[scudo] Reapply "Update secondary cache time-based release logic"

This reapplies commit e5271fe. In
the event that MTE is turned on, in which case released cache entries
may be  inserted into the cache, all released cache entries are
inserted after `LastUnreleasedEntry`. Additionally, when a cache entry
is moved from `Quarantine` to `Entries`, its time field will only be
updated if the cache entry's memory has not been released.

Author: JoshuaMBa

compiler-rt/lib/scudo/standalone/secondary.h

@@ -247,6 +247,7 @@ class MapAllocatorCache {
     // The cache is initially empty
     LRUHead = CachedBlock::InvalidEntry;
     LRUTail = CachedBlock::InvalidEntry;
+    LastUnreleasedEntry = CachedBlock::InvalidEntry;
 
     // Available entries will be retrieved starting from the beginning of the
     // Entries array
@@ -321,9 +322,13 @@ class MapAllocatorCache {
         }
         CachedBlock PrevEntry = Quarantine[QuarantinePos];
         Quarantine[QuarantinePos] = Entry;
-        if (OldestTime == 0)
-          OldestTime = Entry.Time;
         Entry = PrevEntry;
+        // Update the entry time to reflect the time that the
+        // quarantined memory is placed in the Entries array
+        // If an entry has a time value of 0, it has already
+        // been released, so its time value must remain 0
+        if (Entry.Time != 0)
+          Entry.Time = Time;
       }
 
       // All excess entries are evicted from the cache
@@ -334,16 +339,12 @@ class MapAllocatorCache {
       }
 
       insert(Entry);
-
-      if (OldestTime == 0)
-        OldestTime = Entry.Time;
     } while (0);
 
     for (MemMapT &EvictMemMap : EvictionMemMaps)
       unmapCallBack(EvictMemMap);
 
     if (Interval >= 0) {
-      // TODO: Add ReleaseToOS logic to LRU algorithm
       releaseOlderThan(Time - static_cast<u64>(Interval) * 1000000);
     }
   }
@@ -523,22 +524,37 @@ class MapAllocatorCache {
     // Cache should be populated with valid entries when not empty
     DCHECK_NE(AvailableHead, CachedBlock::InvalidEntry);
 
-    u32 FreeIndex = AvailableHead;
+    u16 FreeIndex = AvailableHead;
     AvailableHead = Entries[AvailableHead].Next;
+    Entries[FreeIndex] = Entry;
 
-    if (EntriesCount == 0) {
-      LRUTail = static_cast<u16>(FreeIndex);
+    // Check list order
+    if (EntriesCount > 1)
+      DCHECK_GE(Entries[LRUHead].Time, Entries[Entries[LRUHead].Next].Time);
+
+    // Released entry goes after LastUnreleasedEntry rather than at LRUHead
+    if (Entry.Time == 0 && LastUnreleasedEntry != CachedBlock::InvalidEntry) {
+      Entries[FreeIndex].Next = Entries[LastUnreleasedEntry].Next;
+      Entries[FreeIndex].Prev = LastUnreleasedEntry;
+      Entries[LastUnreleasedEntry].Next = FreeIndex;
+      if (LRUTail == LastUnreleasedEntry) {
+        LRUTail = FreeIndex;
+      } else {
+        Entries[Entries[FreeIndex].Next].Prev = FreeIndex;
+      }
     } else {
-      // Check list order
-      if (EntriesCount > 1)
-        DCHECK_GE(Entries[LRUHead].Time, Entries[Entries[LRUHead].Next].Time);
-      Entries[LRUHead].Prev = static_cast<u16>(FreeIndex);
+      Entries[FreeIndex].Next = LRUHead;
+      Entries[FreeIndex].Prev = CachedBlock::InvalidEntry;
+      if (EntriesCount == 0) {
+        LRUTail = FreeIndex;
+      } else {
+        Entries[LRUHead].Prev = FreeIndex;
+      }
+      LRUHead = FreeIndex;
+      if (LastUnreleasedEntry == CachedBlock::InvalidEntry)
+        LastUnreleasedEntry = FreeIndex;
     }
 
-    Entries[FreeIndex] = Entry;
-    Entries[FreeIndex].Next = LRUHead;
-    Entries[FreeIndex].Prev = CachedBlock::InvalidEntry;
-    LRUHead = static_cast<u16>(FreeIndex);
     EntriesCount++;
 
     // Availability stack should not have available entries when all entries
@@ -552,6 +568,9 @@ class MapAllocatorCache {
 
     Entries[I].invalidate();
 
+    if (I == LastUnreleasedEntry)
+      LastUnreleasedEntry = Entries[LastUnreleasedEntry].Prev;
+
     if (I == LRUHead)
       LRUHead = Entries[I].Next;
     else
@@ -593,35 +612,39 @@ class MapAllocatorCache {
     }
   }
 
-  void releaseIfOlderThan(CachedBlock &Entry, u64 Time) REQUIRES(Mutex) {
-    if (!Entry.isValid() || !Entry.Time)
-      return;
-    if (Entry.Time > Time) {
-      if (OldestTime == 0 || Entry.Time < OldestTime)
-        OldestTime = Entry.Time;
-      return;
-    }
+  inline void release(CachedBlock &Entry) {
+    DCHECK(Entry.Time != 0);
     Entry.MemMap.releaseAndZeroPagesToOS(Entry.CommitBase, Entry.CommitSize);
     Entry.Time = 0;
   }
 
   void releaseOlderThan(u64 Time) EXCLUDES(Mutex) {
     ScopedLock L(Mutex);
-    if (!EntriesCount || OldestTime == 0 || OldestTime > Time)
+    if (!EntriesCount)
       return;
-    OldestTime = 0;
-    for (uptr I = 0; I < Config::getQuarantineSize(); I++)
-      releaseIfOlderThan(Quarantine[I], Time);
-    for (uptr I = 0; I < Config::getEntriesArraySize(); I++)
-      releaseIfOlderThan(Entries[I], Time);
-  }
 
+    // TODO: Add conditional to skip iteration over quarantine
+    // if quarantine is disabled
+    for (uptr I = 0; I < Config::getQuarantineSize(); I++) {
+      CachedBlock &ReleaseEntry = Quarantine[I];
+      if (!ReleaseEntry.isValid() || !ReleaseEntry.Time ||
+          ReleaseEntry.Time > Time)
+        continue;
+      release(ReleaseEntry);
+    }
+
+    // Release oldest entries first by releasing from decommit base
+    while (LastUnreleasedEntry != CachedBlock::InvalidEntry &&
+           Entries[LastUnreleasedEntry].Time <= Time) {
+      release(Entries[LastUnreleasedEntry]);
+      LastUnreleasedEntry = Entries[LastUnreleasedEntry].Prev;
+    }
+  }
   HybridMutex Mutex;
   u32 EntriesCount GUARDED_BY(Mutex) = 0;
   u32 QuarantinePos GUARDED_BY(Mutex) = 0;
   atomic_u32 MaxEntriesCount = {};
   atomic_uptr MaxEntrySize = {};
-  u64 OldestTime GUARDED_BY(Mutex) = 0;
   atomic_s32 ReleaseToOsIntervalMs = {};
   u32 CallsToRetrieve GUARDED_BY(Mutex) = 0;
   u32 SuccessfulRetrieves GUARDED_BY(Mutex) = 0;
@@ -636,6 +659,9 @@ class MapAllocatorCache {
   u16 LRUTail GUARDED_BY(Mutex) = 0;
   // The AvailableHead is the top of the stack of available entries
   u16 AvailableHead GUARDED_BY(Mutex) = 0;
+  // The LastUnreleasedEntry is the least recently used entry that has not
+  // been released
+  u16 LastUnreleasedEntry GUARDED_BY(Mutex) = 0;
 };
 
 template <typename Config> class MapAllocator {