On the other hand, linux does not have any concept of paging in kernel space. so linux doesnt reserve any memory for kernel. but if there is any app that allocates more than 2GB in kernel space, then you will see only 2GB available to userspace. (i.e., physical memory-kernel memory=user memory)

The Linux page allocator, from mm/page_alloc.c, is the main memory allocation mechanism in the Linux kernel. It has to deal with allocations from many parts of the Linux kernel, under many different circumstances. Consequently the Linux page allocator is fairly complex, and easiest to understand in the context of its environment. Because of this, this wiki article begins with an explanation of exactly what the page allocator needs to do, before going into the details of how things are done.