Set Huge Page mode on shadow regions based on no_huge_pages_for_shadow

It turned out that at dynamic shared library mode, the memory access
pattern can increase memory footprint significantly on OS when transparent
hugepages (THP) are enabled. This could cause >70x memory overhead than
running a static linked binary. For example, a static binary with RSS
overhead 300M can use > 23G RSS if it is built dynamically.
/proc/../smaps shows in 6204552 kB RSS 6141952 kB relates to
AnonHugePages.

Also such a high RSS happens in some rate: around 25% runs may use > 23G RSS, the
rest uses in between 6-23G. I guess this may relate to how user memory
is allocated and distributted across huge pages.

THP is a trade-off between time and space. We have a flag
no_huge_pages_for_shadow for sanitizer. It is true by default but DFSan
did not follow this. Depending on if a target is built statically or
dynamically, maybe Clang can set no_huge_pages_for_shadow accordingly
after this change. But it still seems fine to follow the default setting of
no_huge_pages_for_shadow. If time is an issue, and users are fine with
high RSS, this flag can be set to false selectively.

compiler-rt/lib/dfsan/dfsan.cpp

@@ -475,8 +475,10 @@ static void dfsan_init(int argc, char **argv, char **envp) {
 
   ::InitializePlatformEarly();
 
-  if (!MmapFixedNoReserve(ShadowAddr(), UnusedAddr() - ShadowAddr()))
+  if (!MmapFixedSuperNoReserve(ShadowAddr(), UnusedAddr() - ShadowAddr()))
     Die();
+  if (common_flags()->use_madv_dontdump)
+    DontDumpShadowMemory(ShadowAddr(), UnusedAddr() - ShadowAddr());
 
   // Protect the region of memory we don't use, to preserve the one-to-one
   // mapping from application to shadow memory. But if ASLR is disabled, Linux

compiler-rt/test/dfsan/release_shadow_space.c

@@ -1,4 +1,5 @@
-// RUN: %clang_dfsan %s -o %t && %run %t
+// DFSAN_OPTIONS=no_huge_pages_for_shadow=false RUN: %clang_dfsan %s -o %t && %run %t
+// DFSAN_OPTIONS=no_huge_pages_for_shadow=true RUN: %clang_dfsan %s -o %t && %run %t
 
 #include <assert.h>
 #include <sanitizer/dfsan_interface.h>