Remove FBREMAP experiment

Summary:
The flag was removed in https://www.internalfb.com/diff/D28467996 (2c5f68d) but the code was not removed yet.

Changelog: [Internal]

Reviewed By: motiz88

Differential Revision: D33792884

fbshipit-source-id: 0ae5d904ca2ca94437ca63b495d41e47585fde33

ReactCommon/cxxreact/JSBigString.cpp

@@ -50,65 +50,6 @@ JSBigFileString::~JSBigFileString() {
   close(m_fd);
 }
 
-#ifdef WITH_FBREMAP
-// Read and advance the pointer.
-static uint16_t read16(char *&data) {
-  uint16_t result;
-  ::memcpy(&result, data, sizeof(result));
-  data += sizeof(result);
-  return result;
-}
-
-// If the given file has a remapping table header, remap its pages accordingly
-// and return the byte offset from the beginning to the unwrapped payload.
-static off_t maybeRemap(char *data, size_t size, int fd) {
-  // A remapped file's size must be a multiple of its page size, so quickly
-  // filter out files with incorrect size, without touching any pages.
-  static const size_t kMinPageSize = 4096;
-  if (size < kMinPageSize || size % kMinPageSize != 0) {
-    return 0;
-  }
-  const auto begin = data;
-  static const uint8_t kRemapMagic[] = {
-      0xc6, 0x1f, 0xbc, 0x03, 0xc1, 0x03, 0x19, 0x1f, 0xa1, 0xd0, 0xeb, 0x73};
-  if (::memcmp(data, kRemapMagic, sizeof(kRemapMagic)) != 0) {
-    return 0;
-  }
-  data += sizeof(kRemapMagic);
-  const size_t filePS = static_cast<size_t>(1) << read16(data);
-  if (size & (filePS - 1)) {
-    return 0;
-  }
-  {
-    // System page size must be at least as granular as the remapping.
-    // TODO: Consider fallback that reads entire file into memory.
-    const size_t systemPS = sysconf(_SC_PAGESIZE);
-    CHECK(filePS >= systemPS)
-        << "filePS: " << filePS << "systemPS: " << systemPS;
-  }
-  const off_t headerPages = read16(data);
-  uint16_t numMappings = read16(data);
-  size_t curFilePage = headerPages;
-  while (numMappings--) {
-    auto memPage = read16(data) + headerPages;
-    auto numPages = read16(data);
-    if (mmap(
-            begin + memPage * filePS,
-            numPages * filePS,
-            PROT_READ,
-            MAP_FILE | MAP_PRIVATE | MAP_FIXED,
-            fd,
-            curFilePage * filePS) == MAP_FAILED) {
-      CHECK(false) << " memPage: " << memPage << " numPages: " << numPages
-                   << " curFilePage: " << curFilePage << " size: " << size
-                   << " error: " << std::strerror(errno);
-    }
-    curFilePage += numPages;
-  }
-  return headerPages * filePS;
-}
-#endif // WITH_FBREMAP
-
 const char *JSBigFileString::c_str() const {
   if (m_size == 0) {
     return "";
@@ -119,12 +60,6 @@ const char *JSBigFileString::c_str() const {
     CHECK(m_data != MAP_FAILED)
         << " fd: " << m_fd << " size: " << m_size << " offset: " << m_mapOff
         << " error: " << std::strerror(errno);
-#ifdef WITH_FBREMAP
-    // Remapping is only attempted when the entire file was requested.
-    if (m_mapOff == 0 && m_pageOff == 0) {
-      m_pageOff = maybeRemap(const_cast<char *>(m_data), m_size, m_fd);
-    }
-#endif // WITH_FBREMAP
   }
   static const size_t kMinPageSize = 4096;
   CHECK(!(reinterpret_cast<uintptr_t>(m_data) & (kMinPageSize - 1)))