Cache the last used node for faster allocations in VMA

Author: My-Bad-2

kernel/include/memory/vma.hpp

@@ -88,6 +88,7 @@ class VirtualMemoryAllocator {
 
     SpinLock lock;
     VmRegion* root;
+    VmRegion* cached_cursor = nullptr;
     VmRegionAllocator metadata_allocator;
 };
 }  // namespace kernel::memory

kernel/src/memory/vma.cpp

@@ -229,6 +229,18 @@ VmRegion* VirtualMemoryAllocator::find_node(uintptr_t start) {
 }
 
 uintptr_t VirtualMemoryAllocator::find_hole(size_t size, size_t alignment) {
+    if (this->cached_cursor) {
+        // Calculate where the allocation would start if placed after the cursor
+        uintptr_t candidate = align_up(this->cached_cursor->end(), alignment);
+        size_t overhead     = candidate - this->cached_cursor->end();
+
+        // Does the gap after the cursor fit the request?
+        if (this->cached_cursor->gap >= (size + overhead)) {
+            // Voila! We found a spot without touching the tree.
+            return candidate;
+        }
+    }
+
     // Try to find a hole inside the tree
     uintptr_t found = this->find_hole(this->root, size, alignment);
     if (found != 0) {
@@ -284,7 +296,7 @@ uintptr_t VirtualMemoryAllocator::find_hole(VmRegion* node, size_t size, size_t
         return this->find_hole(node->right, size, alignment);
     }
 
-    // We failed, Mr. Stark.
+    // We failed, Mr. Stark
     return 0;
 }
 
@@ -365,6 +377,8 @@ void VirtualMemoryAllocator::insert_region_locked(uintptr_t start, size_t size,
         }
     }
 
+    this->cached_cursor = z;
+
     // Rebalance
     this->insert_fixup(z);
 }