Understanding the relation between user, kernel and physical addresses in pagedir_get_page()

Question

I am confused about the following comment:

/* Looks up the physical address that corresponds to user virtual
address UADDR in PD.  Returns the kernel virtual address
corresponding to that physical address, or a null pointer if
UADDR is unmapped. */

I understand the first sentence which is to find the actual physical address, however, I don't understand why the kernel virtual address correspond to that address is returned. In short, since uaddr is a user virtual address, then why is it related to kernel virtual address?

void *
pagedir_get_page (uint32_t *pd, const void *uaddr)
{
  uint32_t *pte;

  ASSERT (is_user_vaddr (uaddr));

  pte = lookup_page (pd, uaddr, false);
  if (pte != NULL && (*pte & PTE_P) != 0)
    return pte_get_page (*pte) + pg_ofs (uaddr);
  else
    return NULL;
}

Thanks in advance.

Answer 1

There are three kinds of addresses that the comment is talking about:

1. Physical address: this is the real address, I.E. the real exact position in the physical memory of your computer.
2. Kernel virtual address: this is the virtual address at which the kernel sees that physical address.
3. User virtual address: this is the virtual address at which the user space program sees that physical address.

So, with a simple ASCII art, this is the situation:

 User space    
  program 1         Kernel            RAM
+-----------+    +-----------+    +-----------+
| 0xAAAA000 |    | 0x1212000 |    | 0xA7EF000 |
| 0xBBBB000 |    | 0x4398000 |    | 0x0001000 |
| 0xCCCC000 |<---| 0x87FF000 |<---| 0x1234000 |
+-----------+    | ...       |    | ...       |
                 | ...       |    | ...       |
 User space      | ...       |    | ...       |
  program 2      | ...       |    | ...       |
+-----------+    | ...       |    | ...       |
| 0xDDDD000 |    | 0x7FF8000 |    | 0x3FFF000 |
| 0xEEEE000 |    | 0xABCD000 |    | 0x2010000 |
| 0xFFFF000 |<---| 0x98AE000 |<---| 0xA89A000 |
+-----------+    +-----------+    +-----------+

That function, given a valid user space virtual address, makes a lookup and retrieves the associated kernel space virtual address. This lookup can be done since both of them point to the same physical address, and there's a one-to-one correspondence between those. The lookup is done through the Kernel Page Table, which is probably what those two calls to lookup_page(...) and pte_get_page(...) do.