[compiler-rt] [compiler-rt][fuchsia] Preallocate a vmar for sanitizer internals (PR #75256)

[Roland McGrath via llvm-commits]

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@@ -129,6 +129,55 @@ uptr GetMaxVirtualAddress() { return GetMaxUserVirtualAddress(); }
 
 bool ErrorIsOOM(error_t err) { return err == ZX_ERR_NO_MEMORY; }
 
+// For any sanitizer internal that needs to map something which can be unmmaped
+// later, first attempt to map to a pre-allocated vmar. This helps reduce
+// fragmentation from many small anonymous mmap calls. A good value for this
+// vmar size would be the total size of your typical sanitizer internal objects
+// allocated in an "average" process lifetime. Examples of this include:
+// FakeStack, LowLevelAllocator mappings, TwoLevelMap, InternalMmapVector,
+// StackStore, CreateAsanThread, etc.
+//
+// 0xc52000 is ~12.32MB. This is roughly equal to the total sum of sanitizer
+// internal mappings in a run of boot-libc-unittests.
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frobtech wrote:

I don't think we should be referring to individual Fuchsia tree things here.  We can say things more generic like "a large test case".  Let's use a more round number.  If ~12.32MiB is a good number, then 13MiB is a good number, and you can write that as `13 << 20; // 13 MiB` or something more readable like that.


https://github.com/llvm/llvm-project/pull/75256