[compiler-rt] [scudo] Add partial chunk heuristic to retrieval algorithm. (PR #105009)

[Joshua Baehring via llvm-commits]

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@@ -334,61 +345,112 @@ class MapAllocatorCache {
     }
   }
 
-  CachedBlock retrieve(uptr Size, uptr Alignment, uptr HeadersSize,
-                       uptr &EntryHeaderPos) EXCLUDES(Mutex) {
+  CachedBlock retrieve(uptr MaxAllowedFragmentedPages, uptr Size,
+                       uptr Alignment, uptr HeadersSize, uptr &EntryHeaderPos)
+      EXCLUDES(Mutex) {
     const uptr PageSize = getPageSizeCached();
     // 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;
     EntryHeaderPos = 0;
     {
       ScopedLock L(Mutex);
       CallsToRetrieve++;
       if (EntriesCount == 0)
         return {};
-      u32 OptimalFitIndex = 0;
+      u16 RetrievedIndex = CachedBlock::InvalidEntry;
       uptr MinDiff = UINTPTR_MAX;
-      for (u32 I = LRUHead; I != CachedBlock::InvalidEntry;
+
+      //  Since allocation sizes don't always match cached memory chunk sizes
+      //  we allow some memory to be unused (called fragmented bytes). The
+      //  amount of unused bytes is exactly EntryHeaderPos - CommitBase.
+      //
+      //        CommitBase                CommitBase + CommitSize
+      //          V                              V
+      //      +---+------------+-----------------+---+
+      //      |   |            |                 |   |
+      //      +---+------------+-----------------+---+
+      //      ^                ^                     ^
+      //    Guard         EntryHeaderPos          Guard-page-end
+      //    page-begin
+      //
+      //  [EntryHeaderPos, CommitBase + CommitSize) contains the user data as
+      //  well as the header metadata. If EntryHeaderPos - CommitBase exceeds
+      //  MaxAllowedFragmentedPages * PageSize, the cached memory chunk is
+      //  not considered valid for retrieval.
+      for (u16 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;
+        const uptr MaxAllowedFragmentedBytes =
+            MaxAllowedFragmentedPages * PageSize;
         if (HeaderPos > CommitBase + CommitSize)
           continue;
         if (HeaderPos < CommitBase ||
-            AllocPos > CommitBase + PageSize * MaxUnusedCachePages) {
+            AllocPos > CommitBase + MaxAllowedFragmentedBytes) {
----------------
JoshuaMBa wrote:

For now we will keep this logic and set `MaxReleasedCachePages` to 0 while we investigate the effects of this change on the retrieval order.

https://github.com/llvm/llvm-project/pull/105009