fix: invalidate PTEs of evicted pages before eviction occurs to prevent modificationof pages mid-eviction

src/userprog/pagedir.c

@@ -7,7 +7,6 @@
 #include "threads/palloc.h"
 
 static uint32_t *active_pd (void);
-static void invalidate_pagedir (uint32_t *);
 
 /* Creates a new page directory that has mappings for kernel
    virtual addresses, but none for user virtual addresses.
@@ -278,7 +277,7 @@ active_pd (void)
    This function invalidates the TLB if PD is the active page
    directory.  (If PD is not active then its entries are not in
    the TLB, so there is no need to invalidate anything.) */
-static void
+void
 invalidate_pagedir (uint32_t *pd) 
 {
   if (active_pd () == pd) 

src/userprog/pagedir.h

@@ -17,5 +17,6 @@ void pagedir_set_accessed (uint32_t *pd, const void *upage, bool accessed);
 bool pagedir_is_writable (uint32_t *pd, const void *upage);
 void pagedir_set_writable (uint32_t *pd, const void *upage, bool writable);
 void pagedir_activate (uint32_t *pd);
+void invalidate_pagedir (uint32_t *pd);
 
 #endif /* userprog/pagedir.h */

src/vm/frame.c

@@ -93,6 +93,13 @@ frame_alloc (enum palloc_flags flags, void *upage, struct thread *owner)
       ASSERT (victim != NULL); /* get_victim () should never return null. */
 
       /* 2. Swap out victim into disk. */
+      /* Mark page as 'not present' and flag the page directory as having
+         been modified *before* eviction begins to prevent the owner of the
+         victim page from accessing/modifying it mid-eviction. */
+      pagedir_clear_page (owner->pagedir, upage);
+
+      // TODO: Lock PTE of victim page for victim process.
+
       size_t swap_slot = swap_out (victim->frame);
       page_set_swap (victim->owner, victim->upage, swap_slot);
 

src/vm/page.c

@@ -6,8 +6,8 @@
 #define ADDR_START_BIT 12
 
 /* Updates the 'owner' thread's page table entry for virtual address 'upage'
-   to have a present bit of 0 and stores the specified swap slot value in the
-   entry for later retrieval from disk. */
+   to flag the page as being stored in swap, and stores the specified swap slot
+   value in the entry at the address bits for later retrieval from disk. */
 void
 page_set_swap (struct thread *owner, void *upage, size_t swap_slot)
 {
@@ -19,9 +19,7 @@ page_set_swap (struct thread *owner, void *upage, size_t swap_slot)
    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);
+   invalidate_pagedir (owner->pagedir);
 }
 
 /* Returns true iff the page with user address 'upage' owned by 'owner'