refactor(ext4): split into submodules, add io/superblock/bgd with write-back

crates/servers/storage-server/src/fs/ext4/bgd.rs

@@ -0,0 +1,39 @@
+use port_block_device::BlockDevice;
+use super::io::{read_block, write_block, read_u32, write_u32, read_u16, write_u16};
+
+pub struct BlockGroupDescriptor {
+    pub block_bitmap: u32,
+    pub inode_bitmap: u32,
+    pub inode_table: u32,
+    pub free_blocks_count: u16,
+    pub free_inodes_count: u16,
+}
+
+impl BlockGroupDescriptor {
+    pub fn read<B: BlockDevice>(device: &mut B, gdt_block: u32, bg: u32, spb: usize) -> Option<Self> {
+        let mut buf = [0u8; 4096];
+        if !read_block(device, gdt_block as u64, spb, &mut buf) { return None; }
+        let off = (bg as usize) * 32;
+        if off + 32 > buf.len() { return None; }
+        Some(Self {
+            block_bitmap: read_u32(&buf, off),
+            inode_bitmap: read_u32(&buf, off + 4),
+            inode_table: read_u32(&buf, off + 8),
+            free_blocks_count: read_u16(&buf, off + 12),
+            free_inodes_count: read_u16(&buf, off + 14),
+        })
+    }
+
+    pub fn write_back<B: BlockDevice>(&self, device: &mut B, gdt_block: u32, bg: u32, spb: usize) -> bool {
+        let mut buf = [0u8; 4096];
+        if !read_block(device, gdt_block as u64, spb, &mut buf) { return false; }
+        let off = (bg as usize) * 32;
+        if off + 32 > buf.len() { return false; }
+        write_u32(&mut buf, off, self.block_bitmap);
+        write_u32(&mut buf, off + 4, self.inode_bitmap);
+        write_u32(&mut buf, off + 8, self.inode_table);
+        write_u16(&mut buf, off + 12, self.free_blocks_count);
+        write_u16(&mut buf, off + 14, self.free_inodes_count);
+        write_block(device, gdt_block as u64, spb, &buf)
+    }
+}