src/vm/page.c

@@ -12,17 +12,16 @@ void
 page_set_swap (struct thread *owner, void *upage, size_t swap_slot)
 {
    uint32_t *pte = lookup_page (owner->pagedir, upage, false);
-   *pte &= ~PTE_P; //clears the first bit (present bit) to be 0
 
-   //ASSERT (swap_slot < (1 << 20)); //not sure if this is needed
+   /* Store the provided swap slot in the address bits of the page table
-
+      entry, truncating excess bits. */
-   //shifts the swap slot addr to take up bits 31 to 12
+   *pte |= (1 << SWAP_FLAG_BIT);
-   //uses bitwise & to make sure it does not affect flags
-   //then applies it to pte
-   *pte |= (1 << SWAP_FLAG_BIT); // sets the 9th bit
    uint32_t swap_slot_bits = (swap_slot << ADDR_START_BIT) & PTE_ADDR;
    *pte = (*pte & PTE_FLAGS) | swap_slot_bits;
 
+   /* Mark page as 'not present' and flag the page directory as having
+      been modified. */
+   pagedir_clear_page (owner->pagedir, upage);
 }
 
 /* Given that the page with user address 'upage' owned by 'owner' is flagged
@@ -33,11 +32,10 @@ page_get_swap (struct thread *owner, void *upage)
 {
    uint32_t *pte = lookup_page (owner->pagedir, upage, false);
 
-   //these should always be checked and true before using this func
    ASSERT ((*pte & PTE_P) == 0);
-   ASSERT ((*pte & (1 << 9)) == 1);
+   ASSERT ((*pte & (1 << SWAP_FLAG_BIT)) != 0);
 
-   //masks address bits and returns truncated value
+   /* Masks the address bits and returns truncated value. */
-   return ((*pte & PTE_ADDR) >> 12);
+   return ((*pte & PTE_ADDR) >> ADDR_START_BIT);
 }