the cost of vmalloc

Hello, I need to allocate fairly a large piece of memory, about 128KB or 256KB. I tried to use kmalloc, and it sometimes fails, but vmalloc in this case usually still work. But I'm not sure how costly vmalloc is. As far as I know, vmalloc needs to change the page table, and thus might need to invalidate TLB. It seems quite expensive. If it is, maybe I can allocate memory with kmalloc first, and then try to use vmalloc if kmalloc fails. Any suggestions? Thanks, Da