Make paging work ! \o/

11 files changed, 191 insertions, 117 deletions

diff --git a/kernel/config.h b/kernel/config.h
index ad3dfb2..d198295 100644
--- a/kernel/config.h
+++ b/kernel/config.h
@@ -14,9 +14,7 @@
 #endif
 
 
-extern char k_highhalf_addr, k_end_addr;	// defined in linker script : 0xC0000000
-#define K_HIGHHALF_ADDR ((size_t)&k_highhalf_addr)
-#define K_END_ADDR ((size_t)&k_end_addr)
+#define K_HIGHHALF_ADDR ((size_t)0xC0000000)
 
 #define OS_NAME "macrO.Scope"
 #define OS_VERSION "0.0.1"
diff --git a/kernel/include/paging.h b/kernel/include/paging.h
index 7082e2e..2e1f844 100644
--- a/kernel/include/paging.h
+++ b/kernel/include/paging.h
@@ -13,13 +13,11 @@ pagedir_t *get_kernel_pagedir();
 
 void switch_pagedir(pagedir_t *pd);
 
-// The three functions below DO NOT DEPEND on argument pd for
-// addresses above K_HIGHHALF_ADDR, so just pass 0 to map/unmap in kernel space.
-uint32_t pd_get_frame(pagedir_t *pd, size_t vaddr);		// get physical frame for virtual address
-int pd_map_page(pagedir_t *pd,
-				size_t vaddr, uint32_t frame_id,
+// these functions are always relative to the currently mapped page directory
+uint32_t pd_get_frame(size_t vaddr);		// get physical frame for virtual address
+int pd_map_page(size_t vaddr, uint32_t frame_id,
 				uint32_t rw);		// returns nonzero on error
-void pd_unmap_page(pagedir_t *pd, size_t vaddr);	// does nothing if page wasn't mapped
+void pd_unmap_page(size_t vaddr);	// does nothing if page not mapped
 
 pagedir_t *create_pagedir();		// returns zero on error
 void delete_pagedir(pagedir_t *pd);
diff --git a/kernel/include/region.h b/kernel/include/region.h
index 3c64081..1628370 100644
--- a/kernel/include/region.h
+++ b/kernel/include/region.h
@@ -29,9 +29,4 @@ size_t region_alloc(size_t size, uint32_t type, page_fault_handler_t pf);	// ret
 region_info_t *find_region(size_t addr);
 void region_free(size_t addr);
 
-#define N_PAGES_IN_PT_REGION 4
-// special call for use by paging code
-// allocates a region of N_PAGES_IN_PT_REGION pages
-size_t region_alloc_for_pt();	
-
 void dbg_print_region_stats();
diff --git a/kernel/include/sys.h b/kernel/include/sys.h
index a9d2d4c..ff98c60 100644
--- a/kernel/include/sys.h
+++ b/kernel/include/sys.h
@@ -22,6 +22,10 @@ static inline uint16_t inw(uint16_t port) {
 	return ret;
 }
 
+static inline void invlpg(size_t addr) {
+	asm volatile("invlpg (%0)" : : "r"(addr) : "memory");
+}
+
 void panic(const char* message, const char* file, int line);
 void panic_assert(const char* assertion, const char* file, int line);
 #define PANIC(s) panic(s, __FILE__, __LINE__);
@@ -37,6 +41,13 @@ void panic_assert(const char* assertion, const char* file, int line);
 #define PAGE_ALIGN_DOWN(x)	(((size_t)x) & PAGE_MASK) 
 #define PAGE_ALIGN_UP(x)  	((((size_t)x)&(~PAGE_MASK)) == 0 ? ((size_t)x) : (((size_t)x) & PAGE_MASK) + PAGE_SIZE)
 #define PAGE_ID(x)			(((size_t)x) / PAGE_SIZE)
+#define PAGE_SHIFT		12
+#define PT_SHIFT		10
+// PAGE_SHIFT + PT_SHIFT + PT_SHIFT = 32
+#define N_PAGES_IN_PT		1024
+#define PD_MIRROR_ADDR	0xFFC00000		// last 4MB used for PD mirroring
+#define LAST_KERNEL_ADDR	PD_MIRROR_ADDR
+#define FIRST_KERNEL_PT	(K_HIGHHALF_ADDR >> (PAGE_SHIFT+PT_SHIFT))	// must be 768
 
 #define MASK4			0xFFFFFFFC
 #define ALIGN4_UP(x)	((((size_t)x)&(~MASK4)) == 0 ? ((size_t)x) : (((size_t)x) & MASK4) + 4)
diff --git a/kernel/l0/dbglog.c b/kernel/l0/dbglog.c
index 377e6c8..5a7251b 100644
--- a/kernel/l0/dbglog.c
+++ b/kernel/l0/dbglog.c
@@ -38,7 +38,7 @@ static void vga_update_cursor() {
 static void vga_init() {
 	vga_row = 0;
 	vga_column = 0;
-	vga_buffer = (uint16_t*) (&k_highhalf_addr + 0xB8000);
+	vga_buffer = (uint16_t*) (K_HIGHHALF_ADDR + 0xB8000);
 
 	for (size_t y = 0; y < VGA_HEIGHT; y++) {
 		for (size_t x = 0; x < VGA_WIDTH; x++) {
diff --git a/kernel/l0/frame.c b/kernel/l0/frame.c
index 1f16eaf..c646a48 100644
--- a/kernel/l0/frame.c
+++ b/kernel/l0/frame.c
@@ -4,8 +4,8 @@
 // TODO: buddy allocator
 // this is a simple bitmap allocator
 	
-#define INDEX_FROM_BIT(a) (a/(8*4))
-#define OFFSET_FROM_BIT(a) (a%(8*4))
+#define INDEX_FROM_BIT(a) ((a)/(8*4))
+#define OFFSET_FROM_BIT(a) ((a)%(8*4))
 
 static uint32_t *frame_bitset;
 static uint32_t nframes, nused_frames;
@@ -54,9 +54,9 @@ uint32_t frame_alloc(size_t n) {
 }
 
 void frame_free(uint32_t base, size_t n) {
-	for (size_t x = 0; x < n; x++) {
-		uint32_t idx = INDEX_FROM_BIT(base + n);
-		uint32_t ofs = OFFSET_FROM_BIT(base + n);
+	for (size_t i = 0; i < n; i++) {
+		uint32_t idx = INDEX_FROM_BIT(base + i);
+		uint32_t ofs = OFFSET_FROM_BIT(base + i);
 		if (frame_bitset[idx] & (0x1 << ofs)) {
 			frame_bitset[idx] &= ~(0x1 << ofs);
 			nused_frames--;
diff --git a/kernel/l0/idt.c b/kernel/l0/idt.c
index 8a981af..3bfcfc5 100644
--- a/kernel/l0/idt.c
+++ b/kernel/l0/idt.c
@@ -66,11 +66,11 @@ static isr_handler_t ex_handlers[32] = {0};
 
 /*	Called in interrupt.s when an exception fires (interrupt 0 to 31) */
 void idt_exHandler(registers_t *regs) {
-	dbg_printf("/ Exception %i\n", regs->int_no);
-	dbg_printf("| EAX: 0x%p   EBX: 0x%p   ECX: 0x%p   EDX: 0x%p\n", regs->eax, regs->ebx, regs->ecx, regs->edx);
-	dbg_printf("| EDI: 0x%p   ESI: 0x%p   ESP: 0x%p   EBP: 0x%p\n", regs->edi, regs->esi, regs->esp, regs->ebp);
-	dbg_printf("| EIP: 0x%p   CS : 0x%p   DS : 0x%p   SS : 0x%p\n", regs->eip, regs->cs, regs->ds, regs->ss);
-	dbg_printf("\\ EFl: 0x%p   I# : 0x%p   Err: 0x%p\n", regs->eflags, regs->int_no, regs->err_code);
+	/*dbg_printf("/ Exception %i\n", regs->int_no);*/
+	/*dbg_printf("| EAX: 0x%p   EBX: 0x%p   ECX: 0x%p   EDX: 0x%p\n", regs->eax, regs->ebx, regs->ecx, regs->edx);*/
+	/*dbg_printf("| EDI: 0x%p   ESI: 0x%p   ESP: 0x%p   EBP: 0x%p\n", regs->edi, regs->esi, regs->esp, regs->ebp);*/
+	/*dbg_printf("| EIP: 0x%p   CS : 0x%p   DS : 0x%p   SS : 0x%p\n", regs->eip, regs->cs, regs->ds, regs->ss);*/
+	/*dbg_printf("\\ EFl: 0x%p   I# : 0x%p   Err: 0x%p\n", regs->eflags, regs->int_no, regs->err_code);*/
 
 	if (ex_handlers[regs->int_no] != 0) {
 		ex_handlers[regs->int_no](regs);
diff --git a/kernel/l0/kmain.c b/kernel/l0/kmain.c
index f07e243..a221b0e 100644
--- a/kernel/l0/kmain.c
+++ b/kernel/l0/kmain.c
@@ -13,6 +13,19 @@ void breakpoint_handler(registers_t *regs) {
 	dbg_printf("Breakpoint! (int3)\n");
 	BOCHS_BREAKPOINT;
 }
+
+void test_pf_handler(pagedir_t *pd, region_info_t *i, size_t addr) {
+	dbg_printf("0x%p", addr);
+
+	uint32_t f = frame_alloc(1);
+	if (f == 0) PANIC("Out Of Memory");
+	dbg_printf(" -> %i", f);
+
+	int error = pd_map_page(addr, f, 1);
+	if (error) PANIC("Could not map frame (OOM)");
+}
+
+extern char k_end_addr;	// defined in linker script : 0xC0000000
  
 void kmain(struct multiboot_info_t *mbd, int32_t mb_magic) {
 	dbglog_setup();
@@ -35,7 +48,7 @@ void kmain(struct multiboot_info_t *mbd, int32_t mb_magic) {
 	// used for allocation of data structures before malloc is set up
 	// a pointer to this pointer is passed to the functions that might have
 	// to allocate memory ; they just increment it of the allocated quantity
-	void* kernel_data_end = (void*)K_END_ADDR;
+	void* kernel_data_end = (void*)&k_end_addr;
 
 	frame_init_allocator(total_ram, &kernel_data_end);
 	dbg_printf("kernel_data_end: 0x%p\n", kernel_data_end);
@@ -72,6 +85,31 @@ void kmain(struct multiboot_info_t *mbd, int32_t mb_magic) {
 	dbg_printf("Freed region 0x%p\n", s);
 	dbg_print_region_stats();
 
+	// allocate a big region and try to write into it
+	const size_t n = 1000;
+	size_t p0 = region_alloc(n * PAGE_SIZE, REGION_T_HW, test_pf_handler);
+	for (size_t i = 0; i < n; i++) {
+		uint32_t *x = (uint32_t*)(p0 + i * PAGE_SIZE);
+		dbg_printf("[%i : ", i);
+		x[0] = 12;
+		dbg_printf(" : .");
+		x[1] = (i * 20422) % 122;
+		dbg_printf("]\n", i);
+	}
+	// unmap memory
+	for (size_t i = 0; i < n; i++) {
+		uint32_t *x = (uint32_t*)(p0 + i * PAGE_SIZE);
+		ASSERT(x[1] == (i * 20422) % 122);
+
+		size_t f = pd_get_frame((size_t)x);
+		ASSERT(f != 0);
+		pd_unmap_page((size_t)x);
+
+		frame_free(f, 1);
+	}
+	region_free(s);
+
+
 	// TODO:
 	// - setup allocator for physical pages (eg: buddy allocator, see OSDev wiki)
 	// - setup allocator for virtual memory space
diff --git a/kernel/l0/loader.s b/kernel/l0/loader.s
index e4c4611..6ad4ff0 100644
--- a/kernel/l0/loader.s
+++ b/kernel/l0/loader.s
@@ -1,6 +1,6 @@
 [EXTERN kmain]            ; kmain is defined in kmain.c
 [GLOBAL loader]           ; making entry point visible to linker
-[GLOBAL kernel_pagedir]   ; make kernel page directory visible
+[GLOBAL kernel_pd]   ; make kernel page directory visible
 
 ; higher-half kernel setup
 K_HIGHHALF_ADDR     equ 0xC0000000
@@ -25,7 +25,7 @@ multiboot_header:
 
 loader:	
 	; setup the boot page directory used for higher-half
-	mov ecx, kernel_pagedir
+	mov ecx, kernel_pd
 	sub ecx, K_HIGHHALF_ADDR	; access its lower-half address
 	mov cr3, ecx
 
@@ -45,7 +45,7 @@ loader:
 
 [section .data]
 align 0x1000
-kernel_pagedir:
+kernel_pd:
 	; uses 4MB pages
 	; identity-maps the first 4Mb of RAM, and also maps them with offset += k_highhalf_addr
 	dd 0x00000083
@@ -56,7 +56,7 @@ kernel_pagedir:
 [section .text]
 higherhalf:		; now we're running in higher half
 	; unmap first 4Mb
-	mov dword [kernel_pagedir], 0
+	mov dword [kernel_pd], 0
 	invlpg [0]
 
 	mov esp, stack_top          ; set up the stack
diff --git a/kernel/l0/paging.c b/kernel/l0/paging.c
index f076aa1..74f3d09 100644
--- a/kernel/l0/paging.c
+++ b/kernel/l0/paging.c
@@ -2,83 +2,112 @@
 #include <frame.h>
 #include <idt.h>
 #include <dbglog.h>
-
-typedef union page {
-	struct {
-		uint32_t present		: 1;
-		uint32_t rw				: 1;
-		uint32_t user			: 1;
-		uint32_t write_through  : 1;
-		uint32_t disable_cache	: 1;
-		uint32_t accessed		: 1;
-		uint32_t dirty			: 1;	// only PTE
-		uint32_t size_4m		: 1;	// only PDE
-		uint32_t global			: 1;	// only PTE
-		uint32_t rsvd			: 3;
-		uint32_t frame			: 20;
-	};
-	uint32_t as_int32;
-} page_t;
+#include <region.h>
+
+#define PAGE_OF_ADDR(x)		(((size_t)x >> PAGE_SHIFT) % N_PAGES_IN_PT)
+#define PT_OF_ADDR(x)		((size_t)x >> (PAGE_SHIFT + PT_SHIFT))
+
+#define PTE_PRESENT			(1<<0)
+#define PTE_RW				(1<<1)
+#define PTE_USER			(1<<2)
+#define PTE_WRITE_THROUGH	(1<<3)
+#define PTE_DISABLE_CACHE	(1<<4)
+#define PTE_ACCESSED		(1<<5)
+#define PTE_DIRTY			(1<<6)		// only PTE
+#define PTE_SIZE_4M			(1<<7)		// only PDE
+#define PTE_GLOBAL			(1<<8)		// only PTE
+#define PTE_FRAME_SHIFT		12
 
 typedef struct page_table {
-	page_t page[1024];
+	uint32_t page[1024];
 } pagetable_t;
 
 struct page_directory {
-	pagetable_t *pt[1024];	// virtual addresses of each page table
-	pagetable_t *dir;		// virtual address of page directory
 	size_t phys_addr;		// physical address of page directory
+	// to modify a page directory, we first map it
+	// then we can use mirroring to edit it
+	// (the last 4M of the address space are mapped to the PD itself)
+
+	// more info to be stored here, potentially
 };
 
 
 // access kernel page directory page defined in loader.s
 // (this is a correct higher-half address)
-extern pagetable_t kernel_pagedir;
+extern pagetable_t kernel_pd;
+
+// pre-allocate a page table so that we can map the first 4M of kernel memory
+static pagetable_t __attribute__((aligned(PAGE_SIZE))) kernel_pt0;
 
-static pagetable_t __attribute__((aligned(PAGE_SIZE))) kernel_pt768;
-static pagedir_t kernel_pd;
+static pagedir_t kernel_pd_d;
+static pagedir_t *current_pd_d;
 
-static pagedir_t *current_pd;
+#define current_pt ((pagetable_t*)PD_MIRROR_ADDR)
+#define current_pd ((pagetable_t*)(PD_MIRROR_ADDR + (N_PAGES_IN_PT-1)*PAGE_SIZE))
 
 void page_fault_handler(registers_t *regs) {
-	size_t addr;
-	asm volatile("movl %%cr2, %0":"=r"(addr));
-	dbg_printf("Page fault at 0x%p\n", addr);
-	PANIC("PAGE FAULT");
-	// not handled yet
+	size_t vaddr;
+	asm volatile("movl %%cr2, %0":"=r"(vaddr));
+
+	if (vaddr >= K_HIGHHALF_ADDR) {
+		uint32_t pt = PT_OF_ADDR(vaddr);
+
+		if (current_pd != &kernel_pd && vaddr >= K_HIGHHALF_ADDR
+			&& current_pd->page[pt] != kernel_pd.page[pt])
+		{
+			current_pd->page[pt] = kernel_pd.page[pt];
+			invlpg((size_t)(&current_pt[pt]));
+			return;
+		}
+
+		region_info_t *i = find_region(vaddr);
+		if (i == 0) {
+			dbg_printf("Kernel pagefault in non-existing region at 0x%p\n", vaddr);
+			PANIC("Unhandled kernel space page fault");
+		}
+		if (i->pf == 0) {
+			dbg_printf("Kernel pagefault in region with no handler at 0x%p\n", vaddr);
+			PANIC("Unhandled kernel space page fault");
+		}
+		i->pf(current_pd_d, i, vaddr);
+	} else {
+		dbg_printf("Userspace page fault at 0x%p\n", vaddr);
+		PANIC("Unhandled userspace page fault");
+		// not handled yet
+		// TODO
+	}
 }
 
 void paging_setup(void* kernel_data_end) {
 	size_t n_kernel_pages =
 		PAGE_ALIGN_UP((size_t)kernel_data_end - K_HIGHHALF_ADDR)/PAGE_SIZE;
 
-	ASSERT(n_kernel_pages <= 1024);
+	ASSERT(n_kernel_pages <= 1024);	// we use less than 4M for kernel
 
-	// setup kernel_pd structure
-	kernel_pd.dir = &kernel_pagedir;
-	kernel_pd.phys_addr = (size_t)kernel_pd.dir - K_HIGHHALF_ADDR;
-	for (size_t i = 0; i < 1024; i++) kernel_pd.pt[i] = 0;
+	// setup kernel_pd_d structure
+	kernel_pd_d.phys_addr = (size_t)&kernel_pd - K_HIGHHALF_ADDR;
 
-	// setup kernel_pt768
+	// setup kernel_pt0
+	ASSERT(PAGE_OF_ADDR(K_HIGHHALF_ADDR) == 0);	// kernel is 4M-aligned
+	ASSERT(FIRST_KERNEL_PT == 768);
 	for (size_t i = 0; i < n_kernel_pages; i++) {
-		kernel_pt768.page[i].as_int32 = 0;	// clear any junk
-		kernel_pt768.page[i].present = 1;
-		kernel_pt768.page[i].user = 0;
-		kernel_pt768.page[i].rw = 1;
-		kernel_pt768.page[i].frame = i;
+		kernel_pt0.page[i] = (i << PTE_FRAME_SHIFT) | PTE_PRESENT | PTE_RW | PTE_GLOBAL;
 	}
 	for (size_t i = n_kernel_pages; i < 1024; i++){
-		kernel_pt768.page[i].as_int32 = 0;
+		kernel_pt0.page[i] = 0;
 	}
 
-	// replace 4M mapping by kernel_pt768
-	kernel_pd.pt[768] = &kernel_pt768;
-	kernel_pd.dir->page[768].as_int32 =
-		(((size_t)&kernel_pt768 - K_HIGHHALF_ADDR) & PAGE_MASK) | 0x07;
+	// replace 4M mapping by kernel_pt0
+	kernel_pd.page[FIRST_KERNEL_PT] =
+		(((size_t)&kernel_pt0 - K_HIGHHALF_ADDR) & PAGE_MASK) | PTE_PRESENT | PTE_RW;
+	// set up mirroring
+	kernel_pd.page[N_PAGES_IN_PT-1] =
+		(((size_t)&kernel_pd - K_HIGHHALF_ADDR) & PAGE_MASK) | PTE_PRESENT | PTE_RW;
 
-	current_pd = &kernel_pd;
+	invlpg(K_HIGHHALF_ADDR);
 
 	// paging already enabled in loader, nothing to do.
+	switch_pagedir(&kernel_pd_d);
 
 	// disable 4M pages (remove PSE bit in CR4)
 	uint32_t cr4;
@@ -90,45 +119,67 @@ void paging_setup(void* kernel_data_end) {
 }
 
 pagedir_t *get_current_pagedir() {
-	return current_pd;
+	return current_pd_d;
 }
 
 pagedir_t *get_kernel_pagedir() {
-	return &kernel_pd;
+	return &kernel_pd_d;
 }
 
 void switch_pagedir(pagedir_t *pd) {
     asm volatile("movl %0, %%cr3":: "r"(pd->phys_addr));
+	invlpg((size_t)current_pd);
+	current_pd_d = pd;
 }
 
 // ============================== //
 // Mapping and unmapping of pages //
 // ============================== //
 
-uint32_t pd_get_frame(pagedir_t *pd, size_t vaddr) {
-	uint32_t page = vaddr / PAGE_SIZE;
-	uint32_t pt = page / PAGE_SIZE;
-	uint32_t pt_page = page % PAGE_SIZE;
+uint32_t pd_get_frame(size_t vaddr) {
+	uint32_t pt = PT_OF_ADDR(vaddr);
+	uint32_t page = PAGE_OF_ADDR(vaddr);
+
+	pagetable_t *pd = (vaddr >= K_HIGHHALF_ADDR ? &kernel_pd : current_pd);
 
-	if (pd == 0) return 0;
-	if (pd->pt[pt] == 0) return 0;
-	if (!pd->pt[pt]->page[pt_page].present) return 0;
-	return pd->pt[pt]->page[pt_page].frame;
+	if (!pd->page[pt] & PTE_PRESENT) return 0;
+	if (!current_pt[pt].page[page] & PTE_PRESENT) return 0;
+	return current_pt[pt].page[page] >> PTE_FRAME_SHIFT;
 }
 
-int pd_map_page(pagedir_t *pd, size_t vaddr, uint32_t frame_id, uint32_t rw) {
-	return 1; 	// TODO
+int pd_map_page(size_t vaddr, uint32_t frame_id, uint32_t rw) {
+	uint32_t pt = PT_OF_ADDR(vaddr);
+	uint32_t page = PAGE_OF_ADDR(vaddr);
+
+	pagetable_t *pd = (vaddr >= K_HIGHHALF_ADDR ? &kernel_pd : current_pd);
+
+	if (!pd->page[pt] & PTE_PRESENT) {
+		size_t new_pt_frame = frame_alloc(1);
+		if (new_pt_frame == 0) return 1;	// OOM
+
+		current_pd->page[pt] = pd->page[pt] =
+			(new_pt_frame << PTE_FRAME_SHIFT) | PTE_PRESENT | PTE_RW;
+		invlpg((size_t)&current_pt[pt]);
+	}
+
+	current_pt[pt].page[page] =
+		frame_id << PTE_FRAME_SHIFT
+			| PTE_PRESENT
+			| (vaddr < K_HIGHHALF_ADDR ? PTE_USER : PTE_GLOBAL)
+			| (rw ? PTE_RW : 0);
+
+	return 0;
 } 
 
-void pd_unmap_page(pagedir_t *pd, size_t vaddr) {
-	uint32_t page = vaddr / PAGE_SIZE;
-	uint32_t pt = page / PAGE_SIZE;
-	uint32_t pt_page = page % PAGE_SIZE;
+void pd_unmap_page(size_t vaddr) {
+	uint32_t pt = PT_OF_ADDR(vaddr);
+	uint32_t page = PAGE_OF_ADDR(vaddr);
+
+	pagetable_t *pd = (vaddr >= K_HIGHHALF_ADDR ? &kernel_pd : current_pd);
 
-	if (pd == 0) return;
-	if (pd->pt[pt] == 0) return;
-	if (!pd->pt[pt]->page[pt_page].present) return;
-	pd->pt[pt]->page[pt_page].as_int32 = 0;
+	if (!pd->page[pt] & PTE_PRESENT) return;
+	if (!current_pt[pt].page[page] & PTE_PRESENT) return;
+	current_pt[pt].page[page] &= ~PTE_PRESENT;
 
 	// TODO (?) : if pagetable is completely empty, free it
 }
diff --git a/kernel/l0/region.c b/kernel/l0/region.c
index 63e464d..6001346 100644
--- a/kernel/l0/region.c
+++ b/kernel/l0/region.c
@@ -17,7 +17,7 @@ typedef union region_descriptor {
 	} used;
 } descriptor_t;
 
-#define N_RESERVE_DESCRIPTORS 3		// always keep at least 3 unused descriptors
+#define N_RESERVE_DESCRIPTORS 2		// always keep at least 3 unused descriptors
 
 #define N_BASE_DESCRIPTORS 12		// pre-allocate memory for 12 descriptors
 static descriptor_t base_descriptors[N_BASE_DESCRIPTORS];
@@ -210,7 +210,7 @@ void region_allocator_init(void* kernel_data_end) {
 
 	descriptor_t *f0 = &base_descriptors[1];
 	f0->free.addr = PAGE_ALIGN_UP(kernel_data_end);
-	f0->free.size = (0xFFFFF000-f0->free.addr);	// last page cannot be used...
+	f0->free.size = (LAST_KERNEL_ADDR-f0->free.addr);
 	f0->free.next_by_size = 0;
 	f0->free.first_bigger = 0;
 	first_free_region_by_size = first_free_region_by_addr = f0;
@@ -234,8 +234,7 @@ static size_t region_alloc_inner(size_t size, uint32_t type, page_fault_handler_
 					return 0;
 				}
 
-				// this assert is a bit tricky to prove,
-				// but basically it means that the allocation function
+				// this assert basically means that the allocation function
 				// is called less than N_RESERVE_DESCRIPTORS times with
 				// the use_reserve flag before more descriptors
 				// are allocated.
@@ -267,20 +266,6 @@ static size_t region_alloc_inner(size_t size, uint32_t type, page_fault_handler_
 	return 0;	//No big enough block found
 }
 
-bool region_alloc_for_pt_use_reserve = false;
-size_t region_alloc_for_pt() {
-	if (region_alloc_for_pt_use_reserve) {
-		return region_alloc_inner(
-			N_PAGES_IN_PT_REGION * PAGE_SIZE,
-			REGION_T_PAGETABLE,
-			0, true);
-	} else {
-		return region_alloc(
-			N_PAGES_IN_PT_REGION * PAGE_SIZE,
-			REGION_T_PAGETABLE, 0);
-	}
-}
-
 size_t region_alloc(size_t size, uint32_t type, page_fault_handler_t pf) {
 	if (n_unused_descriptors <= N_RESERVE_DESCRIPTORS) {
 		uint32_t frame = frame_alloc(1);
@@ -289,9 +274,7 @@ size_t region_alloc(size_t size, uint32_t type, page_fault_handler_t pf) {
 		size_t descriptor_region = region_alloc_inner(PAGE_SIZE, REGION_T_DESCRIPTORS, 0, true);
 		ASSERT(descriptor_region != 0);
 
-		region_alloc_for_pt_use_reserve = true;
-		int error = pd_map_page(0, descriptor_region, frame, 1);
-		region_alloc_for_pt_use_reserve = false;
+		int error = pd_map_page(descriptor_region, frame, 1);
 		if (error) {
 			// this can happen if we weren't able to allocate a frame for
 			// a new pagetable