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

src/userprog/exception.c

@@ -1,10 +1,18 @@
 #include "userprog/exception.h"
 #include <inttypes.h>
 #include <stdio.h>
+#include "stdbool.h"
 #include "userprog/gdt.h"
 #include "threads/interrupt.h"
 #include "threads/thread.h"
+#include "userprog/pagedir.h"
+#ifdef VM
 #include "vm/stackgrowth.h"
+#include "vm/frame.h"
+#include "vm/page.h"
+#include "devices/swap.h"
+#include "threads/vaddr.h"
+#endif
 
 /* Number of page faults processed. */
 static long long page_fault_cnt;
@@ -147,12 +155,39 @@ page_fault (struct intr_frame *f)
   user = (f->error_code & PF_U) != 0;
 
 #ifdef VM
-  if (user && not_present)
+  struct thread *t = thread_current ();
+  if (user)
     {
+      if (not_present)
+        {
+          /* Check if the non-present user page is in the swap partition.
+             If so, swap it back into main memory, updating the PTE for
+             the faulted virtual address to point to the newly allocated
+             frame. */
+          if (page_in_swap (t, fault_addr))
+            {
+              size_t swap_slot = page_get_swap (t, fault_addr);
+              void *upage = pg_round_down (fault_addr);
+              void *kpage = frame_alloc (0, upage, t);
+              swap_in (kpage, swap_slot);
+
+              bool writeable = pagedir_is_writable (t->pagedir, upage);
+              if (pagedir_set_page (t->pagedir, upage, kpage, writeable)) return;
+            }
+
+          /* Handle user page faults that need to be resolved by dynamic
+             stack growth by checking if this is such a fault and responding
+             accordingly. */
           if (handle_stack_fault (fault_addr, f->esp)) return;
         }
+    }
   else
     { 
+      /* Handle kernel page faults that need to be resolved by dynamic stack
+         growth by checking if this is such a fault and responding
+         accordingly. */
+      if (not_present && handle_stack_fault (fault_addr, t->curr_esp)) return;
+
       f->eip = (void *)f->eax;
       f->eax = 0xffffffff;
       return;

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 +19,17 @@ 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' 
+   is flagged to be in the swap disk via the owner's page table. */
+bool
+page_in_swap (struct thread *owner, void *upage)
+{
+   uint32_t *pte = lookup_page (owner->pagedir, upage, false);
+   return pte != NULL &&
+          (*pte & (1 << SWAP_FLAG_BIT)) != 0;
 }
 
 /* Given that the page with user address 'upage' owned by 'owner' is flagged
@@ -32,8 +40,8 @@ page_get_swap (struct thread *owner, void *upage)
 {
    uint32_t *pte = lookup_page (owner->pagedir, upage, false);
 
+   ASSERT (pte != NULL);
    ASSERT ((*pte & PTE_P) == 0);
-   ASSERT ((*pte & (1 << SWAP_FLAG_BIT)) != 0);
 
    /* Masks the address bits and returns truncated value. */
    return ((*pte & PTE_ADDR) >> ADDR_START_BIT);

src/vm/page.h

@@ -4,6 +4,7 @@
 #include "threads/thread.h"
 
 void page_set_swap (struct thread *, void *, size_t);
+bool page_in_swap (struct thread *, void *);
 size_t page_get_swap (struct thread *, void *);
 
 #endif /* vm/frame.h */