8348400: GenShen: assert(ShenandoahHeap::heap()->is_full_gc_in_progress() || (used_regions_size() <= _max_capacity)) failed: Cannot use more than capacity #

src/hotspot/share/gc/shenandoah/shenandoahFreeSet.cpp

@@ -1310,7 +1310,7 @@ bool ShenandoahFreeSet::flip_to_old_gc(ShenandoahHeapRegion* r) {
     // mutator (they have nothing to steal), but they do have a usable region in their partition. What
     // we want to do here is swap that region from the mutator partition with one from the old collector
     // partition.
-    // 1. Find an unusable trash region in the old collector partition
+    // 1. Find a temporarily unusable trash region in the old collector partition
     ShenandoahRightLeftIterator iterator(&_partitions, ShenandoahFreeSetPartitionId::OldCollector, true);
     idx_t unusable_trash = -1;
     for (unusable_trash = iterator.current(); iterator.has_next(); unusable_trash = iterator.next()) {
@@ -1321,20 +1321,24 @@ bool ShenandoahFreeSet::flip_to_old_gc(ShenandoahHeapRegion* r) {
     }
 
     if (unusable_trash != -1) {
-      // 2. Move it to the mutator partition
+      const size_t unusable_capacity = alloc_capacity(unusable_trash);
+      // 2. Move the (temporarily) unusable trash region we found to the mutator partition
       _partitions.move_from_partition_to_partition(unusable_trash,
                                                    ShenandoahFreeSetPartitionId::OldCollector,
-                                                   ShenandoahFreeSetPartitionId::Mutator, region_capacity);
+                                                   ShenandoahFreeSetPartitionId::Mutator, unusable_capacity);
 
-      // 3. Move this usable region to the old collector partition
+      // 3. Move this usable region from the mutator partition to the old collector partition
       _partitions.move_from_partition_to_partition(idx,
                                                    ShenandoahFreeSetPartitionId::Mutator,
                                                    ShenandoahFreeSetPartitionId::OldCollector, region_capacity);
 
       _partitions.assert_bounds();
 
       // 4. Do not adjust capacities for generations, we just swapped the regions that have already
-      // been accounted for.
+      // been accounted for. However, we should adjust the evacuation reserves as those may have changed.
+      shenandoah_assert_heaplocked();
+      const size_t reserve = _heap->old_generation()->get_evacuation_reserve();
+      _heap->old_generation()->set_evacuation_reserve(reserve - unusable_capacity + region_capacity);
       return true;
     }
   }

src/hotspot/share/gc/shenandoah/shenandoahFreeSet.hpp

@@ -323,9 +323,11 @@ class ShenandoahFreeSet : public CHeapObj<mtGC> {
   //
   // Typical usage: During evacuation, the GC may find it needs more memory than had been reserved at the start of evacuation to
   // hold evacuated objects.  If this occurs and memory is still available in the Mutator's free set, we will flip a region from
-  // the Mutator free set into the Collector or OldCollector free set.
+  // the Mutator free set into the Collector or OldCollector free set. The conditions to move this region are checked by
+  // the caller, so the given region is always moved.
   void flip_to_gc(ShenandoahHeapRegion* r);
 
+  // Return true if and only if the given region is successfully flipped to the old partition
   bool flip_to_old_gc(ShenandoahHeapRegion* r);
 
   // Handle allocation for mutator.