[compiler-rt] [scudo] Added LRU eviction policy to secondary cache. (PR #99409)

[Christopher Ferris via llvm-commits]

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@@ -213,23 +225,39 @@ template <typename Config> class MapAllocatorCache {
     if (Config::getDefaultReleaseToOsIntervalMs() != INT32_MIN)
       ReleaseToOsInterval = Config::getDefaultReleaseToOsIntervalMs();
     setOption(Option::ReleaseInterval, static_cast<sptr>(ReleaseToOsInterval));
+
+    // The cache is initially empty
+    LRUHead = CachedBlock::InvalidEntry;
+    LRUTail = CachedBlock::InvalidEntry;
+
+    // Available entries will be retrieved starting from the beginning of the
+    // Entries array
+    AvailableHead = 0;
+    for (u32 I = 0; I < Config::getEntriesArraySize() - 1; I++)
+      Entries[I].Next = static_cast<u16>(I + 1);
+
+    Entries[Config::getEntriesArraySize() - 1].Next = CachedBlock::InvalidEntry;
   }
 
   void store(const Options &Options, LargeBlock::Header *H) EXCLUDES(Mutex) {
     if (!canCache(H->CommitSize))
       return unmap(H);
 
-    bool EntryCached = false;
-    bool EmptyCache = false;
     const s32 Interval = atomic_load_relaxed(&ReleaseToOsIntervalMs);
-    const u64 Time = getMonotonicTimeFast();
-    const u32 MaxCount = atomic_load_relaxed(&MaxEntriesCount);
+    u64 Time;
     CachedBlock Entry;
+
+    // Usually only one entry will be evicted from the cache.
+    // Only in the rare event that the cache shrinks in real-time
+    // due to a decrease in the configurable value MaxEntriesCount
+    // will more than one cache entry be evicted
+    Vector<MemMapT, 1U> EvictionMemMaps;
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cferris1000 wrote:

I'd move this right in front of the do loop down below so it's more obvious where this is used. I'd also add a comment explaining that you using this to gather the maps to evict and then doing it outside of the lock.

https://github.com/llvm/llvm-project/pull/99409