fix: bugs

kernel/src/mem/pmm.rs

@@ -8,7 +8,6 @@ pub struct BitmapPMM {
     ref_counts:  &'static mut [u16],
     total_pages: usize,
     used_pages:  usize,
-    /// Byte index hint: next search starts here to amortise O(N) scans.
     last_byte:   usize,
 }
 
@@ -59,6 +58,7 @@ impl BitmapPMM {
             last_byte:  0,
         };
 
+        // Free all USABLE pages (gap pages not covered by any entry stay locked).
         for entry in mmap.entries() {
             if entry.entry_type == limine::memory_map::EntryType::USABLE {
                 for addr in (entry.base..entry.base + entry.length).step_by(PAGE_SIZE as usize) {
@@ -67,6 +67,7 @@ impl BitmapPMM {
             }
         }
 
+        // Metadata pages sit inside a USABLE region — lock them back.
         let meta_end = (meta_phys + total_meta_size as u64 + PAGE_SIZE - 1) & !(PAGE_SIZE - 1);
         for addr in (meta_phys..meta_end).step_by(PAGE_SIZE as usize) {
             pmm.lock_frame(PhysAddr(addr));