Fix block bitmap area calculation (#638)

* Fix block bitmap area calculation

* Update documentation

* Take reserved blocks into account for disk usage

* Fix out of space errors

* Fix off by one errors

* Remove pub attributes

doc/filesystem.md

@@ -10,9 +10,9 @@ A hard drive is separated in blocks of 512 bytes, grouped into 4 areas:
     +------------+
     | Superblock | (2 blocks)
     +------------+
-    | Bitmap     | (n / (8 * 512) blocks)
+    | Bitmap     | (n blocks)
     +------------+
-    | Data       | (n blocks)
+    | Data       | (n * 512 * 8 blocks)
     +------------+
 
 The first area contains the bootloader and the kernel, the second is a

src/sys/fs/bitmap_block.rs

@@ -42,6 +42,10 @@ impl BitmapBlock {
 
     pub fn next_free_addr() -> Option<u32> {
         let sb = SuperBlock::read();
+        if sb.alloc_count() == sb.block_count() {
+            return None;
+        }
+
         let n = sb.block_size();
         let m = sb.block_count() / n / 8;
         for i in 0..m {

src/sys/fs/dir.rs

@@ -120,15 +120,18 @@ impl Dir {
         if entry_len > space_left {
             match entries.block.alloc_next() {
                 None => return None, // Disk is full
-                Some(new_block) => {
-                    entries.block = new_block;
+                Some(block) => {
+                    entries.block = block;
                     entries.block_offset = 0;
                 }
             }
         }
 
         // Create a new entry
-        let entry_block = LinkedBlock::alloc().unwrap();
+        let entry_block = match LinkedBlock::alloc() {
+            None => return None,
+            Some(block) => block,
+        };
         let entry_kind = kind as u8;
         let entry_addr = entry_block.addr();
         let entry_size = 0u32;

src/sys/fs/mod.rs

@@ -26,7 +26,7 @@ use super_block::SuperBlock;
 
 use alloc::string::{String, ToString};
 
-pub const VERSION: u8 = 1;
+pub const VERSION: u8 = 2;
 
 // TODO: Move that to API
 #[derive(Clone, Copy)]
@@ -157,11 +157,11 @@ pub fn canonicalize(path: &str) -> Result<String, ()> {
 }
 
 pub fn disk_size() -> usize {
-    (SuperBlock::read().block_count as usize) * BLOCK_SIZE
+    (SuperBlock::read().block_count() as usize) * BLOCK_SIZE
 }
 
 pub fn disk_used() -> usize {
-    (SuperBlock::read().alloc_count as usize) * BLOCK_SIZE
+    (SuperBlock::read().alloc_count() as usize) * BLOCK_SIZE
 }
 
 pub fn disk_free() -> usize {

src/sys/fs/super_block.rs

@@ -12,8 +12,8 @@ pub struct SuperBlock {
     signature: &'static [u8; 8],
     version: u8,
     block_size: u32,
-    pub block_count: u32,
-    pub alloc_count: u32,
+    block_count: u32,
+    alloc_count: u32,
 }
 
 impl SuperBlock {
@@ -27,13 +27,18 @@ impl SuperBlock {
 
     pub fn new() -> Option<Self> {
         if let Some(ref dev) = *super::block_device::BLOCK_DEVICE.lock() {
-            Some(Self {
+            let mut sb = Self {
                 signature: SIGNATURE,
                 version: super::VERSION,
                 block_size: dev.block_size() as u32,
                 block_count: dev.block_count() as u32,
                 alloc_count: 0,
-            })
+            };
+
+            // Reserved blocks
+            sb.alloc_count = sb.data_area();
+
+            Some(sb)
         } else {
             None
         }
@@ -78,16 +83,33 @@ impl SuperBlock {
         self.block_count
     }
 
+    pub fn alloc_count(&self) -> u32 {
+        self.alloc_count
+    }
+
     pub fn bitmap_area(&self) -> u32 {
         SUPERBLOCK_ADDR + 2
     }
 
     pub fn data_area(&self) -> u32 {
-        let bs = super::BITMAP_SIZE as u32;
-        let total = self.block_count;
-        let offset = self.bitmap_area();
-        let rest = (total - offset) * bs / (bs + 1);
-        self.bitmap_area() + rest / bs
+        let s = self.block_size * 8;
+        let n = self.block_count;
+        let a = self.bitmap_area();
+
+        if self.version == 1 {
+            a + ((n - a) / (s + 1)) // Incorrect formula fixed in v2
+        } else {
+            let mut p; // Previous bitmap count
+            let mut b = 0; // Bitmap count
+            loop {
+                p = b;
+                b = (n - (a + b) + s - 1) / s;
+                if b == p {
+                    break;
+                }
+            }
+            a + b
+        }
     }
 }
 
@@ -99,6 +121,6 @@ pub fn inc_alloc_count() {
 
 pub fn dec_alloc_count() {
     let mut sb = SuperBlock::read();
-    sb.alloc_count -= 1;
+    sb.alloc_count -= 1; // FIXME: Use saturating substraction
     sb.write();
 }