LibC: Support in-place realloc for mmap backed allocations

When shrinking a mmap based allocation, we can just unmap the trailing
pages that will no longer be needed. When extending an allocation we try
to MMAP_FIXED_NOREPLACE the need extra data to avoid memcpy between old
and new allocation. I'm not sure if the extension logic is really worth
it but it is just 15 lines of code to avoid possibly very large memcpys.

userspace/libraries/LibC/malloc.cpp

@@ -313,47 +313,78 @@ void free(void* ptr)
 	munmap(static_cast<uint8_t*>(ptr) - header.offset, header.mmap_size);
 }
 
-static size_t allocation_size(void* ptr)
-{
-	pthread_mutex_lock(&s_allocator_lock);
-	for (size_t i = 0; i < s_allocator_count; i++)
-	{
-		if (!s_allocators[i].contains(ptr))
-			continue;
-		const size_t size = s_allocators[i].allocation_size(ptr);
-		pthread_mutex_unlock(&s_allocator_lock);
-		return size;
-	}
-	pthread_mutex_unlock(&s_allocator_lock);
-
-	const auto& header = static_cast<MmapAllocationHeader*>(ptr)[-1];
-	return header.mmap_size - header.offset;
-}
-
 void* realloc(void* ptr, size_t size)
 {
 	if (ptr == nullptr)
 		return malloc(size);
 
+	bool was_mmapped = true;
+	size_t old_alloc_size = 0;
+
 	pthread_mutex_lock(&s_allocator_lock);
 	for (size_t i = 0; i < s_allocator_count; i++)
 	{
 		if (!s_allocators[i].contains(ptr))
 			continue;
-		if (!s_allocators[i].resize(ptr, size))
-			break;
-		pthread_mutex_unlock(&s_allocator_lock);
-		return ptr;
+		if (s_allocators[i].resize(ptr, size))
+		{
+			pthread_mutex_unlock(&s_allocator_lock);
+			return ptr;
+		}
+		was_mmapped = false;
+		old_alloc_size = s_allocators[i].allocation_size(ptr);
+		break;
 	}
 	pthread_mutex_unlock(&s_allocator_lock);
 
-	// TODO: maybe an in-place realloc for mmap-backed allocations?
+	if (was_mmapped)
+	{
+		auto& header = static_cast<MmapAllocationHeader*>(ptr)[-1];
+
+		old_alloc_size = header.mmap_size - header.offset;
+
+		if (size == old_alloc_size)
+			return ptr;
+
+		const size_t page_size = getpagesize();
+
+		size_t new_mmap_size = header.offset + size;
+		if (const auto rem = new_mmap_size % page_size)
+			new_mmap_size += page_size - rem;
+
+		if (size < old_alloc_size)
+		{
+			// TODO: should we drop back to bitmap based allocations if the new size is < mmap threshold?
+			const size_t shrink_bytes = header.mmap_size - new_mmap_size;
+			munmap(static_cast<uint8_t*>(ptr) - header.offset + new_mmap_size, shrink_bytes);
+			s_mmap_bytes -= shrink_bytes;
+			header.mmap_size = new_mmap_size;
+			return ptr;
+		}
+
+		const size_t extend_bytes = new_mmap_size - header.mmap_size;
+
+		void* extend_data = mmap(
+			static_cast<uint8_t*>(ptr) - header.offset + header.mmap_size,
+			extend_bytes,
+			PROT_READ | PROT_WRITE,
+			MAP_ANONYMOUS | MAP_PRIVATE | MAP_FIXED_NOREPLACE,
+			-1, 0
+		);
+
+		if (extend_data != MAP_FAILED)
+		{
+			s_mmap_bytes += extend_bytes;
+			header.mmap_size = new_mmap_size;
+			return ptr;
+		}
+	}
 
 	void* new_ptr = malloc(size);
 	if (new_ptr == nullptr)
 		return nullptr;
 
-	memcpy(new_ptr, ptr, BAN::Math::min(allocation_size(ptr), size));
+	memcpy(new_ptr, ptr, BAN::Math::min(old_alloc_size, size));
 
 	free(ptr);
 
@@ -449,7 +480,20 @@ size_t malloc_usable_size(void* ptr)
 {
 	if (ptr == nullptr)
 		return 0;
-	return allocation_size(ptr);
+
+	pthread_mutex_lock(&s_allocator_lock);
+	for (size_t i = 0; i < s_allocator_count; i++)
+	{
+		if (!s_allocators[i].contains(ptr))
+			continue;
+		const size_t size = s_allocators[i].allocation_size(ptr);
+		pthread_mutex_unlock(&s_allocator_lock);
+		return size;
+	}
+	pthread_mutex_unlock(&s_allocator_lock);
+
+	const auto& header = static_cast<MmapAllocationHeader*>(ptr)[-1];
+	return header.mmap_size - header.offset;
 }
 
 struct mallinfo mallinfo(void)