[memory] coalesce redundant remappings of MultiPageLevel (#3857)

there is no need to call mmap() over the mapped region as the OS will automatically map it via lazy paging

basically the mmap() and virtualAlloc on a region already allocated is a no-op (FOR THIS SPECIFIC USECASE)

Signed-off-by: lizzie <lizzie@eden-emu.dev>
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/3857
Reviewed-by: crueter <crueter@eden-emu.dev>

src/common/multi_level_page_table.inc

@@ -13,13 +13,13 @@
 namespace Common {
 
 template <typename BaseAddr>
-MultiLevelPageTable<BaseAddr>::MultiLevelPageTable(std::size_t address_space_bits_,
-                                                   std::size_t first_level_bits_,
-                                                   std::size_t page_bits_)
-    : address_space_bits{address_space_bits_},
-      first_level_bits{first_level_bits_}, page_bits{page_bits_} {
+MultiLevelPageTable<BaseAddr>::MultiLevelPageTable(std::size_t address_space_bits_, std::size_t first_level_bits_, std::size_t page_bits_)
+    : address_space_bits{address_space_bits_}
+    , first_level_bits{first_level_bits_}
+    , page_bits{page_bits_}
+{
     if (page_bits == 0) {
-      return;
+        return;
     }
     first_level_shift = address_space_bits - first_level_bits;
     first_level_chunk_size = (1ULL << (first_level_shift - page_bits)) * sizeof(BaseAddr);
@@ -30,12 +30,9 @@ MultiLevelPageTable<BaseAddr>::MultiLevelPageTable(std::size_t address_space_bit
     void* base{VirtualAlloc(nullptr, alloc_size, MEM_RESERVE, PAGE_READWRITE)};
 #else
     void* base{mmap(nullptr, alloc_size, PROT_READ | PROT_WRITE, MAP_PRIVATE | MAP_ANONYMOUS, -1, 0)};
-
-    if (base == MAP_FAILED) {
+    if (base == MAP_FAILED)
         base = nullptr;
-    }
 #endif
-
     ASSERT(base);
     base_ptr = reinterpret_cast<BaseAddr*>(base);
 }
@@ -56,29 +53,21 @@ template <typename BaseAddr>
 void MultiLevelPageTable<BaseAddr>::ReserveRange(u64 start, std::size_t size) {
     const u64 new_start = start >> first_level_shift;
     const u64 new_end = (start + size) >> first_level_shift;
-    for (u64 i = new_start; i <= new_end; i++) {
-        if (!first_level_map[i]) {
+    for (u64 i = new_start; i <= new_end; i++)
+        if (!first_level_map[i])
             AllocateLevel(i);
-        }
-    }
 }
 
 template <typename BaseAddr>
-void MultiLevelPageTable<BaseAddr>::AllocateLevel(u64 level) {
-    void* ptr = reinterpret_cast<char *>(base_ptr) + level * first_level_chunk_size;
+void MultiLevelPageTable<BaseAddr>::AllocateLevel(u64 index) {
+    void* ptr = reinterpret_cast<char *>(base_ptr) + index * first_level_chunk_size;
 #ifdef _WIN32
-    void* base{VirtualAlloc(ptr, first_level_chunk_size, MEM_COMMIT, PAGE_READWRITE)};
+    void* base = VirtualAlloc(ptr, first_level_chunk_size, MEM_COMMIT, PAGE_READWRITE);
+    ASSERT(base);
 #else
-    void* base{mmap(ptr, first_level_chunk_size, PROT_READ | PROT_WRITE,
-                    MAP_ANONYMOUS | MAP_PRIVATE, -1, 0)};
-
-    if (base == MAP_FAILED) {
-        base = nullptr;
-    }
+    void* base = ptr;
 #endif
-    ASSERT(base);
-
-    first_level_map[level] = base;
+    first_level_map[index] = base;
 }
 
 } // namespace Common