fix bug in impl Drop for FrameTracker, for the correctly mapping/unmapping display buffer in app

os/src/mm/frame_allocator.rs

@@ -7,6 +7,7 @@ use lazy_static::*;
 
 pub struct FrameTracker {
     pub ppn: PhysPageNum,
+    pub nodrop: bool,
 }
 
 impl FrameTracker {
@@ -16,10 +17,10 @@ impl FrameTracker {
         for i in bytes_array {
             *i = 0;
         }
-        Self { ppn }
+        Self { ppn, nodrop: false }
     }
-    pub fn new_nowrite(ppn: PhysPageNum) -> Self {
-        Self { ppn }
+    pub fn new_noalloc(ppn: PhysPageNum) -> Self {
+        Self { ppn, nodrop: true }
     }
 }
 
@@ -31,6 +32,9 @@ impl Debug for FrameTracker {
 
 impl Drop for FrameTracker {
     fn drop(&mut self) {
+        if self.nodrop {
+            return;
+        }
         frame_dealloc(self.ppn);
     }
 }

os/src/mm/memory_set.rs

@@ -290,6 +290,9 @@ impl MapArea {
                 ppn = frame.ppn;
                 self.data_frames.insert(vpn, frame);
             }
+            MapType::Noalloc => {
+                panic!("Noalloc should not be mapped");
+            }
         }
         let pte_flags = PTEFlags::from_bits(self.map_perm.bits).unwrap();
         page_table.map(vpn, ppn, pte_flags);
@@ -307,7 +310,7 @@ impl MapArea {
     }
     pub fn map_noalloc(&mut self, page_table: &mut PageTable,ppn_range:PPNRange) {
         for (vpn,ppn) in core::iter::zip(self.vpn_range,ppn_range) {
-            self.data_frames.insert(vpn, FrameTracker::new_nowrite(ppn));
+            self.data_frames.insert(vpn, FrameTracker::new_noalloc(ppn));
             let pte_flags = PTEFlags::from_bits(self.map_perm.bits).unwrap();
             page_table.map(vpn, ppn, pte_flags);
         }
@@ -346,6 +349,7 @@ impl MapArea {
 pub enum MapType {
     Identical,
     Framed,
+    Noalloc,
 }
 
 bitflags! {

os/src/syscall/gui.rs

@@ -22,7 +22,7 @@ pub fn sys_framebuffer() -> isize {
         MapArea::new(
             (FB_VADDR as usize).into(),
             (FB_VADDR + len as usize).into(),
-            MapType::Framed,
+            MapType::Noalloc,
             MapPermission::R | MapPermission::W | MapPermission::U,
         ),
         PPNRange::new(fb_ppn, fb_end_ppn),