[scudo] Separated committed and decommitted entries.

[JoshuaMBa]

Initially, the LRU list stored all mapped entries with no distinction between the committed (non-madvise()'d) entries and decommitted (madvise()'d) entries. Now these two types of entries re separated into two lists, allowing future cache logic to branch depending on whether or not entries are committed or decommitted. Furthermore, the retrieval algorithm will prioritize committed entries over decommitted entries. Specifically, committed entries that satisfy the MaxUnusedCachePages requirement are retrieved before optimal-fit, decommitted entries.

This commit addresses the compiler errors raised here.

[llvmbot]

@llvm/pr-subscribers-compiler-rt-sanitizer

Author: Joshua Baehring (JoshuaMBa)

Changes

Initially, the LRU list stored all mapped entries with no distinction between the committed (non-madvise()'d) entries and decommitted (madvise()'d) entries. Now these two types of entries re separated into two lists, allowing future cache logic to branch depending on whether or not entries are committed or decommitted. Furthermore, the retrieval algorithm will prioritize committed entries over decommitted entries. Specifically, committed entries that satisfy the MaxUnusedCachePages requirement are retrieved before optimal-fit, decommitted entries.

This commit addresses the compiler errors raised here.

---

Full diff: https://github.com/llvm/llvm-project/pull/101409.diff

1 Files Affected:

• (modified) compiler-rt/lib/scudo/standalone/secondary.h (+156-92)

diff --git a/compiler-rt/lib/scudo/standalone/secondary.h b/compiler-rt/lib/scudo/standalone/secondary.h
index d8505742d6054..4e39126f00453 100644
--- a/compiler-rt/lib/scudo/standalone/secondary.h
+++ b/compiler-rt/lib/scudo/standalone/secondary.h
@@ -180,6 +180,14 @@ template <typename T> class NonZeroLengthArray<T, 0> {
 
 template <typename Config> class MapAllocatorCache {
 public:
+  typedef enum { COMMITTED = 0, DECOMMITTED = 1, NONE } EntryListT;
+
+  // TODO: Refactor the intrusive list to support non-pointer link type
+  typedef struct {
+    u16 Head;
+    u16 Tail;
+  } ListInfo;
+
   void getStats(ScopedString *Str) {
     ScopedLock L(Mutex);
     uptr Integral;
@@ -197,13 +205,18 @@ template <typename Config> class MapAllocatorCache {
                 SuccessfulRetrieves, CallsToRetrieve, Integral, Fractional);
     Str->append("Cache Entry Info (Most Recent -> Least Recent):\n");
 
-    for (u32 I = LRUHead; I != CachedBlock::InvalidEntry; I = Entries[I].Next) {
-      CachedBlock &Entry = Entries[I];
-      Str->append("  StartBlockAddress: 0x%zx, EndBlockAddress: 0x%zx, "
-                  "BlockSize: %zu %s\n",
-                  Entry.CommitBase, Entry.CommitBase + Entry.CommitSize,
-                  Entry.CommitSize, Entry.Time == 0 ? "[R]" : "");
-    }
+    auto printList = [&](EntryListT ListType) REQUIRES(Mutex) {
+      for (u32 I = EntryLists[ListType].Head; I != CachedBlock::InvalidEntry;
+           I = Entries[I].Next) {
+        CachedBlock &Entry = Entries[I];
+        Str->append("  StartBlockAddress: 0x%zx, EndBlockAddress: 0x%zx, "
+                    "BlockSize: %zu %s\n",
+                    Entry.CommitBase, Entry.CommitBase + Entry.CommitSize,
+                    Entry.CommitSize, Entry.Time == 0 ? "[R]" : "");
+      }
+    };
+    printList(COMMITTED);
+    printList(DECOMMITTED);
   }
 
   // Ensure the default maximum specified fits the array.
@@ -227,8 +240,10 @@ template <typename Config> class MapAllocatorCache {
     setOption(Option::ReleaseInterval, static_cast<sptr>(ReleaseToOsInterval));
 
     // The cache is initially empty
-    LRUHead = CachedBlock::InvalidEntry;
-    LRUTail = CachedBlock::InvalidEntry;
+    EntryLists[COMMITTED].Head = CachedBlock::InvalidEntry;
+    EntryLists[COMMITTED].Tail = CachedBlock::InvalidEntry;
+    EntryLists[DECOMMITTED].Head = CachedBlock::InvalidEntry;
+    EntryLists[DECOMMITTED].Tail = CachedBlock::InvalidEntry;
 
     // Available entries will be retrieved starting from the beginning of the
     // Entries array
@@ -310,15 +325,19 @@ template <typename Config> class MapAllocatorCache {
       // All excess entries are evicted from the cache
       while (needToEvict()) {
         // Save MemMaps of evicted entries to perform unmap outside of lock
-        EvictionMemMaps.push_back(Entries[LRUTail].MemMap);
-        remove(LRUTail);
+        EntryListT EvictionListType;
+        if (EntryLists[DECOMMITTED].Tail == CachedBlock::InvalidEntry)
+          EvictionListType = COMMITTED;
+        else
+          EvictionListType = DECOMMITTED;
+        remove(EntryLists[EvictionListType].Tail, EvictionListType);
       }
 
-      insert(Entry);
+      insert(Entry, (Entry.Time == 0) ? DECOMMITTED : COMMITTED);
 
       if (OldestTime == 0)
         OldestTime = Entry.Time;
-    } while (0);
+    } while (0); // ScopedLock L(Mutex);
 
     for (MemMapT &EvictMemMap : EvictionMemMaps)
       EvictMemMap.unmap(EvictMemMap.getBase(), EvictMemMap.getCapacity());
@@ -335,56 +354,69 @@ template <typename Config> class MapAllocatorCache {
     // 10% of the requested size proved to be the optimal choice for
     // retrieving cached blocks after testing several options.
     constexpr u32 FragmentedBytesDivisor = 10;
-    bool Found = false;
     CachedBlock Entry;
     uptr EntryHeaderPos = 0;
+    uptr OptimalFitIndex = CachedBlock::InvalidEntry;
     {
       ScopedLock L(Mutex);
       CallsToRetrieve++;
       if (EntriesCount == 0)
         return false;
-      u32 OptimalFitIndex = 0;
       uptr MinDiff = UINTPTR_MAX;
-      for (u32 I = LRUHead; I != CachedBlock::InvalidEntry;
-           I = Entries[I].Next) {
-        const uptr CommitBase = Entries[I].CommitBase;
-        const uptr CommitSize = Entries[I].CommitSize;
-        const uptr AllocPos =
-            roundDown(CommitBase + CommitSize - Size, Alignment);
-        const uptr HeaderPos = AllocPos - HeadersSize;
-        if (HeaderPos > CommitBase + CommitSize)
-          continue;
-        if (HeaderPos < CommitBase ||
-            AllocPos > CommitBase + PageSize * MaxUnusedCachePages) {
-          continue;
-        }
-        Found = true;
-        const uptr Diff = HeaderPos - CommitBase;
-        // immediately use a cached block if it's size is close enough to the
-        // requested size.
-        const uptr MaxAllowedFragmentedBytes =
-            (CommitBase + CommitSize - HeaderPos) / FragmentedBytesDivisor;
-        if (Diff <= MaxAllowedFragmentedBytes) {
+      EntryListT OptimalFitListType = NONE;
+      auto FindAvailableEntry = [&](EntryListT ListType) REQUIRES(Mutex) {
+        for (uptr I = EntryLists[ListType].Head; I != CachedBlock::InvalidEntry;
+             I = Entries[I].Next) {
+          const uptr CommitBase = Entries[I].CommitBase;
+          const uptr CommitSize = Entries[I].CommitSize;
+          const uptr AllocPos =
+              roundDown(CommitBase + CommitSize - Size, Alignment);
+          const uptr HeaderPos = AllocPos - HeadersSize;
+          if (HeaderPos > CommitBase + CommitSize)
+            continue;
+          if (HeaderPos < CommitBase ||
+              AllocPos > CommitBase + PageSize * MaxUnusedCachePages)
+            continue;
+
+          const uptr Diff = HeaderPos - CommitBase;
+          // immediately use a cached block if it's size is close enough to
+          // the requested size.
+          const uptr MaxAllowedFragmentedBytes =
+              (CommitBase + CommitSize - HeaderPos) / FragmentedBytesDivisor;
+          if (Diff <= MaxAllowedFragmentedBytes) {
+            OptimalFitIndex = I;
+            EntryHeaderPos = HeaderPos;
+            OptimalFitListType = ListType;
+            return Entries[OptimalFitIndex];
+          }
+
+          // keep track of the smallest cached block
+          // that is greater than (AllocSize + HeaderSize)
+          if (Diff > MinDiff)
+            continue;
           OptimalFitIndex = I;
+          MinDiff = Diff;
+          OptimalFitListType = ListType;
           EntryHeaderPos = HeaderPos;
-          break;
         }
-        // keep track of the smallest cached block
-        // that is greater than (AllocSize + HeaderSize)
-        if (Diff > MinDiff)
-          continue;
-        OptimalFitIndex = I;
-        MinDiff = Diff;
-        EntryHeaderPos = HeaderPos;
-      }
-      if (Found) {
-        Entry = Entries[OptimalFitIndex];
-        remove(OptimalFitIndex);
-        SuccessfulRetrieves++;
-      }
-    }
-    if (!Found)
-      return false;
+        CachedBlock FoundEntry;
+        if (OptimalFitIndex != CachedBlock::InvalidEntry)
+          FoundEntry = Entries[OptimalFitIndex];
+        return FoundEntry;
+      };
+
+      // Prioritize valid fit from COMMITTED entries over
+      // optimal fit from DECOMMITTED entries
+      Entry = FindAvailableEntry(COMMITTED);
+      if (!Entry.isValid())
+        Entry = FindAvailableEntry(DECOMMITTED);
+
+      if (!Entry.isValid())
+        return false;
+
+      remove(OptimalFitIndex, OptimalFitListType);
+      SuccessfulRetrieves++;
+    } // ScopedLock L(Mutex);
 
     *H = reinterpret_cast<LargeBlock::Header *>(
         LargeBlock::addHeaderTag<Config>(EntryHeaderPos));
@@ -448,10 +480,15 @@ template <typename Config> class MapAllocatorCache {
         Quarantine[I].invalidate();
       }
     }
-    for (u32 I = LRUHead; I != CachedBlock::InvalidEntry; I = Entries[I].Next) {
-      Entries[I].MemMap.setMemoryPermission(Entries[I].CommitBase,
-                                            Entries[I].CommitSize, 0);
-    }
+    auto disableLists = [&](EntryListT EntryList) REQUIRES(Mutex) {
+      for (u32 I = EntryLists[EntryList].Head; I != CachedBlock::InvalidEntry;
+           I = Entries[I].Next) {
+        Entries[I].MemMap.setMemoryPermission(Entries[I].CommitBase,
+                                              Entries[I].CommitSize, 0);
+      }
+    };
+    disableLists(COMMITTED);
+    disableLists(DECOMMITTED);
     QuarantinePos = -1U;
   }
 
@@ -466,7 +503,7 @@ template <typename Config> class MapAllocatorCache {
     return (EntriesCount >= atomic_load_relaxed(&MaxEntriesCount));
   }
 
-  void insert(const CachedBlock &Entry) REQUIRES(Mutex) {
+  void insert(const CachedBlock &Entry, EntryListT ListType) REQUIRES(Mutex) {
     DCHECK_LT(EntriesCount, atomic_load_relaxed(&MaxEntriesCount));
 
     // Cache should be populated with valid entries when not empty
@@ -475,66 +512,86 @@ template <typename Config> class MapAllocatorCache {
     u32 FreeIndex = AvailableHead;
     AvailableHead = Entries[AvailableHead].Next;
 
-    if (EntriesCount == 0) {
-      LRUTail = static_cast<u16>(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] = Entry;
-    Entries[FreeIndex].Next = LRUHead;
-    Entries[FreeIndex].Prev = CachedBlock::InvalidEntry;
-    LRUHead = static_cast<u16>(FreeIndex);
+    pushFront(FreeIndex, ListType);
     EntriesCount++;
 
+    if (Entries[EntryLists[ListType].Head].Next != CachedBlock::InvalidEntry) {
+      DCHECK_GE(Entries[EntryLists[ListType].Head].Time,
+                Entries[Entries[EntryLists[ListType].Head].Next].Time);
+    }
     // Availability stack should not have available entries when all entries
     // are in use
     if (EntriesCount == Config::getEntriesArraySize())
       DCHECK_EQ(AvailableHead, CachedBlock::InvalidEntry);
   }
 
-  void remove(uptr I) REQUIRES(Mutex) {
-    DCHECK(Entries[I].isValid());
-
-    Entries[I].invalidate();
-
-    if (I == LRUHead)
-      LRUHead = Entries[I].Next;
+  // Joins the entries adjacent to Entries[I], effectively
+  // unlinking Entries[I] from the list
+  void unlink(uptr I, EntryListT ListType) REQUIRES(Mutex) {
+    if (I == EntryLists[ListType].Head)
+      EntryLists[ListType].Head = Entries[I].Next;
     else
       Entries[Entries[I].Prev].Next = Entries[I].Next;
 
-    if (I == LRUTail)
-      LRUTail = Entries[I].Prev;
+    if (I == EntryLists[ListType].Tail)
+      EntryLists[ListType].Tail = Entries[I].Prev;
     else
       Entries[Entries[I].Next].Prev = Entries[I].Prev;
+  }
 
+  // Invalidates Entries[I], removes Entries[I] from list, and pushes
+  // Entries[I] onto the stack of available entries
+  void remove(uptr I, EntryListT ListType) REQUIRES(Mutex) {
+    DCHECK(Entries[I].isValid());
+
+    Entries[I].invalidate();
+
+    unlink(I, ListType);
     Entries[I].Next = AvailableHead;
     AvailableHead = static_cast<u16>(I);
     EntriesCount--;
 
     // Cache should not have valid entries when not empty
     if (EntriesCount == 0) {
-      DCHECK_EQ(LRUHead, CachedBlock::InvalidEntry);
-      DCHECK_EQ(LRUTail, CachedBlock::InvalidEntry);
+      DCHECK_EQ(EntryLists[COMMITTED].Head, CachedBlock::InvalidEntry);
+      DCHECK_EQ(EntryLists[COMMITTED].Tail, CachedBlock::InvalidEntry);
+      DCHECK_EQ(EntryLists[DECOMMITTED].Head, CachedBlock::InvalidEntry);
+      DCHECK_EQ(EntryLists[DECOMMITTED].Tail, CachedBlock::InvalidEntry);
     }
   }
 
+  inline void pushFront(uptr I, EntryListT ListType) REQUIRES(Mutex) {
+    if (EntryLists[ListType].Tail == CachedBlock::InvalidEntry)
+      EntryLists[ListType].Tail = static_cast<u16>(I);
+    else
+      Entries[EntryLists[ListType].Head].Prev = static_cast<u16>(I);
+
+    Entries[I].Next = EntryLists[ListType].Head;
+    Entries[I].Prev = CachedBlock::InvalidEntry;
+    EntryLists[ListType].Head = static_cast<u16>(I);
+  }
+
   void empty() {
     MemMapT MapInfo[Config::getEntriesArraySize()];
     uptr N = 0;
     {
       ScopedLock L(Mutex);
-      for (uptr I = 0; I < Config::getEntriesArraySize(); I++) {
-        if (!Entries[I].isValid())
-          continue;
-        MapInfo[N] = Entries[I].MemMap;
-        remove(I);
-        N++;
-      }
+      auto emptyList = [&](EntryListT ListType) REQUIRES(Mutex) {
+        for (uptr I = EntryLists[ListType].Head;
+             I != CachedBlock::InvalidEntry;) {
+          uptr ToRemove = I;
+          I = Entries[I].Next;
+          MapInfo[N] = Entries[ToRemove].MemMap;
+          remove(ToRemove, ListType);
+          N++;
+        }
+      };
+      emptyList(COMMITTED);
+      emptyList(DECOMMITTED);
       EntriesCount = 0;
+      for (uptr I = 0; I < Config::getEntriesArraySize(); I++)
+        DCHECK(!Entries[I].isValid());
     }
     for (uptr I = 0; I < N; I++) {
       MemMapT &MemMap = MapInfo[I];
@@ -561,8 +618,13 @@ template <typename Config> class MapAllocatorCache {
     OldestTime = 0;
     for (uptr I = 0; I < Config::getQuarantineSize(); I++)
       releaseIfOlderThan(Quarantine[I], Time);
-    for (uptr I = 0; I < Config::getEntriesArraySize(); I++)
+    for (uptr I = 0; I < Config::getEntriesArraySize(); I++) {
+      if (Entries[I].isValid() && Entries[I].Time && Entries[I].Time <= Time) {
+        unlink(I, COMMITTED);
+        pushFront(I, DECOMMITTED);
+      }
       releaseIfOlderThan(Entries[I], Time);
+    }
   }
 
   HybridMutex Mutex;
@@ -579,10 +641,12 @@ template <typename Config> class MapAllocatorCache {
   NonZeroLengthArray<CachedBlock, Config::getQuarantineSize()>
       Quarantine GUARDED_BY(Mutex) = {};
 
-  // The LRUHead of the cache is the most recently used cache entry
-  u16 LRUHead GUARDED_BY(Mutex) = 0;
-  // The LRUTail of the cache is the least recently used cache entry
-  u16 LRUTail GUARDED_BY(Mutex) = 0;
+  // EntryLists stores the head and tail indices of all
+  // lists being used to store valid cache entries.
+  // Currently there are lists storing COMMITTED and DECOMMITTED entries.
+  // COMMITTED entries are those that are not madvise()'d
+  // DECOMMITTED entries are those that are madvise()'d
+  ListInfo EntryLists[2] GUARDED_BY(Mutex) = {};
   // The AvailableHead is the top of the stack of available entries
   u16 AvailableHead GUARDED_BY(Mutex) = 0;
 };

[cferris1000]

Just a couple of nits.

[cferris1000]

LGTM

[ChiaHungDuan]

It would be better to have the end-to-end test before this change and let's make sure the leaking bug is caught by the test

[github-actions]

✅ With the latest revision this PR passed the C/C++ code formatter.

[cferris1000]

LGTM.