title: set_memory_uc() function of linux API
body: <p>I know some virtual address from user space, I want to uncache the space. I try to use the API set_memory_uc in a kernel module. </p>
  
  <p>I look at the kernel code and found this API accepting virtual address as first argument, but in this API it uses __pa() to convert it to PA. However, __pa() only valid for kernel space virtual address right? So I wonder if I can use set_memory_uc with user space virtual address as argument.</p>
  
  <p>What I expect is a function like :</p>
  
  <p>uncache(VA)   VA is a user space virtual address, this function uncache one page according to the virtual address (I will mlock the page first).</p>
  
  <p>Thank you!</p>