phys: use a bitmap to track pages

src/arena.zig

@@ -1,5 +1,7 @@
 //! Simple bump allocator arena.
 
+const std = @import("std");
+
 const phys_memory = @import("mem/phys.zig");
 const log = @import("debug.zig").log;
 const mem = @import("mem.zig");

src/mem/phys.zig

@@ -43,21 +43,22 @@ const PhysicalMemoryManager = struct {
     page_array: []Page,
     offset: u64 = 0,
     last_free: usize = 0,
+    bitmap: []u64, // Each bit represents a page
 
     const RECORDS_PER_PAGE: usize = vmm.PAGE_SIZE / @sizeOf(Page);
 
     fn alloc_page(self: *@This()) ?mem.PhysicalAddress {
-        for (self.last_free..self.page_array.len) |i| {
-            if (self.page_array[i].refcount == 0) {
-                self.page_array[i].refcount += 1;
-                self.last_free = (i + 1) % self.page_array.len;
+        for (self.last_free..self.bitmap.len * 64) |i| {
+            if (!self.is_page_used(i)) {
+                self.set_page_used(i);
+                self.last_free = (i + 1) % (self.bitmap.len * 64);
                 return .{ .raw = self.offset + i * vmm.PAGE_SIZE };
             }
         }
         for (0..self.last_free) |i| {
-            if (self.page_array[i].refcount == 0) {
-                self.page_array[i].refcount += 1;
-                self.last_free = (i + 1) % self.page_array.len;
+            if (!self.is_page_used(i)) {
+                self.set_page_used(i);
+                self.last_free = (i + 1) % (self.bitmap.len * 64);
                 return .{ .raw = self.offset + i * vmm.PAGE_SIZE };
             }
         }
@@ -107,25 +108,44 @@ const PhysicalMemoryManager = struct {
 
     fn free_page(self: *@This(), page: mem.PhysicalAddress) void {
         const index = self.valid_index(page);
-        if (self.page_array[index].refcount == 0) {
+        if (!self.is_page_used(index)) {
             log.panic("free_page: double free of page 0x{x} detected", .{page.raw});
         }
-        self.page_array[index].refcount -= 1;
-        if (self.page_array[index].refcount == 0) {
-            self.last_free = index;
-        }
+        self.clear_page_used(index);
+        self.last_free = index;
     }
 
     fn get_page(self: *@This(), page: mem.PhysicalAddress) *Page {
         const index = self.valid_index(page);
         return &self.page_array[index];
     }
+
+    fn is_page_used(self: *@This(), index: usize) bool {
+        const word_index = index / 64;
+        const bit_index = index % 64;
+        return (self.bitmap[word_index] & (@as(u64, 1) << @intCast(bit_index))) != 0;
+    }
+
+    fn set_page_used(self: *@This(), index: usize) void {
+        const word_index = index / 64;
+        const bit_index = index % 64;
+        self.bitmap[word_index] |= (@as(u64, 1) << @intCast(bit_index));
+    }
+
+    fn clear_page_used(self: *@This(), index: usize) void {
+        const word_index = index / 64;
+        const bit_index = index % 64;
+        self.bitmap[word_index] &= ~(@as(u64, 1) << @intCast(bit_index));
+    }
 };
 
 var g_memory_regions: std.BoundedArray(MemoryRegion, 16) = .{};
 var g_reserved_regions: std.BoundedArray(MemoryRegion, 16) = .{};
 var g_physical_memory_lock = Spinlock{};
-var g_physical_memory = PhysicalMemoryManager{ .page_array = undefined };
+var g_physical_memory = PhysicalMemoryManager{
+    .page_array = undefined,
+    .bitmap = undefined,
+};
 
 /// Adds an available memory region to the list.
 ///
@@ -198,6 +218,21 @@ fn alloc_page_array(page_count: usize) []Page {
     @panic("TODO");
 }
 
+fn alloc_bitmap(pages: usize) []u64 {
+    // Round up to the upper u64 that has at least `pages` bits
+    const bitmap_entries = (pages + 63) / 64;
+    for (g_memory_regions.constSlice()) |region| {
+        if (alloc_from_region(&region, "bitmap", bitmap_entries * @sizeOf(u64))) |phys_address| {
+            const vaddr = (mem.PhysicalAddress{ .raw = phys_address }).virtualize();
+            const ptr: [*]u64 = @ptrFromInt(vaddr);
+            const slice: []u64 = ptr[0..bitmap_entries];
+            @memset(slice, 0); // Clear the bitmap
+            return slice;
+        }
+    }
+    @panic("Failed to allocate bitmap");
+}
+
 /// Initializes the physical memory management.
 ///
 /// # Notes
@@ -218,10 +253,13 @@ pub fn init() void {
     }
 
     const memory_pages = (memory_end - memory_start) / vmm.PAGE_SIZE; // == bitmap bits required
+    const bitmap = alloc_bitmap(memory_pages);
+
     const page_array_pages = (memory_pages + PhysicalMemoryManager.RECORDS_PER_PAGE - 1) //
-        / PhysicalMemoryManager.RECORDS_PER_PAGE;
+    / PhysicalMemoryManager.RECORDS_PER_PAGE;
 
     const page_array = alloc_page_array(page_array_pages);
+
     var available_pages: usize = 0;
 
     for (g_memory_regions.constSlice()) |region| {
@@ -248,6 +286,7 @@ pub fn init() void {
 
     g_physical_memory.page_array = page_array;
     g_physical_memory.offset = memory_start;
+    g_physical_memory.bitmap = bitmap;
 }
 
 fn trace_allocation(count: usize, page: ?mem.PhysicalAddress) void {