Update page_load to create and use the shared read-only pages of executables, w/ G

src/threads/thread.c

@@ -134,9 +134,12 @@ thread_start (void)
                   t))
     PANIC ("Failed to initialise child results table for main thread.");
 
+  /* Initialise the shared files table and lock. */
   if (!hash_init (&shared_files, shared_file_hash, shared_file_less, NULL))
     PANIC ("Failed to initialise shared pages table.");
 
+  lock_init (&shared_files_lock);
+
   /* Create the idle thread. */
   struct semaphore idle_started;
   sema_init (&idle_started, 0);

src/vm/page.c

@@ -3,9 +3,12 @@
 #include "filesys/file.h"
 #include "threads/malloc.h"
 #include "threads/palloc.h"
+#include "threads/vaddr.h"
 #include "userprog/process.h"
 #include "vm/frame.h"
 
+static struct shared_page_entry *shared_page_insert (struct file *file,
+                                                     void *upage, void *frame);
 static struct shared_file_entry *shared_file_get (struct file *file);
 static unsigned shared_page_hash (const struct hash_elem *e, void *aux UNUSED);
 static bool shared_page_less (const struct hash_elem *a_,
@@ -73,6 +76,23 @@ page_get (void *upage)
 bool
 page_load (struct page_entry *page, bool writable)
 {
+  /* If the page is read-only, we want to check if it is a shared page already
+     loaded into memory. If it is, we can just map the page to the frame. */
+  if (!page->writable)
+    {
+      ASSERT (page->read_bytes == PGSIZE);
+
+      struct shared_page_entry *shared_page =
+        shared_page_get (page->file, page->upage);
+
+      if (shared_page != NULL)
+        {
+          /* Map the page to the shared frame for this read-only portion. */
+          if (!install_page (page->upage, shared_page->frame, false))
+            return false;
+        }
+    }
+
   /* 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. */
@@ -81,7 +101,7 @@ page_load (struct page_entry *page, bool writable)
     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))
     {
       frame_free (frame);
       return false;
@@ -100,6 +120,16 @@ page_load (struct page_entry *page, bool writable)
   /* Zero out the remaining bytes in the frame. */
   memset (frame + page->read_bytes, 0, page->zero_bytes);
 
+  /* If the page is read-only, we need to add it to the shared pages table. */
+  if (!page->writable)
+    {
+      if (shared_page_insert (page->file, page->upage, frame) == NULL)
+        {
+          frame_free (frame);
+          return false;
+        }
+    }
+
   /* Mark the page as loaded successfully. */
   return true;
 }
@@ -190,21 +220,32 @@ shared_page_get (struct file *file, void *upage)
   return hash_entry (e, struct shared_page_entry, elem);
 }
 
-struct shared_page_entry *
+static struct shared_page_entry *
 shared_page_insert (struct file *file, void *upage, void *frame)
 {
   struct shared_file_entry *shared_file = shared_file_get (file);
+
+  /* Allocate a new shared_page_entry first for easier error handling. */
+  struct shared_page_entry *shared_page
+    = malloc (sizeof (struct shared_page_entry));
+  if (shared_page == NULL)
+    return NULL;
+
   /* If shared file doesn't exist in table, also create it. */
   if (shared_file == NULL)
     {
       shared_file = malloc (sizeof (struct shared_file_entry));
       if (shared_file == NULL)
-        return NULL;
+        {
+          free (shared_page);
+          return NULL;
+        }
 
       shared_file->file = file;
       shared_file->ref_count = 0;
       if (!hash_init (&shared_file->pages, shared_page_hash, shared_page_less, NULL))
         {
+          free (shared_page);
           free (shared_file);
           return NULL;
         }
@@ -212,11 +253,6 @@ shared_page_insert (struct file *file, void *upage, void *frame)
       hash_insert (&shared_files, &shared_file->elem);
     }
 
-  struct shared_page_entry *shared_page
-    = malloc (sizeof (struct shared_page_entry));
-  if (shared_page == NULL)
-    return NULL;
-
   shared_page->upage = upage;
   shared_page->frame = frame;
   hash_insert (&shared_file->pages, &shared_page->elem);

src/vm/page.h

@@ -2,9 +2,11 @@
 #define VM_PAGE_H
 
 #include "threads/thread.h"
+#include "threads/synch.h"
 #include "filesys/off_t.h"
 
 struct hash shared_files;
+struct lock shared_files_lock;
 
 enum page_type {
   PAGE_EXECUTABLE,