drm/asahi: alloc: Do not allocate memory to free memory

The existing garbage mechanism could allocate a relatively unbounded vec
when freeing garbage, which was hurting memory exhaustion scenarios.
The only reason we need that buffer is to move garbage out of the lock
so we can drop it without deadlocks. Replace it with a 128-size
pre-allocated garbage buffer, and loop around reusing it.

Signed-off-by: Asahi Lina <lina@asahilina.net>

Author: hoshinolina

drivers/gpu/drm/asahi/alloc.rs

@@ -720,6 +720,7 @@ pub(crate) struct HeapAllocator {
     guard_nodes: Vec<mm::Node<HeapAllocatorInner, HeapAllocationInner>>,
     mm: mm::Allocator<HeapAllocatorInner, HeapAllocationInner>,
     name: CString,
+    garbage: Option<Vec<mm::Node<HeapAllocatorInner, HeapAllocationInner>>>,
 }
 
 impl HeapAllocator {
@@ -773,6 +774,15 @@ impl HeapAllocator {
             guard_nodes: Vec::new(),
             mm,
             name,
+            garbage: if keep_garbage {
+                Some({
+                    let mut v = Vec::new();
+                    v.try_reserve(128)?;
+                    v
+                })
+            } else {
+                None
+            },
         })
     }
 
@@ -1038,29 +1048,31 @@ impl Allocator for HeapAllocator {
         })
     }
 
-    fn collect_garbage(&mut self, count: usize) {
-        // Take the garbage out of the inner block, so we can safely drop it without deadlocking
-        let mut garbage = Vec::new();
-
-        if garbage.try_reserve(count).is_err() {
-            dev_crit!(
-                self.dev,
-                "HeapAllocator[{}]:collect_garbage: failed to reserve space\n",
-                &*self.name,
-            );
-            return;
-        }
+    fn collect_garbage(&mut self, mut count: usize) {
+        if let Some(garbage) = self.garbage.as_mut() {
+            garbage.clear();
+
+            while count > 0 {
+                let block = count.min(garbage.capacity());
+                assert!(block > 0);
+
+                // Take the garbage out of the inner block, so we can safely drop it without deadlocking
+                self.mm.with_inner(|inner| {
+                    if let Some(g) = inner.garbage.as_mut() {
+                        for node in g.drain(0..block) {
+                            inner.total_garbage -= node.size() as usize;
+                            garbage
+                                .try_push(node)
+                                .expect("try_push() failed after reserve()");
+                        }
+                    }
+                });
 
-        self.mm.with_inner(|inner| {
-            if let Some(g) = inner.garbage.as_mut() {
-                for node in g.drain(0..count) {
-                    inner.total_garbage -= node.size() as usize;
-                    garbage
-                        .try_push(node)
-                        .expect("try_push() failed after reserve()");
-                }
+                count -= block;
+                // Now drop it
+                garbage.clear();
             }
-        });
+        }
     }
 }