What is the difference between vmalloc and kmalloc

Question

I've googled around and found most people advocating the use of kmalloc, as you're guaranteed to get contiguous physical blocks of memory. However, it also seems as though kmalloc can fail if a contiguous physical block that you want can't be found.
What are the advantages of having a contiguous block of memory? Specifically, why would I need to have a contiguous physical block of memory in a system call? Is there any reason I couldn't just use vmalloc?
Finally, if I were to allocate memory during the handling of a system call, should I specify GFP_ATOMIC? Is a system call executed in an atomic context?

GFP_ATOMIC
The allocation is high-priority and does not sleep. This is the flag to use in interrupt handlers, bottom halves and other situations where you cannot sleep.

GFP_KERNEL This is a normal allocation and might block. This is the flag to use in process context code when it is safe to sleep.

Answer 1

The kmalloc() & vmalloc() functions are a simple interface for obtaining kernel memory in byte-sized chunks.

1. The kmalloc() function guarantees that the pages are physically contiguous (and virtually contiguous).

2. The vmalloc() function works in a similar fashion to kmalloc(), except it allocates memory that is only virtually contiguous and not necessarily physically contiguous.