CareerCup

Obviously, when we are running a program in userspace, we don't have access to the kernel virtual memory addresses. In that situation, the question is moot.

When we are running code in kernel mode, say, as a kernel module or driver, we normally access kernel virtual memory. But if the situation comes where our kernel mode code has to interact with some userspace component, we must be extremely careful that we translate "kernel virtual addresses" to "user virtual addresses" before passing them off, and vice-versa.

copy_to_user() copy_from_user() requires user virtual address

explain how to convert kernel virtual address to user virtual address with a simple example..

explain how to convert kernel virtual address to user virtual address with a simple example..

All answers above doesn't address why ? : Kernel doesn't map entire physical ram into its address space , so when it wants to use virtual address which is not mapped inside kernel address space it will temparory map those address into special region call pkmap, this time it needs to use user virtual address.

I think they might try to talk about swap. User virtual can swap out while kernel virtual can't. So you can not use user virtual in kernel. But you can use kernel virtual in user as long as the same pages are mapped to user address space.

In kernel space, memory allocated from high memory are virtual memories. These memory has to be released at earliest since, it may be needed by other process or drivers. Other than that, any memory allocated at kernel space will be logical memory.

kmalloc -> contiguous memory
vmalloc -> non contiguous memory

Read this in detail to understand the memory stuff..
kmalloc -> contiguous + logical address
vmalloc -> NON contiguous + virtual address

kmalloc VS. vmalloc?

There are two types of virtual addresses: user mode and kernel mode. All user applications/drivers runs in user virtual address space while the kernel mode drivers and OS runs in kernel mode virtual address space. Kernel mode drivers can access user virtual address but vice versa is not true. However kernel mode drivers accessing user virtual addresses can lead to problem. Whenever user mode app/driver wants to run, a process will be created which will provide user virtual address space. So even if app crashes, it will not affect the while system address space but of kernel mode driver crashes , it can lead to system crash.