utils/bitmap: permit construction of empty bitmap allocators

There are cases where it is useful to initialize a bitmap allocator with
all bits clear (signifying that the bitmap is empty) as well as
initializing the allocator with all bits set (signifying that the bitmap
is full).

Signed-off-by: Jon Lange <jlange@microsoft.com>

fuzz/fuzz_targets/bitmap_allocator.rs

@@ -149,8 +149,8 @@ impl TestBitmapAllocator for BitmapAllocator1024 {
 }
 
 fuzz_target!(|actions: Vec<BmaAction>| {
-    let mut bma64 = BitmapAllocator64::new();
-    let mut bma1024 = BitmapAllocator1024::new();
+    let mut bma64 = BitmapAllocator64::new_full();
+    let mut bma1024 = BitmapAllocator1024::new_full();
     for action in actions.iter() {
         let bma64_before = bma64;
         let bma1024_before = bma1024.clone();

kernel/src/mm/virtualrange.rs

@@ -36,7 +36,7 @@ impl VirtualRange {
             start_virt: VirtAddr::null(),
             page_count: 0,
             page_shift: PAGE_SHIFT,
-            bits: BitmapAllocator1024::new(),
+            bits: BitmapAllocator1024::new_full(),
         }
     }
 

kernel/src/utils/bitmap_allocator.rs

@@ -32,10 +32,14 @@ pub struct BitmapAllocator64 {
 }
 
 impl BitmapAllocator64 {
-    pub const fn new() -> Self {
+    pub const fn new_full() -> Self {
         Self { bits: u64::MAX }
     }
 
+    pub const fn new_empty() -> Self {
+        Self { bits: 0 }
+    }
+
     #[cfg(fuzzing)]
     pub fn get_bits(&self) -> u64 {
         self.bits
@@ -101,10 +105,17 @@ pub struct BitmapAllocatorTree<T: BitmapAllocator + Debug> {
 }
 
 impl BitmapAllocatorTree<BitmapAllocator64> {
-    pub const fn new() -> Self {
+    pub const fn new_full() -> Self {
         Self {
             bits: u16::MAX,
-            child: [BitmapAllocator64::new(); 16],
+            child: [BitmapAllocator64::new_full(); 16],
+        }
+    }
+
+    pub const fn new_empty() -> Self {
+        Self {
+            bits: 0,
+            child: [BitmapAllocator64::new_empty(); 16],
         }
     }
 
@@ -362,7 +373,7 @@ mod tests {
 
     #[test]
     fn tree_set_all() {
-        let mut b = BitmapAllocatorTree::<BitmapAllocator64>::new();
+        let mut b = BitmapAllocatorTree::<BitmapAllocator64>::new_full();
         b.set(0, 64 * 16, false);
         for i in 0..16 {
             assert_eq!(b.child[i].bits, 0);
@@ -373,7 +384,7 @@ mod tests {
 
     #[test]
     fn tree_clear_all() {
-        let mut b = BitmapAllocatorTree::<BitmapAllocator64>::new();
+        let mut b = BitmapAllocatorTree::<BitmapAllocator64>::new_full();
         b.set(0, 64 * 16, true);
         for i in 0..16 {
             assert_eq!(b.child[i].bits, u64::MAX);
@@ -384,7 +395,7 @@ mod tests {
 
     #[test]
     fn tree_set_some() {
-        let mut b = BitmapAllocatorTree::<BitmapAllocator64>::new();
+        let mut b = BitmapAllocatorTree::<BitmapAllocator64>::new_full();
 
         // First child
         b.set(0, BitmapAllocatorTree::<BitmapAllocator64>::CAPACITY, false);
@@ -430,17 +441,26 @@ mod tests {
 
     #[test]
     fn tree_alloc_simple() {
-        let mut b = BitmapAllocatorTree::<BitmapAllocator64>::new();
+        let mut b = BitmapAllocatorTree::<BitmapAllocator64>::new_full();
         b.set(0, BitmapAllocatorTree::<BitmapAllocator64>::CAPACITY, false);
         for i in 0..256 {
             assert_eq!(b.alloc(1, 0), Some(i));
         }
         assert_eq!(b.used(), 256);
     }
 
+    #[test]
+    fn tree_alloc_empty_simple() {
+        let mut b = BitmapAllocatorTree::<BitmapAllocator64>::new_empty();
+        for i in 0..256 {
+            assert_eq!(b.alloc(1, 0), Some(i));
+        }
+        assert_eq!(b.used(), 256);
+    }
+
     #[test]
     fn tree_alloc_aligned() {
-        let mut b = BitmapAllocatorTree::<BitmapAllocator64>::new();
+        let mut b = BitmapAllocatorTree::<BitmapAllocator64>::new_full();
         b.set(0, BitmapAllocatorTree::<BitmapAllocator64>::CAPACITY, false);
         // Alignment of 1 << 5 bits : 32 bit alignment
         assert_eq!(b.alloc(1, 5), Some(0));
@@ -454,7 +474,7 @@ mod tests {
 
     #[test]
     fn tree_alloc_large_aligned() {
-        let mut b = BitmapAllocatorTree::<BitmapAllocator64>::new();
+        let mut b = BitmapAllocatorTree::<BitmapAllocator64>::new_full();
         b.set(0, BitmapAllocatorTree::<BitmapAllocator64>::CAPACITY, false);
         // Alignment of 1 << 4 bits : 16 bit alignment
         assert_eq!(b.alloc(500, 4), Some(0));
@@ -464,7 +484,7 @@ mod tests {
 
     #[test]
     fn tree_alloc_out_of_space() {
-        let mut b = BitmapAllocatorTree::<BitmapAllocator64>::new();
+        let mut b = BitmapAllocatorTree::<BitmapAllocator64>::new_full();
         b.set(0, BitmapAllocatorTree::<BitmapAllocator64>::CAPACITY, false);
         // Alignment of 1 << 4 bits : 16 bit alignment
         assert_eq!(b.alloc(1000, 4), Some(0));
@@ -477,7 +497,7 @@ mod tests {
 
     #[test]
     fn tree_free_space() {
-        let mut b = BitmapAllocatorTree::<BitmapAllocator64>::new();
+        let mut b = BitmapAllocatorTree::<BitmapAllocator64>::new_full();
         b.set(0, BitmapAllocatorTree::<BitmapAllocator64>::CAPACITY, false);
         // Alignment of 1 << 4 bits : 16 bit alignment
         assert_eq!(
@@ -492,7 +512,7 @@ mod tests {
 
     #[test]
     fn tree_free_multiple() {
-        let mut b = BitmapAllocatorTree::<BitmapAllocator64>::new();
+        let mut b = BitmapAllocatorTree::<BitmapAllocator64>::new_full();
         b.set(0, BitmapAllocatorTree::<BitmapAllocator64>::CAPACITY, true);
         b.free(0, 16);
         b.free(765, 16);