Only use hugepage when orderfile is present on Chrome OS

This patches removes old hugepage mechanism that works without
orderfiles. So now, enabling orderfile == enabling THP.

Bug: 1100803
Test: Build chrome os locally
Change-Id: Ib4ff215b10ccc4c3c51347357a0cba7f51ec6793
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2369335
Commit-Queue: Tiancong Wang <tcwang@google.com>
Reviewed-by: George Burgess <gbiv@chromium.org>
Cr-Commit-Position: refs/heads/master@{#800740}

chromeos/hugepage_text/hugepage_text.cc

@@ -16,12 +16,7 @@
 #include "base/logging.h"
 
 // CHROMEOS_ORDERFILE_USE is a flag intended to use orderfile
-// to link Chrome. The else part of macro check in this code is to
-// make sure when the flag is turned off, the code works the same
-// as before.
-// We plan to turn it on in the future for Chrome OS.
-// Therefore, before it's deployed, by default, we turn it off until
-// the testing is done.
+// to link Chrome. Only when orderfile is used, we will use hugepages.
 
 // These function are here to delimit the start and end of the symbols
 // ordered by orderfile.
@@ -56,6 +51,12 @@ namespace chromeos {
 #define MADV_HUGEPAGE 14
 #endif
 
+#ifdef CHROMEOS_ORDERFILE_USE
+constexpr static bool kIsOrderfileEnabled = true;
+#else
+constexpr static bool kIsOrderfileEnabled = false;
+#endif
+
 const base::Feature kCrOSHugepageRemapAndLockZygote{
     "CrOSHugepageRemapAndLockInZygote", base::FEATURE_ENABLED_BY_DEFAULT};
 
@@ -219,39 +220,6 @@ static void RemapHugetlbTextWithOrderfileLayout(void* vaddr,
   MremapHugetlbText(reinterpret_cast<void*>(mapping_start), hsize);
 }
 
-// Top level text remapping function, without orderfile (old code).
-//
-// Inputs: vaddr, the starting virtual address to remap to hugepage
-//         segsize, size of the memory segment to remap in bytes
-// Return: none
-// Effect: physical backing page changed from small page to hugepage. If there
-//         are error condition, the remaping operation is aborted.
-static void RemapHugetlbTextWithoutOrderfileLayout(void* vaddr,
-                                                   const size_t segsize) {
-  // The number of hugepages to use if no orderfile is specified
-  const int kNumHugePages = 15;
-
-  // remove unaligned head regions
-  uintptr_t head_gap =
-      (kHpageSize - reinterpret_cast<uintptr_t>(vaddr) % kHpageSize) %
-      kHpageSize;
-  uintptr_t addr = reinterpret_cast<uintptr_t>(vaddr) + head_gap;
-
-  if (segsize < head_gap)
-    return;
-
-  size_t hsize = segsize - head_gap;
-  hsize = hsize & kHpageMask;
-
-  if (hsize > kHpageSize * kNumHugePages)
-    hsize = kHpageSize * kNumHugePages;
-
-  if (hsize == 0)
-    return;
-
-  MremapHugetlbText(reinterpret_cast<void*>(addr), hsize);
-}
-
 // For a given ELF program header descriptor, iterates over all segments within
 // it and find the first segment that has PT_LOAD and is executable, call
 // RemapHugetlbText().
@@ -277,16 +245,7 @@ static int FilterElfHeader(struct dl_phdr_info* info, size_t size, void* data) {
         info->dlpi_phdr[i].p_flags == (PF_R | PF_X)) {
       vaddr = bit_cast<void*>(info->dlpi_addr + info->dlpi_phdr[i].p_vaddr);
       segsize = info->dlpi_phdr[i].p_filesz;
-      // The following function is conditionally compiled, so use
-      // the statements to avoid compiler warnings of unused functions
-      (void)RemapHugetlbTextWithOrderfileLayout;
-      (void)RemapHugetlbTextWithoutOrderfileLayout;
-#ifdef CHROMEOS_ORDERFILE_USE
       RemapHugetlbTextWithOrderfileLayout(vaddr, segsize);
-#else
-      RemapHugetlbTextWithoutOrderfileLayout(vaddr, segsize);
-#endif
-      // Only re-map the first text segment.
       return 1;
     }
   }
@@ -299,7 +258,10 @@ static int FilterElfHeader(struct dl_phdr_info* info, size_t size, void* data) {
 // the hugepages. Any errors will cause the failing piece of this to be rolled
 // back, so nothing world-ending can come from this function (hopefully ;) ).
 void InitHugepagesAndMlockSelf(void) {
-  if (base::FeatureList::IsEnabled(chromeos::kCrOSHugepageRemapAndLockZygote)) {
+  // The following function is conditionally compiled, so use
+  // the statements to avoid compiler warnings of unused functions
+  if (kIsOrderfileEnabled &&
+      base::FeatureList::IsEnabled(chromeos::kCrOSHugepageRemapAndLockZygote)) {
     dl_iterate_phdr(FilterElfHeader, 0);
   }
 }