Persistently mapped memory

If you need to map memory on host, it may happen that two allocations are assigned to the same VkDeviceMemory block, so if you map them both at the same time, it will cause error because mapping single memory block multiple times is illegal in Vulkan.

It is safer, more convenient and more efficient to use special feature designed for that: persistently mapped memory. Allocations made with VMA_ALLOCATION_CREATE_PERSISTENT_MAP_BIT flag set in VmaAllocationCreateInfo::flags are returned from device memory blocks that stay mapped all the time, so you can just access CPU pointer to it. VmaAllocationInfo::pMappedData pointer is already offseted to the beginning of particular allocation. Example:

VkBufferCreateInfo bufCreateInfo = { VK_STRUCTURE_TYPE_BUFFER_CREATE_INFO };
bufCreateInfo.size = 1024;
bufCreateInfo.usage = VK_BUFFER_USAGE_TRANSFER_SRC_BIT;

VmaAllocationCreateInfo allocCreateInfo = {};
allocCreateInfo.usage = VMA_MEMORY_USAGE_CPU_ONLY;
allocCreateInfo.flags = VMA_ALLOCATION_CREATE_PERSISTENT_MAP_BIT;

VkBuffer buf;
VmaAllocation alloc;
VmaAllocationInfo allocInfo;
vmaCreateBuffer(allocator, &bufCreateInfo, &allocCreateInfo, &buf, &alloc, &allocInfo);

.// Buffer is immediately mapped. You can access its memory.
memcpy(allocInfo.pMappedData, myData, 1024);

Memory in Vulkan doesn't need to be unmapped before using it e.g. for transfers, but if you are not sure whether it's HOST_COHERENT (here is surely is because it's created with VMA_MEMORY_USAGE_CPU_ONLY), you should check it. If it's not, you should call vkInvalidateMappedMemoryRanges() before reading and vkFlushMappedMemoryRanges() after writing to mapped memory on CPU. Example:

VkMemoryPropertyFlags memFlags;
vmaGetMemoryTypeProperties(allocator, allocInfo.memoryType, &memFlags);
if((memFlags & VK_MEMORY_PROPERTY_HOST_COHERENT_BIT) == 0)
{
    VkMappedMemoryRange memRange = { VK_STRUCTURE_TYPE_MAPPED_MEMORY_RANGE };
    memRange.memory = allocInfo.deviceMemory;
    memRange.offset = allocInfo.offset;
    memRange.size   = allocInfo.size;
    vkFlushMappedMemoryRanges(device, 1, &memRange);
}

On AMD GPUs on Windows, Vulkan memory from the type that has both DEVICE_LOCAL and HOST_VISIBLE flags should not be mapped for the time of any call to vkQueueSubmit() or vkQueuePresent(). Although legal, that would cause performance degradation because WDDM migrates such memory to system RAM. To ensure this, you can unmap all persistently mapped memory using just one function call. For details, see function vmaUnmapPersistentlyMappedMemory(), vmaMapPersistentlyMappedMemory().