= entry::USER_ACCESSIBLE; }
+
   auto entry::calculate_pointed_to_frame() const -> std::optional<allocator::physical_frame>
   {
     if (contains_flags(PRESENT))
@@ -51,6 +55,7 @@ namespace teachos::arch::memory::paging
   {
     exception_handling::assert((frame.start_address() & ~PHYSICAL_ADDRESS_MASK) == 0,
                                "[Paging Entry] Start address is not aligned with page");
+
     flags = frame.start_address() | additional_flags.to_ulong();
   }
 
diff --git a/arch/x86_64/src/memory/paging/temporary_page.cpp b/arch/x86_64/src/memory/paging/temporary_page.cpp
index 152241d..8e73523 100644
--- a/arch/x86_64/src/memory/paging/temporary_page.cpp
+++ b/arch/x86_64/src/memory/paging/temporary_page.cpp
@@ -4,16 +4,16 @@
 
 namespace teachos::arch::memory::paging
 {
-  auto temporary_page::map_table_frame(allocator::physical_frame frame,
-                                       active_page_table & active_table) -> page_table_handle
+  auto temporary_page::map_table_frame(allocator::physical_frame frame, active_page_table & active_table)
+      -> page_table_handle
   {
     page_table_handle handle{reinterpret_cast<page_table *>(map_to_frame(frame, active_table)),
                              page_table_handle::LEVEL1};
     return handle;
   }
 
-  auto temporary_page::map_to_frame(allocator::physical_frame frame,
-                                    active_page_table & active_table) -> virtual_address
+  auto temporary_page::map_to_frame(allocator::physical_frame frame, active_page_table & active_table)
+      -> virtual_address
   {
     exception_handling::assert(!active_table.translate_page(page).has_value(),
                                "[Temporary page] Page is already mapped");