Index: include/llvm/Support/CachePruning.h
===================================================================
--- include/llvm/Support/CachePruning.h
+++ include/llvm/Support/CachePruning.h
@@ -54,6 +54,11 @@
   /// This defaults to 1000000 because with that many files there are
   /// diminishing returns on the effectiveness of the cache, and file
   /// systems have a limit on total number of files.
+  /// Notably ext4 on Linux, with the default setting (block
+  /// size is 4096 and large_dir is off), there is a per-directory entry limit
+  /// of 508*510*floor(4096/(40+8))~=20M for average filename length of 40. Due
+  /// to the inbalance of the htree structure, certain filenames cannot be
+  /// created while others can before hitting the limit.
   uint64_t MaxSizeFiles = 1000000;
 };