feat: implement synchronisation to protecting access to PTEs of SPTs during eviction

src/vm/page.c

@@ -1,10 +1,12 @@
 #include "page.h"
+#include <stdint.h>
 #include <string.h>
 #include <stdio.h>
 #include "filesys/file.h"
 #include "threads/pte.h"
 #include "threads/malloc.h"
 #include "threads/palloc.h"
+#include "devices/swap.h"
 #include "userprog/process.h"
 #include "userprog/pagedir.h"
 #include "vm/frame.h"
@@ -33,22 +35,65 @@ page_less (const struct hash_elem *a_, const struct hash_elem *b_,
   return a->upage < b->upage;
 }
 
-/* Allocate and insert a new page entry into the thread's page table. */
+static void page_flag_swap (uint32_t *pte, bool set);
+static void page_set_swap (struct thread *owner, uint32_t *pte,
+                           size_t swap_slot);
+
+/* 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 *
-page_insert (struct file *file, off_t ofs, void *upage, uint32_t read_bytes,
-             uint32_t zero_bytes, bool writable, enum page_type type)
+page_insert_swapped (void *upage, void *kpage, struct thread *owner)
+{
+
+  /* 1. Initialize swapped page entry. */
+  struct page_entry *page = malloc(sizeof (struct page_entry));
+  if (page == NULL)
+    return NULL;
+
+  page->upage = upage;
+  lock_init (&page->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);
+  ASSERT (pte != NULL);
+
+  page_flag_swap (pte, true);
+  lock_acquire (&page->lock);
+  struct hash_elem *elem = hash_insert (&owner->pages, &page->elem);
+  ASSERT(elem == NULL);
+
+  pagedir_clear_page (owner->pagedir, upage);
+
+  size_t swap_slot = swap_out (kpage);
+  page_set_swap (owner, pte, swap_slot);
+
+  lock_release (&page->lock);
+  return page;
+}
+
+/* Allocate and insert a new page entry into the user process thread's
+   SPT representing a file page. */
+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 type)
 {
   struct page_entry *page = malloc(sizeof (struct page_entry));
   if (page == NULL)
     return NULL;
 
+  page->type = type;
   page->file = file;
   page->offset = ofs;
   page->upage = upage;
   page->read_bytes = read_bytes;
   page->zero_bytes = zero_bytes;
   page->writable = writable;
-  page->type = type;
 
   hash_insert (&thread_current ()->pages, &page->elem);
   return page;
@@ -71,7 +116,7 @@ page_get (void *upage)
 }
 
 bool
-page_load (struct page_entry *page, bool writable)
+page_load_file (struct page_entry *page, bool writable)
 {
   /* 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
@@ -114,14 +159,21 @@ page_cleanup (struct hash_elem *e, void *aux UNUSED)
   free (hash_entry (e, struct page_entry, elem));
 }
 
-/* Updates the 'owner' thread's page table entry for virtual address 'upage'
-   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. */
+/* Flags the provided page table entry as representing a swapped out page. */
 void
-page_set_swap (struct thread *owner, void *upage, size_t swap_slot)
-{
-  uint32_t *pte = lookup_page (owner->pagedir, upage, false);
+page_flag_swap (uint32_t *pte, bool set)
+  {
+    if (set)
+      *pte |= (1 << SWAP_FLAG_BIT);
+    else
+      *pte &= ~(1 << SWAP_FLAG_BIT);
+  }
 
+/* Sets the address bits of the page table entry to the provided swap slot
+   value. To be used for later retrieval of the swap slot when page faulting. */
+static void
+page_set_swap (struct thread *owner, uint32_t *pte, size_t swap_slot)
+{
   /* Store the provided swap slot in the address bits of the page table
      entry, truncating excess bits. */
   *pte |= (1 << SWAP_FLAG_BIT);
@@ -143,7 +195,7 @@ page_in_swap (struct thread *owner, void *upage)
 
 /* Given that the page with user address 'upage' owned by 'owner' is flagged
    to be in the swap disk via the owner's page table, returns its stored
-   swap slot. Otherwise panics the kernel. */
+   swap slot and marks the PTE as not being in swap. */
 size_t
 page_get_swap (struct thread *owner, void *upage)
 {
@@ -153,5 +205,6 @@ page_get_swap (struct thread *owner, void *upage)
    ASSERT ((*pte & PTE_P) == 0);
 
    /* Masks the address bits and returns truncated value. */
+   page_flag_swap (pte, false);
    return ((*pte & PTE_ADDR) >> ADDR_START_BIT);
 }