fix: implement ptable_lock for synchronising the SPT structure for insertions/accesses

Write back mmap file pages to file upon eviction

See merge request lab2425_autumn/pintos_22!59

src/threads/init.c

@@ -33,6 +33,7 @@
 #endif
 #ifdef VM
 #include "vm/frame.h"
+#include "vm/page.h"
 #include "devices/swap.h"
 #endif
 #ifdef FILESYS
@@ -104,6 +105,7 @@ main (void)
   paging_init ();
 #ifdef VM
   frame_init ();
+  shared_file_pages_init ();
 #endif
 
   /* Segmentation. */

src/threads/thread.c

@@ -265,11 +265,24 @@ thread_create (const char *name, int priority,
 #ifdef USERPROG
   /* Initialize the thread's file descriptor table. */
   t->fd_counter = MINIMUM_USER_FD;
+  bool success = hash_init (&t->open_files, fd_hash, fd_less, NULL);
+  if (success)
+    {
+      success = hash_init (&t->child_results, process_result_hash,
+                           process_result_less, t);
+      if (!success)
+        hash_destroy (&t->open_files, NULL);
+#ifdef VM
+      else
+        {
+          success = init_pages (&t->pages);
+          if (!success)
+            hash_destroy (&t->child_results, NULL);
+        }
+#endif
+    }
 
-  if (!hash_init (&t->open_files, fd_hash, fd_less, NULL)
+  if (!success)
-      || !hash_init (&t->child_results, process_result_hash,
-                     process_result_less, t)
-      || !hash_init (&t->pages, page_hash, page_less, NULL))
     {
       palloc_free_page (t);
       free (t->result);
@@ -723,6 +736,10 @@ init_thread (struct thread *t, const char *name, int nice, int priority,
   t->recent_cpu = recent_cpu;
   t->priority = t->base_priority;
 
+#ifdef VM
+  lock_init (&t->ptable_lock);
+#endif
+
   old_level = intr_disable ();
   list_push_back (&all_list, &t->allelem);
   intr_set_level (old_level);

src/threads/thread.h

@@ -137,6 +137,9 @@ struct thread
 
     struct hash pages;                  /* Table of open user pages. */
 
+    struct lock ptable_lock;           /* Protects access to the process's
+                                           page directory and SPT. */
+
     /* Memory mapped files for user virtual memory. */
     struct hash mmap_files;             /* List of memory mapped files. */
     unsigned int mmap_counter;          /* Counter for memory mapped files. */

src/userprog/exception.c

@@ -4,19 +4,19 @@
 #include "stdbool.h"
 #include "threads/synch.h"
 #include "userprog/gdt.h"
-#include "userprog/pagedir.h"
-#include "userprog/process.h"
 #include "threads/interrupt.h"
-#include "threads/palloc.h"
 #include "threads/thread.h"
-#include "threads/vaddr.h"
+#ifdef VM
 #include "vm/frame.h"
 #include "vm/page.h"
 #include "devices/swap.h"
+#include "threads/vaddr.h"
 #include "userprog/pagedir.h"
+#endif
 
 #define MAX_STACK_SIZE (8 * 1024 * 1024) // 8MB
 #define MAX_STACK_OFFSET 32 // 32 bytes offset below stack pointer (ESP)
+
 /* Number of page faults processed. */
 static long long page_fault_cnt;
 
@@ -169,6 +169,9 @@ page_fault (struct intr_frame *f)
    So we attempt to grow the stack. If this does not work, we check our SPT to
    see if the page is expected to have data loaded in memory. */
   void *upage = pg_round_down (fault_addr);
+
+  printf ("FATHER, I FAULT AT %p!\n", fault_addr);
+
   if (not_present && is_user_vaddr (upage) && upage != NULL)
     {
       if (fetch_page (upage, write))
@@ -188,6 +191,7 @@ page_fault (struct intr_frame *f)
     return;
   }
 
+
   /* To implement virtual memory, delete the rest of the function
      body, and replace it with code that brings in the page to
      which fault_addr refers. */
@@ -250,17 +254,21 @@ grow_stack (void *upage)
 bool
 fetch_page (void *upage, bool write)
 {
+  struct thread *t = thread_current ();
   /* Check if the page is in the supplemental page table. That is, it is a page
      that is expected to be in memory. */
-  struct page_entry *page = page_get (upage);
+  lock_acquire (&t->ptable_lock);
+  struct page_entry *page = page_get (t, upage);
+  lock_release (&t->ptable_lock);
   if (page == NULL)
+    {
       return false;
+    }
 
   /* 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. */
-  struct thread *t = thread_current ();
   if (page_in_swap (t, upage))
     {
       /* NOTE: This code should be refactored and moved into helper functions
@@ -275,9 +283,6 @@ fetch_page (void *upage, bool write)
 
       bool writeable = pagedir_is_writable (t->pagedir, upage);
 
-    /* TODO: When this returns false we should quit the page fault,
-      but currently we continue and check the stack conditions in the
-      page fault handler. */
       return pagedir_set_page (t->pagedir, upage, kpage, writeable);
     }
 
@@ -290,7 +295,8 @@ fetch_page (void *upage, bool write)
   switch (page->type) {
     case PAGE_MMAP:
     case PAGE_FILE:
-      success = page_load_file (page, page->writable);
+    case PAGE_SHARED:
+      success = page_load_file (page);
       break;
     default:
       return false;

src/userprog/pagedir.c

@@ -2,9 +2,12 @@
 #include <stdbool.h>
 #include <stddef.h>
 #include <string.h>
+#include "devices/swap.h"
 #include "threads/init.h"
 #include "threads/pte.h"
 #include "threads/palloc.h"
+#include "vm/frame.h"
+#include "vm/page.h"
 
 static uint32_t *active_pd (void);
 
@@ -39,8 +42,14 @@ pagedir_destroy (uint32_t *pd)
         uint32_t *pte;
         
         for (pte = pt; pte < pt + PGSIZE / sizeof *pte; pte++)
-          if (*pte & PTE_P) 
+          {
-            palloc_free_page (pte_get_page (*pte));
+            if (page_is_shared_pte (pte))
+              continue;
+            else if (page_in_swap_pte (pte))
+              swap_drop (page_get_swap_pte (pte));
+            else if (*pte & PTE_P)
+              frame_free (pte_get_page (*pte));
+          }
         palloc_free_page (pt);
       }
   palloc_free_page (pd);

src/userprog/syscall.c

@@ -10,6 +10,7 @@
 #include "threads/synch.h"
 #include "userprog/process.h"
 #include "userprog/pagedir.h"
+#include "vm/frame.h"
 #include "vm/page.h"
 #include "vm/mmap.h"
 #include <stdio.h>
@@ -52,9 +53,14 @@ static mapid_t syscall_mmap (int fd, void *addr);
 static void syscall_munmap (mapid_t mapping);
 
 static struct open_file *fd_get_file (int fd);
-static void validate_user_pointer (const void *ptr, size_t size,
+static void validate_user_ptr (const void *start, size_t size,
-                                   bool check_write);
+                                   bool write);
-static void validate_user_string (const char *str, bool check_write);
+static void validate_and_pin_user_ptr (const void *start, size_t size,
+                                       bool write);
+static void validate_and_pin_user_str (const char *ptr);
+static void unpin_user_ptr (const void *start, size_t size);
+
+static void unpin_user_str (const char *ptr);
 static int get_user (const uint8_t *);
 static bool put_user (uint8_t *, uint8_t);
 
@@ -107,7 +113,7 @@ static void
 syscall_handler (struct intr_frame *f)
 {
   /* First, read the system call number from the stack. */
-  validate_user_pointer (f->esp, sizeof (uintptr_t), false);
+  validate_user_ptr (f->esp, sizeof (uintptr_t), false);
   uintptr_t syscall_number = *(int *)f->esp;
   thread_current ()->curr_esp = f->esp;
 
@@ -118,7 +124,7 @@ syscall_handler (struct intr_frame *f)
   struct syscall_arguments syscall = syscall_lookup[syscall_number];
 
   /* Next, read and copy the arguments from the stack pointer. */
-  validate_user_pointer (f->esp + sizeof (uintptr_t),
+  validate_user_ptr (f->esp + sizeof (uintptr_t),
                          syscall.arity * sizeof (uintptr_t), false);
   uintptr_t args[MAX_SYSCALL_ARGS] = { 0 };
   for (int i = 0; i < syscall.arity && i < MAX_SYSCALL_ARGS; i++)
@@ -151,9 +157,11 @@ syscall_exit (int status)
 static pid_t
 syscall_exec (const char *cmd_line)
 {
-  validate_user_string (cmd_line, false);
+  validate_and_pin_user_str (cmd_line);
+  pid_t pid = process_execute (cmd_line);
+  unpin_user_str (cmd_line);
 
-  return process_execute (cmd_line); /* Returns the PID of the new process */
+  return pid;
 }
 
 /* Handles the syscall of wait. Effectively a wrapper for process_wait as the
@@ -170,13 +178,15 @@ syscall_wait (pid_t pid)
 static bool
 syscall_create (const char *file, unsigned initial_size)
 {
-  validate_user_string (file, false);
+  validate_and_pin_user_str (file);
 
   /* Acquire the file system lock to prevent race conditions. */
   lock_acquire (&filesys_lock);
   bool status = filesys_create (file, initial_size);
   lock_release (&filesys_lock);
 
+  unpin_user_str (file);
+
   /* Return the status of the file creation. */
   return status;
 }
@@ -187,13 +197,15 @@ syscall_create (const char *file, unsigned initial_size)
 static bool
 syscall_remove (const char *file)
 {
-  validate_user_string (file, false);
+  validate_and_pin_user_str (file);
 
   /* Acquire the file system lock to prevent race conditions. */
   lock_acquire (&filesys_lock);
   bool status = filesys_remove (file);
   lock_release (&filesys_lock);
 
+  unpin_user_str (file);
+
   /* Return the status of the file removal. */
   return status;
 }
@@ -205,13 +217,15 @@ syscall_remove (const char *file)
 static int
 syscall_open (const char *file)
 {
-  validate_user_string (file, false);
+  validate_and_pin_user_str (file);
 
   /* Acquire the file system lock to prevent race conditions. */
   lock_acquire (&filesys_lock);
   struct file *ptr = filesys_open (file);
   lock_release (&filesys_lock);
 
+  unpin_user_str (file);
+
   /* If the file could not be opened, return failure. */
   if (ptr == NULL)
     return EXIT_FAILURE;
@@ -271,10 +285,11 @@ syscall_read (int fd, void *buffer, unsigned size)
   if (fd < STDIN_FILENO || fd == STDOUT_FILENO)
     return EXIT_FAILURE;
 
-  validate_user_pointer (buffer, size, true);
-
   if (fd == STDIN_FILENO)
     {
+      /* Validate the user buffer. */
+      validate_user_ptr (buffer, size, true);
+
       /* Reading from the console. */
       char *write_buffer = buffer;
       for (unsigned i = 0; i < size; i++)
@@ -292,13 +307,19 @@ syscall_read (int fd, void *buffer, unsigned size)
       if (file_info == NULL)
         return EXIT_FAILURE;
 
+      /* Validate the user buffer, and pin the pages to prevent eviction. */
+      validate_and_pin_user_ptr (buffer, size, true);
+
       /* Acquire the file system lock to prevent race-conditions. */
       lock_acquire (&filesys_lock);
-      int bytes_written = file_read (file_info->file, buffer, size);
+      int bytes_read = file_read (file_info->file, buffer, size);
       lock_release (&filesys_lock);
 
+      /* Unpin the pages to allow eviction. */
+      unpin_user_ptr (buffer, size);
+
       /* Return the number of bytes read. */
-      return bytes_written;
+      return bytes_read;
     }
 }
 
@@ -314,10 +335,11 @@ syscall_write (int fd, const void *buffer, unsigned size)
   if (fd <= 0)
     return 0;
 
-  validate_user_pointer (buffer, size, false);
-
   if (fd == STDOUT_FILENO)
     {
+      /* Validate the user buffer. */
+      validate_user_ptr (buffer, size, false);
+
       /* Writing to the console. */
       putbuf (buffer, size);
 
@@ -333,13 +355,19 @@ syscall_write (int fd, const void *buffer, unsigned size)
       if (file_info == NULL)
         return 0;
 
+      /* Validate the user buffer, and pin the pages to prevent eviction. */
+      validate_and_pin_user_ptr (buffer, size, false);
+
       /* Acquire the file system lock to prevent race conditions. */
       lock_acquire (&filesys_lock);
-      int bytes = file_write (file_info->file, buffer, size);
+      int bytes_written = file_write (file_info->file, buffer, size);
       lock_release (&filesys_lock);
 
+      /* Unpin the pages to allow eviction. */
+      unpin_user_ptr (buffer, size);
+
       /* Return the number of bytes written. */
-      return bytes;
+      return bytes_written;
     }
 }
 
@@ -426,17 +454,21 @@ syscall_mmap (int fd, void *addr)
   if (file_size == 0)
     return MMAP_FAILURE;
 
-  /* Ensure that the mmap page doesn't overlap with the stack. */
+  /* ensures the page for mmap does not overlap with the stack */
-  if (addr >= (thread_current ()->curr_esp) - PGSIZE)
+  if (addr >= (thread_current ()->curr_esp - PGSIZE))
     return MMAP_FAILURE;
 
   /* Check and ensure that there is enough space in the user virtual memory to
      hold the entire file. */
+  lock_acquire (&thread_current ()->ptable_lock);
   for (off_t ofs = 0; ofs < file_size; ofs += PGSIZE)
     {
-      if (page_get (addr + ofs) != NULL)
+      if (page_get (thread_current (), addr + ofs) != NULL)
+        {
+          lock_release (&thread_current ()->ptable_lock);
           return MMAP_FAILURE;
         }
+    }
 
   /* Map the file data into the user virtual memory starting from addr. */
   for (off_t ofs = 0; ofs < file_size; ofs += PGSIZE)
@@ -445,15 +477,20 @@ syscall_mmap (int fd, void *addr)
       off_t zero_bytes = PGSIZE - read_bytes;
 
       if (page_insert_file (file, ofs, addr + ofs, read_bytes, zero_bytes, true,
-                            PAGE_MMAP) == NULL)
+                       PAGE_FILE) == NULL)
+        {
+          lock_release (&thread_current ()->ptable_lock);
           return MMAP_FAILURE;
         }
+    }
+  lock_release (&thread_current ()->ptable_lock);
 
   /* Create a new mapping for the file. */
   struct mmap_entry *mmap = mmap_insert (file, addr);
   if (mmap == NULL)
       return MMAP_FAILURE;
 
+
   return mmap->mapping;
 }
 
@@ -532,55 +569,128 @@ fd_get_file (int fd)
   return hash_entry (e, struct open_file, elem);
 }
 
+/* Helper function that validates a block of memory and optionally pins frames.
+   thread_exit() if the memory is invalid. Used only by the two helper functions
+   validate_user_ptr and validate_and_pin_user_ptr. See the comments for those
+    functions for more details on each. */
+static void
+validate_user_ptr_helper (const void *start, size_t size, bool write, bool pin)
+{
+  if (size == 0)
+    return;
+
+  /* ptr < ptr + size - 1, so sufficient to check that (ptr + size -1) is a
+     valid user virtual memory address. */
+  void *end = start + size - 1;
+  if (!is_user_vaddr (end))
+    syscall_exit (EXIT_FAILURE);
+
+  for (const void *ptr = pg_round_down (start); ptr <= end; ptr += PGSIZE)
+  {
+    int result;
+
+    /* Check read access to pointer. */
+    if ((result = get_user (ptr)) == -1)
+      syscall_exit (EXIT_FAILURE);
+
+    /* Check write access to pointer (if required). */
+    if (write && !put_user ((uint8_t *)ptr, result))
+      syscall_exit (EXIT_FAILURE);
+
+    /* If pin is set, pin the frame to prevent eviction. */
+    if (pin)
+    {
+      void *kpage = pagedir_get_page(thread_current()->pagedir, ptr);
+      if (kpage == NULL)
+      {
+        // If it was evicted, try to load it back in.
+        ptr -= PGSIZE;
+        continue;
+      }
+
+      frame_pin(kpage);
+    }
+  }
+}
+
 /* Validates if a block of memory starting at PTR and of size SIZE bytes is
    fully contained within valid user virtual memory. thread_exit () if the
    memory is invalid.
    If the size is 0, the function does no checks and returns PTR. */
 static void
-validate_user_pointer (const void *ptr, size_t size, bool check_write)
+validate_user_ptr (const void *start, size_t size, bool write)
 {
-  if (size == 0)
+  validate_user_ptr_helper (start, size, write, false);
-    return;
+}
-  /* ptr < ptr + size - 1, so sufficient to check that (ptr + size -1) is a
+
-     valid user virtual memory address. */
+/* Validates if a block of memory starting at PTR and of size SIZE bytes is
-  void *last = ptr + size - 1;
+   fully contained within valid user virtual memory. thread_exit () if the
-  if (!is_user_vaddr (last))
+   memory is invalid. The function also checks if the memory is writable if
-    syscall_exit (EXIT_FAILURE);
+   WRITE flag is set.
-  ptr = pg_round_down (ptr);
+
-  while (ptr <= last)
+   The function attempts to preload the pages in case they are not in memory
+   yet (e.g., in a swap, lazy loading). If this is successful, the frame pages
+   are pinned to prevent eviction prior to access.
+
+   As such, a call to this function MUST be followed by a call to
+   unpin_user_ptr (START, SIZE) to unpin the pages and allow eviction.
+
+   If the size is 0, the function does no checks and returns PTR. */
+static void
+validate_and_pin_user_ptr (const void *start, size_t size, bool write)
 {
-      int result;
+  validate_user_ptr_helper (start, size, write, true);
-      /* Check read access to pointer. */
+}
-      if ((result = get_user (ptr)) == -1)
+
-        syscall_exit (EXIT_FAILURE);
+/* Unpins all the pages containing a block of memory starting at START and of
-      /* Check write access to pointer (if required). */
+   size SIZE bytes.
-      if (check_write && !put_user (ptr, result))
+
-        syscall_exit (EXIT_FAILURE);
+   Pre: The pages were previously pinned by validate_and_pin_user_ptr (START,
-      ptr += PGSIZE;
+        SIZE). */
+static void
+unpin_user_ptr (const void *start, size_t size)
+{
+  void *end = start + size - 1;
+
+  /* We don't need to do any checks as this function is always called after
+     validate_and_pin_user_ptr. */
+  /* Go through all pages in the block range, unpinning the frames. */
+  for (void *ptr = pg_round_down (start); ptr <= end; ptr += PGSIZE)
+  {
+    void *kpage = pagedir_get_page (thread_current ()->pagedir, ptr);
+    ASSERT (kpage != NULL);
+
+    frame_unpin (kpage);
   }
 }
 
 /* Validates of a C-string starting at ptr is fully contained within valid
    user virtual memory. thread_exit () if the memory is invalid. */
 static void
-validate_user_string (const char *ptr, bool check_write)
+validate_and_pin_user_str (const char *ptr)
 {
   size_t offset = (uintptr_t) ptr % PGSIZE;
 
   for (;;)
     {
-      void *page = pg_round_down (ptr);
-
-      if (!is_user_vaddr (page))
-        syscall_exit (EXIT_FAILURE);
       if (!is_user_vaddr (ptr))
         syscall_exit (EXIT_FAILURE);
-      int result;
+
-      if ((result = get_user ((const uint8_t *)ptr)) == -1)
+      if (get_user ((const uint8_t *)ptr) == -1)
-        syscall_exit (EXIT_FAILURE);
-      if (check_write && !put_user ((uint8_t *)ptr, result))
         syscall_exit (EXIT_FAILURE);
 
+      /* Pin the frame to prevent eviction. */
+      void *page = pg_round_down (ptr);
+      void *kpage = pagedir_get_page (thread_current ()->pagedir, page);
+      if (kpage == NULL)
+        {
+          // If it was evicted, attempt to reload.
+          ptr -= PGSIZE;
+          continue;
+        }
+
+      frame_pin (kpage);
+
       while (offset < PGSIZE)
         {
           if (*ptr == '\0')
@@ -591,7 +701,36 @@ validate_user_string (const char *ptr, bool check_write)
         }
 
       offset = 0;
+    }
+}
 
+/* Unpins all the pages containing a C-string starting at PTR.
+
+   Pre: The pages were previously pinned by validate_and_pin_user_str (PTR).
+        PTR points to a valid C string that ends with '\0'. */
+static void
+unpin_user_str (const char *ptr)
+{
+  size_t offset = (uintptr_t)ptr % PGSIZE;
+  const char *str_ptr = ptr;
+
+  for (;;)
+  {
+    void *page = pg_round_down(str_ptr);
+    void *kpage = pagedir_get_page(thread_current()->pagedir, page);
+    ASSERT(kpage != NULL);
+    frame_unpin (kpage);
+
+    /* Scan until end of string or page */
+    while (offset < PGSIZE)
+    {
+      if (*str_ptr == '\0')
+        return;  /* Found end of string */
+      str_ptr++;
+      offset++;
+    }
+
+    offset = 0;
   }
 }
 

src/vm/frame.c

@@ -6,6 +6,7 @@
 #include "page.h"
 #include "filesys/file.h"
 #include "threads/malloc.h"
+#include "threads/thread.h"
 #include "threads/vaddr.h"
 #include "userprog/pagedir.h"
 #include "userprog/syscall.h"
@@ -36,7 +37,9 @@ struct frame_metadata
   {
      void *frame;  /* The kernel virtual address holding the frame. */
      void *upage; /* The user virtual address pointing to the frame. */
-     struct thread *owner; /* Pointer to the thread that owns the frame. */
+     struct list owners;          /* List of threads that own the frame. */
+     bool pinned;                 /* Indicates wheter the frame should be
+                                     considered as an eviction candidate.*/
      struct hash_elem hash_elem;  /* Tracks the position of the frame metadata
                                      within 'frame_table', whose key is the 
                                      kernel virtual address of the frame. */
@@ -50,7 +53,10 @@ hash_less_func frame_metadata_less;
 
 static struct list_elem *lru_next (struct list_elem *e);
 static struct list_elem *lru_prev (struct list_elem *e);
+static struct frame_metadata *frame_metadata_get (void *frame);
 static struct frame_metadata *get_victim (void);
+static void free_owners (struct list *owners);
+static struct frame_metadata *frame_metadata_find (void *frame);
 
 /* Initialize the frame system by initializing the frame (hash) table with
    the frame_metadata hashing and comparison functions, as well as initializing
@@ -93,7 +99,8 @@ frame_alloc (enum palloc_flags flags, void *upage, struct thread *owner)
       ASSERT (victim != NULL); /* get_victim () should never return null. */
 
       /* 2. Handle victim page writing based on its type. */
-      struct page_entry *victim_page = page_get (victim->upage);
+      lock_acquire (&owner->ptable_lock);
+      struct page_entry *victim_page = page_get (thread_current (), victim->upage);
       if (victim_page != NULL && victim_page->type == PAGE_MMAP)
         {
           /* If it was a memory-mapped file page, we just write it back
@@ -104,13 +111,17 @@ frame_alloc (enum palloc_flags flags, void *upage, struct thread *owner)
               file_write_at (victim_page->file, victim->upage,
                              victim_page->read_bytes, victim_page->offset);
               lock_release (&filesys_lock);
+              lock_release (&owner->ptable_lock);
             }
-
         }
       else
+        {
           /* Otherwise, insert the page into swap. */
-        page_insert_swapped (victim->upage, victim->frame, victim->owner);
+          page_insert_swapped (victim->upage, victim->frame, &victim->owners);
+        }
 
+      /* Free victim's owners. */
+      free_owners (&victim->owners);
 
       /* If zero flag is set, zero out the victim page. */
       if (flags & PAL_ZERO)
@@ -131,6 +142,8 @@ frame_alloc (enum palloc_flags flags, void *upage, struct thread *owner)
       /* Must own lru_lock here, as otherwise there is a race condition
          with next_victim either being NULL or uninitialized. */
       frame_metadata = malloc (sizeof (struct frame_metadata));
+      if (frame_metadata == NULL)
+        PANIC ("Couldn't allocate memory for frame metadata!\n");
       frame_metadata->frame = frame;
 
       /* Newly allocated frames are pushed to the back of the circular queue
@@ -150,31 +163,55 @@ frame_alloc (enum palloc_flags flags, void *upage, struct thread *owner)
       hash_insert (&frame_table, &frame_metadata->hash_elem);
     }
 
+  struct frame_owner *frame_owner = malloc (sizeof (struct frame_owner));
+  if (frame_owner == NULL)
+    PANIC ("Couldn't allocate memory for frame owner!\n");
+  frame_owner->owner = owner;
+  list_init (&frame_metadata->owners);
+  list_push_back (&frame_metadata->owners, &frame_owner->elem);
   frame_metadata->upage = upage;
-  frame_metadata->owner = owner;
+  frame_metadata->pinned = false;
   lock_release (&lru_lock);
-
   return frame_metadata->frame;
 }
 
+void
+frame_pin (void *frame)
+{
+  struct frame_metadata *frame_metadata = frame_metadata_get (frame);
+  if (frame_metadata == NULL)
+    PANIC ("Attempted to pin a frame at an unallocated kernel address '%p'\n",
+           frame);
+
+  frame_metadata->pinned = true;
+}
+
+void
+frame_unpin (void *frame)
+{
+  struct frame_metadata *frame_metadata = frame_metadata_get (frame);
+  if (frame_metadata == NULL)
+    PANIC ("Attempted to unpin a frame at an unallocated kernel address '%p'\n",
+           frame);
+
+  frame_metadata->pinned = false;
+}
+
 /* Attempt to deallocate a frame for a user process by removing it from the
    frame table as well as lru_list, and freeing the underlying page
    memory & metadata struct. Panics if the frame isn't active in memory. */
 void
 frame_free (void *frame)
   {
-    struct frame_metadata key_metadata;
+    struct frame_metadata *frame_metadata = frame_metadata_find (frame);
-    key_metadata.frame = frame;
+    if (frame_metadata == NULL)
-
+      PANIC ("Attempted to free a frame at kernel address %p, "
-    struct hash_elem *e =
+             "but this address is not allocated!\n",
-      hash_delete (&frame_table, &key_metadata.hash_elem);
+             frame);
-    if (e == NULL) PANIC ("Attempted to free a frame at kernel address %p, "
-                          "but this address is not allocated!\n", frame);
-
-    struct frame_metadata *frame_metadata =
-      hash_entry (e, struct frame_metadata, hash_elem);
 
+    free_owners (&frame_metadata->owners);
     lock_acquire (&lru_lock);
+    hash_delete (&frame_table, &frame_metadata->hash_elem);
     list_remove (&frame_metadata->list_elem);
 
     /* If we're freeing the frame marked as the next victim, update
@@ -193,33 +230,125 @@ frame_free (void *frame)
     palloc_free_page (frame);
   }
 
-/* TODO: Account for page aliases when checking accessed bit. */
+/* Add a thread to a frame's frame_metadata owners list. */
-/* A pre-condition for calling this function is that the calling thread
+bool
-   owns lru_lock and that lru_list is non-empty. */
+frame_owner_insert (void *frame, struct thread *owner)
+{
+  struct frame_metadata *frame_metadata = frame_metadata_find (frame);
+  if (frame_metadata == NULL)
+    return false;
+
+  struct frame_owner *frame_owner = malloc (sizeof (struct frame_owner));
+  if (frame_owner == NULL)
+    return false;
+  frame_owner->owner = owner;
+  list_push_back (&frame_metadata->owners, &frame_owner->elem);
+  return true;
+}
+
+/* Remove and deallocate a frame owner from the frame_metadata owners list.
+  */
+void
+frame_owner_remove (void *frame, struct thread *owner)
+{
+  struct frame_metadata *frame_metadata = frame_metadata_find (frame);
+  if (frame_metadata == NULL)
+    PANIC ("Attempted to remove an owner from a frame at kernel "
+            "address %p, but this address is not allocated!\n",
+            frame);
+
+  struct list_elem *oe;
+  for (oe = list_begin (&frame_metadata->owners);
+        oe != list_end (&frame_metadata->owners);)
+    {
+      struct frame_owner *frame_owner
+          = list_entry (oe, struct frame_owner, elem);
+      oe = list_next (oe);
+      if (frame_owner->owner == owner)
+        {
+          list_remove (&frame_owner->elem);
+          free (frame_owner);
+          return;
+        }
+    }
+  NOT_REACHED ();
+}
+
+/* Find a frame_metadata entry in the frame table. */
+static struct frame_metadata *
+frame_metadata_find (void *frame)
+{
+  struct frame_metadata key_metadata;
+  key_metadata.frame = frame;
+
+  struct hash_elem *e = hash_find (&frame_table, &key_metadata.hash_elem);
+  if (e == NULL)
+    return NULL;
+  return hash_entry (e, struct frame_metadata, hash_elem);
+}
+
+/* Obtain the next frame that should be evicted following the clock (second
+   chance) algorithm, ignoring pinned frames. A pre-condition for calling this 
+   function is that the calling thread owns lru_lock and that lru_list is 
+   non-empty. */
 static struct frame_metadata *
 get_victim (void)
   {
-    struct list_elem *e = next_victim;
+    struct list_elem *ve = next_victim;
     struct frame_metadata *frame_metadata;
-    uint32_t *pd;
+    bool found = false;
-    void *upage;
+    while (!found)
-    for (;;)
       {
-        frame_metadata = list_entry (e, struct frame_metadata, list_elem);
+        frame_metadata = list_entry (ve, struct frame_metadata, list_elem);
-        pd = frame_metadata->owner->pagedir;
-        upage = frame_metadata->upage;
-        e = lru_next (e);
 
-        if (!pagedir_is_accessed (pd, upage))
+        ve = lru_next (ve);
-          break;
+        struct list_elem *oe;
 
+        /* Skip pinned frames */
+        if (frame_metadata->pinned)
+          continue;
+
+        /* Returns once a frame that was not accessed by any owner is found. */
+        found = true;
+        for (oe = list_begin (&frame_metadata->owners);
+             oe != list_end (&frame_metadata->owners); oe = list_next (oe))
+          {
+            struct frame_owner *frame_owner
+                = list_entry (oe, struct frame_owner, elem);
+
+            lock_acquire (&frame_owner->owner->ptable_lock);
+            /* TODO: Account for death of frame_owner here! */
+
+            uint32_t *pd = frame_owner->owner->pagedir;
+            void *upage = frame_metadata->upage;
+
+            if (pagedir_is_accessed (pd, upage))
+              {
+                found = false;
                 pagedir_set_accessed (pd, upage, false);
               }
 
-    next_victim = e;
+            lock_release (&frame_owner->owner->ptable_lock);
+          }
+      }
+
+    next_victim = ve;
     return frame_metadata;
   }
 
+static void
+free_owners (struct list *owners)
+{
+  struct list_elem *oe;
+  for (oe = list_begin (owners); oe != list_end (owners);)
+    {
+      struct frame_owner *frame_owner
+          = list_entry (oe, struct frame_owner, elem);
+      oe = list_remove (oe);
+      free (frame_owner);
+    }
+}
+
 /* Hash function for frame metadata, used for storing entries in the
    frame table. */
 unsigned
@@ -246,6 +375,18 @@ frame_metadata_less (const struct hash_elem *a_, const struct hash_elem *b_,
   return a->frame < b->frame;
 }
 
+static struct frame_metadata *
+frame_metadata_get (void *frame)
+{
+  struct frame_metadata key_metadata;
+  key_metadata.frame = frame;
+
+  struct hash_elem *e = hash_find (&frame_table, &key_metadata.hash_elem);
+  if (e == NULL) return NULL;
+
+  return hash_entry (e, struct frame_metadata, hash_elem);
+}
+
 /* Returns the next recently used element after the one provided, which
    is achieved by iterating through lru_list like a circular queue
    (wrapping around the list at the tail). */

src/vm/frame.h

@@ -4,8 +4,19 @@
 #include "threads/thread.h"
 #include "threads/palloc.h"
 
+struct frame_owner
+{
+  struct thread *owner;  /* The thread that owns the frame. */
+  struct list_elem elem; /* List element for the list of owners. */
+};
+
 void frame_init (void);
 void *frame_alloc (enum palloc_flags, void *, struct thread *);
+void frame_pin (void *frame);
+void frame_unpin (void *frame);
 void frame_free (void *frame);
 
+bool frame_owner_insert (void *frame, struct thread *owner);
+void frame_owner_remove (void *frame, struct thread *owner);
+
 #endif /* vm/frame.h */

src/vm/mmap.c

@@ -1,5 +1,6 @@
 #include "mmap.h"
 #include "page.h"
+#include "threads/thread.h"
 #include "threads/vaddr.h"
 #include "threads/malloc.h"
 #include "userprog/syscall.h"
@@ -65,12 +66,13 @@ mmap_unmap (struct mmap_entry *mmap)
   /* Free all the pages associated with the mapping, writing back to the file
      if necessary. */
   off_t length = file_length (mmap->file);
+  lock_acquire (&thread_current ()->ptable_lock);
   for (off_t ofs = 0; ofs < length; ofs += PGSIZE)
   {
     void *upage = mmap->upage + ofs;
 
     /* Get the SPT page entry for this page. */
-    struct page_entry *page = page_get(upage);
+    struct page_entry *page = page_get(thread_current (), upage);
     if (page == NULL)
       continue;
 
@@ -85,6 +87,7 @@ mmap_unmap (struct mmap_entry *mmap)
     /* Remove the page from the supplemental page table. */
     hash_delete (&thread_current ()->pages, &page->elem);
   }
+  lock_release (&thread_current ()->ptable_lock);
 
   file_close (mmap->file);
   free (mmap);

src/vm/page.c

@@ -6,18 +6,45 @@
 #include "threads/pte.h"
 #include "threads/malloc.h"
 #include "threads/palloc.h"
+#include "threads/synch.h"
 #include "devices/swap.h"
+#include "threads/thread.h"
 #include "userprog/process.h"
 #include "userprog/pagedir.h"
 #include "vm/frame.h"
 
 #define SWAP_FLAG_BIT 9
+#define SHARED_FLAG_BIT 10
 #define ADDR_START_BIT 12
 
+struct hash shared_file_pages;
+struct lock shared_file_pages_lock;
+
+static unsigned page_hash (const struct hash_elem *e, void *aux UNUSED);
+static bool page_less (const struct hash_elem *a_, const struct hash_elem *b_,
+                       void *aux UNUSED);
+
+static void page_flag_shared (struct thread *owner, void *upage, bool shared);
+static unsigned shared_file_page_hash (const struct hash_elem *e,
+                                       void *aux UNUSED);
+static bool shared_file_page_less (const struct hash_elem *a_,
+                                   const struct hash_elem *b_,
+                                   void *aux UNUSED);
+static struct shared_file_page *shared_file_page_get (struct file *file,
+                                                      void *upage);
+
+/* Initialise a supplementary page table. */
+bool
+init_pages (struct hash *pages)
+{
+  ASSERT (pages != NULL);
+  return hash_init (pages, page_hash, page_less, NULL);
+}
+
 /* Hashing function needed for the SPT table. Returns a hash for an entry,
    based on its upage. */
-unsigned
+static unsigned
-page_hash (const struct hash_elem *e, UNUSED void *aux)
+page_hash (const struct hash_elem *e, void *aux UNUSED)
 {
   struct page_entry *page = hash_entry (e, struct page_entry, elem);
   return hash_ptr (page->upage);
@@ -25,7 +52,7 @@ page_hash (const struct hash_elem *e, UNUSED void *aux)
 
 /* Comparator function for the SPT table. Compares two entries based on their
    upages. */
-bool
+static bool
 page_less (const struct hash_elem *a_, const struct hash_elem *b_,
            void *aux UNUSED)
 {
@@ -43,11 +70,34 @@ static void page_set_swap (struct thread *owner, uint32_t *pte,
 /* Swap out 'owner' process's 'upage' stored at 'kpage'. Then, allocate and
    insert a new page entry into the user process thread's SPT representing
    this swapped out page. */
-struct page_entry *
+bool
-page_insert_swapped (void *upage, void *kpage, struct thread *owner)
+page_insert_swapped (void *upage, void *kpage, struct list *owners)
 {
+  struct file *exec_file = NULL;
+  struct list_elem *e;
+  for (e = list_begin (owners); e != list_end (owners); e = list_next (e))
+    {
+      struct thread *owner = list_entry (e, struct frame_owner, elem)->owner;
+      if (!lock_held_by_current_thread (&owner->ptable_lock))
+        {
+          lock_acquire (&owner->ptable_lock);
+        }
+
+      uint32_t *pte = lookup_page (owner->pagedir, upage, false);
+      if (exec_file != NULL || page_is_shared_pte (pte))
+        {
+          ASSERT (page_is_shared_pte (pte));
+          pagedir_clear_page (owner->pagedir, upage);
+          exec_file = owner->exec_file;
+          ASSERT (exec_file != NULL);
+
+          lock_release (&owner->ptable_lock);
+          continue;
+        }
+      ASSERT (list_size (owners) == 1);
+
       /* 1. Initialize swapped page entry. */
-  struct page_entry *page = page_get (upage);
+      struct page_entry *page = page_get (owner, upage);
       if (page == NULL)
         {
           page = malloc (sizeof (struct page_entry));
@@ -57,14 +107,13 @@ page_insert_swapped (void *upage, void *kpage, struct thread *owner)
           lock_init (&page->lock);
           hash_insert (&owner->pages, &page->elem);
         }
+      lock_release (&owner->ptable_lock);
 
       /* Mark page as 'swapped' 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. */
       /* TODO: We need to stop the process from destroying pagedir mid-eviction,
         as this could render the page table entry invalid. */
-  uint32_t *pte = lookup_page (owner->pagedir, upage, false);
-
       page_flag_swap (pte, true);
       lock_acquire (&page->lock);
       pagedir_clear_page (owner->pagedir, upage);
@@ -73,7 +122,16 @@ page_insert_swapped (void *upage, void *kpage, struct thread *owner)
       page_set_swap (owner, pte, swap_slot);
 
       lock_release (&page->lock);
-  return page;
+    }
+  if (exec_file != NULL)
+    {
+      lock_acquire (&shared_file_pages_lock);
+      struct shared_file_page *sfp = shared_file_page_get (exec_file, upage);
+      sfp->frame = NULL;
+      sfp->swap_slot = swap_out (kpage);
+      lock_release (&shared_file_pages_lock);
+    }
+  return true;
 }
 
 /* Allocate and insert a new page entry into the user process thread's
@@ -83,6 +141,25 @@ page_insert_file (struct file *file, off_t ofs, void *upage,
                   uint32_t read_bytes, uint32_t zero_bytes, bool writable,
                   enum page_type type)
 {
+  bool ptlock_held =
+    lock_held_by_current_thread(&thread_current ()->ptable_lock);
+
+  /* If page exists, just update it. */
+  if (!ptlock_held)
+    lock_acquire (&thread_current ()->ptable_lock);
+
+  struct page_entry *existing = page_get (thread_current (), upage);
+
+  if (existing != NULL)
+    {
+      ASSERT (existing->read_bytes == read_bytes);
+      ASSERT (existing->zero_bytes == zero_bytes);
+      existing->writable = existing->writable || writable;
+      lock_release (&thread_current ()->ptable_lock);
+      return existing;
+    }
+  lock_release (&thread_current ()->ptable_lock);
+
   struct page_entry *page = malloc(sizeof (struct page_entry));
   if (page == NULL)
     return NULL;
@@ -96,41 +173,105 @@ page_insert_file (struct file *file, off_t ofs, void *upage,
   page->writable = writable;
   lock_init (&page->lock);
 
+  lock_acquire (&thread_current ()->ptable_lock);
   hash_insert  (&thread_current ()->pages, &page->elem);
+  
+  if (!ptlock_held)
+    lock_release (&thread_current ()->ptable_lock);
+
   return page;
 }
 
 /* Gets a page_entry from the starting address of the page. Returns NULL if no
-   such page_entry exists in the hash map.*/
+   such page_entry exists in the hash map. Must only be called on a thread
+   whose ptable_lock you own. */
 struct page_entry *
-page_get (void *upage)
+page_get (struct thread *thread, void *upage)
 {
+  ASSERT (lock_held_by_current_thread (&thread->ptable_lock));
+
   struct page_entry fake_page_entry;
   fake_page_entry.upage = upage;
 
   struct hash_elem *e
-    = hash_find (&thread_current ()->pages, &fake_page_entry.elem);
+    = hash_find (&thread->pages, &fake_page_entry.elem);
   if (e == NULL)
+    {
       return NULL;
+    }
 
-  return hash_entry (e, struct page_entry, elem);
+  struct page_entry *pe = hash_entry (e, struct page_entry, elem);
+
+  return pe;
 }
 
 bool
-page_load_file (struct page_entry *page, bool writable)
+page_load_file (struct page_entry *page)
 {
   /* Allocate a frame for the page. If a frame allocation fails, then
      frame_alloc should try to evict a page. If it is still NULL, the OS
      panics as this should not happen if eviction is working correctly. */
   struct thread *t = thread_current ();
+  bool shareable = !page->writable && file_compare (page->file, t->exec_file);
+  shareable = false;
+  if (shareable)
+    {
+      lock_acquire (&shared_file_pages_lock);
+      struct shared_file_page *sfp
+          = shared_file_page_get (page->file, page->upage);
+      if (sfp != NULL)
+        {
+          /* Frame exists, just install it. */
+          if (sfp->frame != NULL)
+            {
+              if (!install_page (page->upage, sfp->frame, page->writable))
+                {
+                  lock_release (&shared_file_pages_lock);
+                  return false;
+                }
+              /* First time adding the shared page, so add thread as owner. */
+              if (page->type != PAGE_SHARED)
+                {
+                  frame_owner_insert (sfp->frame, t);
+                }
+            }
+            /* Shared page is in swap. Load it. */
+            else
+              {
+                void *frame = frame_alloc (PAL_USER, page->upage, t);
+                if (frame == NULL)
+                  PANIC (
+                      "Could not allocate a frame to load page into memory.");
+                swap_in (frame, sfp->swap_slot);
+
+                if (!install_page (page->upage, frame, false))
+                  {
+                    frame_free (frame);
+                    lock_release (&shared_file_pages_lock);
+                    return false;
+                  }
+              }
+            page_flag_shared (t, page->upage, true);
+            if (page->type != PAGE_SHARED)
+              {
+                sfp->ref_count++;
+                page->type = PAGE_SHARED;
+              }
+            lock_release (&shared_file_pages_lock);
+            return true;
+        }
+    }
+
   void *frame = frame_alloc (PAL_USER, page->upage, t);
   pagedir_set_accessed (t->pagedir, page->upage, true);
   if (frame == NULL)
     PANIC ("Could not allocate a frame to load page into memory.");
 
   /* Map the page to the frame. */
-  if (!install_page (page->upage, frame, writable))
+  if (!install_page (page->upage, frame, page->writable))
     {
+      if (shareable)
+        lock_release (&shared_file_pages_lock);
       frame_free (frame);
       return false;
     }
@@ -141,6 +282,8 @@ page_load_file (struct page_entry *page, bool writable)
   file_seek (page->file, page->offset);
   if (file_read (page->file, frame, page->read_bytes) != (int) page->read_bytes)
   {
+    if (shareable)
+      lock_release (&shared_file_pages_lock);
     frame_free (frame);
     return false;
   }
@@ -148,6 +291,27 @@ page_load_file (struct page_entry *page, bool writable)
   /* Zero out the remaining bytes in the frame. */
   memset (frame + page->read_bytes, 0, page->zero_bytes);
 
+  /* If file page is read-only, make it shared. */
+  if (shareable)
+    {
+      struct shared_file_page *sfp = malloc (sizeof (struct shared_file_page));
+      if (sfp == NULL)
+        {
+          lock_release (&shared_file_pages_lock);
+          frame_free (frame);
+          return false;
+        }
+      sfp->file = page->file;
+      sfp->upage = page->upage;
+      sfp->frame = frame;
+      sfp->swap_slot = 0;
+      sfp->ref_count = 1;
+      hash_insert (&shared_file_pages, &sfp->elem);
+      page_flag_shared (t, page->upage, true);
+      page->type = PAGE_SHARED;
+      lock_release (&shared_file_pages_lock);
+    }
+
   /* Mark the page as loaded successfully. */
   return true;
 }
@@ -157,7 +321,28 @@ page_load_file (struct page_entry *page, bool writable)
 void
 page_cleanup (struct hash_elem *e, void *aux UNUSED)
 {
-  free (hash_entry (e, struct page_entry, elem));
+  struct page_entry *page = hash_entry (e, struct page_entry, elem);
+  if (page->type == PAGE_SHARED)
+    {
+      lock_acquire (&shared_file_pages_lock);
+      struct shared_file_page *sfp
+          = shared_file_page_get (page->file, page->upage);
+      ASSERT (sfp != NULL);
+      if (sfp->frame != NULL)
+        frame_owner_remove (sfp->frame, thread_current ());
+      sfp->ref_count--;
+      if (sfp->ref_count == 0)
+        {
+          hash_delete (&shared_file_pages, &sfp->elem);
+          if (sfp->frame != NULL)
+            frame_free (sfp->frame);
+          else
+            swap_drop (sfp->swap_slot);
+          free (sfp);
+        }
+      lock_release (&shared_file_pages_lock);
+    }
+  free (page);
 }
 
 /* Flags the provided page table entry as representing a swapped out page. */
@@ -190,8 +375,14 @@ bool
 page_in_swap (struct thread *owner, void *upage)
 {
    uint32_t *pte = lookup_page (owner->pagedir, upage, false);
-   return pte != NULL &&
+   return page_in_swap_pte (pte);
-          (*pte & (1 << SWAP_FLAG_BIT)) != 0;
+}
+
+/* Returns true iff the page table entry is marked to be in the swap disk. */
+bool
+page_in_swap_pte (uint32_t *pte)
+{
+  return pte != NULL && (*pte & (1 << SWAP_FLAG_BIT)) != 0;
 }
 
 /* Given that the page with user address 'upage' owned by 'owner' is flagged
@@ -209,3 +400,82 @@ page_get_swap (struct thread *owner, void *upage)
    page_flag_swap (pte, false);
    return ((*pte & PTE_ADDR) >> ADDR_START_BIT);
 }
+
+/* Returns the swap slot stored in a PTE. */
+size_t
+page_get_swap_pte (uint32_t *pte)
+{
+  ASSERT (pte != NULL);
+  ASSERT ((*pte & PTE_P) == 0);
+  return ((*pte & PTE_ADDR) >> ADDR_START_BIT);
+}
+
+/* Flags the provided page table entry as representing a shared page. */
+static void
+page_flag_shared (struct thread *owner, void *upage, bool shared)
+{
+  uint32_t *pte = lookup_page (owner->pagedir, upage, false);
+  ASSERT (pte != NULL);
+
+  if (shared)
+    *pte |= (1 << SHARED_FLAG_BIT);
+  else
+    *pte &= ~(1 << SHARED_FLAG_BIT);
+}
+
+/* Returns true iff the page table entry is marked to be shared. */
+bool
+page_is_shared_pte (uint32_t *pte)
+{
+  return pte != NULL && (*pte & (1 << SHARED_FLAG_BIT)) != 0;
+}
+
+/* Initializes the shared file pages hash table. */
+void
+shared_file_pages_init ()
+{
+  if (!hash_init (&shared_file_pages, shared_file_page_hash,
+                  shared_file_page_less, NULL))
+    PANIC ("Failed to initialize shared file pages hash table.");
+  lock_init (&shared_file_pages_lock);
+}
+
+/* Hash function for shared file pages, used for storing entries in the
+   shared file pages table. */
+static unsigned
+shared_file_page_hash (const struct hash_elem *e, void *aux UNUSED)
+{
+  struct shared_file_page *sfp = hash_entry (e, struct shared_file_page, elem);
+  void *inode = file_get_inode (sfp->file);
+  void *upage = sfp->upage;
+  void *bytes[2] = { inode, upage };
+  return hash_bytes (bytes, sizeof (bytes));
+}
+
+/* 'less_func' comparison function for shared file pages, used for comparing
+   the keys of the shared file pages table. */
+static bool
+shared_file_page_less (const struct hash_elem *a_, const struct hash_elem *b_,
+                       void *aux UNUSED)
+{
+  const struct shared_file_page *a
+      = hash_entry (a_, struct shared_file_page, elem);
+  const struct shared_file_page *b
+      = hash_entry (b_, struct shared_file_page, elem);
+
+  return !file_compare (a->file, b->file) || a->upage < b->upage;
+}
+
+static struct shared_file_page *
+shared_file_page_get (struct file *file, void *upage)
+{
+  struct shared_file_page fake_sfp;
+  fake_sfp.file = file;
+  fake_sfp.upage = upage;
+
+  struct hash_elem *e = hash_find (&shared_file_pages, &fake_sfp.elem);
+  if (e == NULL)
+    return NULL;
+
+  return hash_entry (e, struct shared_file_page, elem);
+}

src/vm/page.h

@@ -5,12 +5,16 @@
 #include "threads/synch.h"
 #include "filesys/off_t.h"
 
-enum page_type {
+enum page_type
+{
   PAGE_FILE,
-  PAGE_MMAP
+  PAGE_MMAP,
+  PAGE_EMPTY,
+  PAGE_SHARED
 };
 
-struct page_entry {
+struct page_entry
+{
   enum page_type type; /* Type of Data that should go into the page */
   void *upage;         /* Start Address of the User Page (Key of hash table). */
 
@@ -29,19 +33,32 @@ struct page_entry {
   struct hash_elem elem; /* An elem for the hash table. */
 };
 
-unsigned page_hash (const struct hash_elem *e, void *aux);
+struct shared_file_page
-bool page_less (const struct hash_elem *a_, const struct hash_elem *b_,
+{
-                void *aux);
+  struct file *file;
-struct page_entry *page_insert_swapped (void *upage, void* kpage,
+  void *upage;
-                                        struct thread *owner);
+  void *frame;
+  size_t swap_slot;
+  int ref_count;
+
+  struct hash_elem elem;
+};
+
+bool init_pages (struct hash *pages);
+bool page_insert_swapped (void *upage, void *kpage, struct list *owners);
 struct page_entry *page_insert_file (struct file *file, off_t ofs, void *upage,
                                      uint32_t read_bytes, uint32_t zero_bytes,
                                      bool writable, enum page_type);
-struct page_entry *page_get (void *upage);
+struct page_entry *page_get (struct thread *thread, void *upage);
-bool page_load_file (struct page_entry *page, bool writable);
+bool page_load_file (struct page_entry *page);
 void page_cleanup (struct hash_elem *e, void *aux);
 
 bool page_in_swap (struct thread *, void *);
+bool page_in_swap_pte (uint32_t *pte);
 size_t page_get_swap (struct thread *owner, void *upage);
+size_t page_get_swap_pte (uint32_t *pte);
 
-#endif /* vm/frame.h */
+bool page_is_shared_pte (uint32_t *pte);
+void shared_file_pages_init (void);
+
+#endif