void
page_check(void)
{
	struct Page *pp, *pp0, *pp1, *pp2;
	struct Page_list fl;
	pte_t *ptep;

	// should be able to allocate three pages
	pp0 = pp1 = pp2 = 0;
	assert(page_alloc(&pp0) == 0);
	assert(page_alloc(&pp1) == 0);
	assert(page_alloc(&pp2) == 0);

	assert(pp0);
	assert(pp1 && pp1 != pp0);
	assert(pp2 && pp2 != pp1 && pp2 != pp0);

	// temporarily steal the rest of the free pages
	fl = page_free_list;
	LIST_INIT(&page_free_list);

	// should be no free memory
	assert(page_alloc(&pp) == -E_NO_MEM);

	// DEP Extra tests 
	// Fill pp1 with bogus data and check for invalid tlb entries
	memset(page2kva(pp1), 0xFFFFFFFF, PGSIZE);

	// there is no page allocated at address 0
	assert(page_lookup(boot_pgdir, (void *) 0x0, &ptep) == NULL);

	// there is no free memory, so we can't allocate a page table 
	assert(page_insert(boot_pgdir, pp1, 0x0, 0) < 0);

	// free pp0 and try again: pp0 should be used for page table
	page_free(pp0);
	assert(page_insert(boot_pgdir, pp1, 0x0, 0) == 0);
	tlb_invalidate(boot_pgdir, 0x0);
	// DEP Should have shot down invalid TLB entry - let's check
	{
	  int *x = 0x0;
	  assert(*x == 0xFFFFFFFF);
	}
	assert(PTE_ADDR(boot_pgdir[0]) == page2pa(pp0));
	assert(check_va2pa(boot_pgdir, 0x0) == page2pa(pp1));
	assert(pp1->pp_ref == 1);
	assert(pp0->pp_ref == 1);

	// should be able to map pp2 at PGSIZE because pp0 is already allocated for page table
	assert(page_insert(boot_pgdir, pp2, (void*) PGSIZE, 0) == 0);
	assert(check_va2pa(boot_pgdir, PGSIZE) == page2pa(pp2));
	assert(pp2->pp_ref == 1);

	// DEP Extra test:
	// Make sure that pgdir_walk returns a pointer to the pte and
	// not the table or some other garbage
	{
	  pte_t *p = KADDR(PTE_ADDR(boot_pgdir[PDX(PGSIZE)]));
	  assert(pgdir_walk(boot_pgdir, (void *)PGSIZE, 0) == &p[PTX(PGSIZE)]);
	}

	// should be no free memory
	assert(page_alloc(&pp) == -E_NO_MEM);

	// should be able to map pp2 at PGSIZE because it's already there
	assert(page_insert(boot_pgdir, pp2, (void*) PGSIZE, PTE_U) == 0);
	assert(check_va2pa(boot_pgdir, PGSIZE) == page2pa(pp2));
	assert(pp2->pp_ref == 1);

	// DEP Extra test:
	// Make sure that we actually changed the permission on pp2 when we re-mapped it
	{
	  pte_t *p = pgdir_walk(boot_pgdir, (void*)PGSIZE, 0);
	  assert(((*p) & PTE_U) == PTE_U);
	}


	// pp2 should NOT be on the free list
	// could happen in ref counts are handled sloppily in page_insert
	assert(page_alloc(&pp) == -E_NO_MEM);

	// should not be able to map at PTSIZE because need free page for page table
	assert(page_insert(boot_pgdir, pp0, (void*) PTSIZE, 0) < 0);

	// insert pp1 at PGSIZE (replacing pp2)
	assert(page_insert(boot_pgdir, pp1, (void*) PGSIZE, 0) == 0);

	// should have pp1 at both 0 and PGSIZE, pp2 nowhere, ...
	assert(check_va2pa(boot_pgdir, 0) == page2pa(pp1));
	assert(check_va2pa(boot_pgdir, PGSIZE) == page2pa(pp1));
	// ... and ref counts should reflect this
	assert(pp1->pp_ref == 2);
	assert(pp2->pp_ref == 0);

	// pp2 should be returned by page_alloc
	assert(page_alloc(&pp) == 0 && pp == pp2);

	// unmapping pp1 at 0 should keep pp1 at PGSIZE
	page_remove(boot_pgdir, 0x0);
	assert(check_va2pa(boot_pgdir, 0x0) == ~0);
	assert(check_va2pa(boot_pgdir, PGSIZE) == page2pa(pp1));
	assert(pp1->pp_ref == 1);
	assert(pp2->pp_ref == 0);

	// unmapping pp1 at PGSIZE should free it
	page_remove(boot_pgdir, (void*) PGSIZE);
	assert(check_va2pa(boot_pgdir, 0x0) == ~0);
	assert(check_va2pa(boot_pgdir, PGSIZE) == ~0);
	assert(pp1->pp_ref == 0);
	assert(pp2->pp_ref == 0);

	// so it should be returned by page_alloc
	assert(page_alloc(&pp) == 0 && pp == pp1);

	// should be no free memory
	assert(page_alloc(&pp) == -E_NO_MEM);

	// forcibly take pp0 back
	assert(PTE_ADDR(boot_pgdir[0]) == page2pa(pp0));
	boot_pgdir[0] = 0;
	assert(pp0->pp_ref == 1);
	pp0->pp_ref = 0;

	// DEP Extra test:
	// Catch invalid pointer addition in pgdir_walk - i.e. pgdir + PDX(va)
	{
	  // Give back pp0 for a bit
	  page_free(pp0);

	  void * va = (void *)((PGSIZE * NPDENTRIES) + PGSIZE);
	  pte_t *p2 = pgdir_walk(boot_pgdir, va, 1);
	  pte_t *p = KADDR(PTE_ADDR(boot_pgdir[PDX(va)]));
	  assert(p2 == &p[PTX(va)]);

	  // Clean up again
	  boot_pgdir[PDX(va)] = 0;
	  pp0->pp_ref = 0;
	}


	// give free list back
	page_free_list = fl;

	// free the pages we took
	page_free(pp0);
	page_free(pp1);
	page_free(pp2);

	cprintf("page_check() succeeded!\n");
}