Revert of Make text section 2MB aligned for hugepages. (patchset #7 id:120001 of https://codereview.chromium.org/1619713007/ )

Reason for revert:
Causing failures on x86-alex, peach_pit, and daisy_skate (render process crash in login screen).

Original issue's description:
> Make text section 2MB aligned for hugepages.
>
> If we do not use the option '-Wl,--section-start=.text=2001000' at
> link time, the performance gain on hugepages is gone. This is probably
> because the original text section is not 2MB aligned and the after
> mremap, the hugepage is not preserved.
>
> BUG=chromium:569963
> TEST=The performance gain is back on micro benchmarks.
>
> Committed: https://crrev.com/1384534a94e6324d74906f0cbc1e707606c063f2
> Cr-Commit-Position: refs/heads/master@{#373673}

TBR=rickyz@chromium.org,cmtice@chromium.org,kenchen@google.com,llozano@chromium.org,shenhan@chromium.org,yunlian@chromium.org,yunlian@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.
BUG=chromium:569963

Review URL: https://codereview.chromium.org/1687033002

Cr-Commit-Position: refs/heads/master@{#374687}

1 files changed, 12 insertions, 9 deletions

diff --git a/chromeos/hugepage_text/hugepage_text.cc b/chromeos/hugepage_text/hugepage_text.cc
index 4856660..ccae53f 100644
--- a/chromeos/hugepage_text/hugepage_text.cc
+++ b/chromeos/hugepage_text/hugepage_text.cc
@@ -36,6 +36,11 @@ const int kMmapFlags = (MAP_ANONYMOUS | MAP_SHARED);
 const int kMmapHtlbFlags = (kMmapFlags | MAP_HUGETLB);
 const int kMremapFlags = (MREMAP_MAYMOVE | MREMAP_FIXED);
 
+// The number of hugepages we want to use to map chrome text section
+// to hugepages. With the help of AutoFDO, the hot functions are grouped
+// in to a small area of the binary.
+const int kNumHugePages = 15;
+
 // mremap syscall is always supported for small page segment on all kernels.
 // However, it is not the case for hugepage.
 // If not used carefully, mremap() a hugepage segment directly onto small page
@@ -124,9 +129,10 @@ static void NoAsanAlignedMemcpy(void* dst, void* src, size_t size) {
 // Effect: physical backing page changed from small page to hugepage. If there
 //         are error condition, the remapping operation is aborted.
 static void MremapHugetlbText(void* vaddr, const size_t hsize) {
-  DCHECK_EQ(0ul, reinterpret_cast<uintptr_t>(vaddr) & ~kHpageMask);
   void* haddr = MAP_FAILED;
-  if (HugetlbMremapSupported()) {
+
+  if ((reinterpret_cast<intptr_t>(vaddr) & ~kHpageMask) == 0 &&
+      HugetlbMremapSupported()) {
     // Try anon hugepage from static hugepage pool only if the source address
     // is hugepage aligned, otherwise, mremap below has non-recoverable error.
     haddr = mmap(NULL, hsize, kProtection, kMmapHtlbFlags, 0, 0);
@@ -166,18 +172,15 @@ static void MremapHugetlbText(void* vaddr, const size_t hsize) {
 // Effect: physical backing page changed from small page to hugepage. If there
 //         are error condition, the remaping operation is aborted.
 static void RemapHugetlbText(void* vaddr, const size_t segsize) {
-  // remove unaligned head regions
-  size_t head_gap = (kHpageSize - reinterpret_cast<size_t>(vaddr) % kHpageSize)
-                    % kHpageSize;
-  size_t addr = reinterpret_cast<size_t>(vaddr) + head_gap;
-
-  int hsize = segsize - head_gap;
+  int hsize = segsize;
+  if (segsize > kHpageSize * kNumHugePages)
+    hsize = kHpageSize * kNumHugePages;
   hsize = hsize & kHpageMask;
 
   if (hsize == 0)
     return;
 
-  MremapHugetlbText(reinterpret_cast<void *>(addr), hsize);
+  MremapHugetlbText(vaddr, hsize);
 }
 
 // For a given ELF program header descriptor, iterates over all segments within