the cost of vmalloc

[Zheng Da]

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