Support experimental re-mmap wrappers

Summary: Add experimental support for reordering the pages of a file that is mmap:ed by JSBigFileString. The wrapper is auto-detected (by checking file size and magic header) and transparently reorders the pages.

Reviewed By: ridiculousfish

Differential Revision: D14721397

fbshipit-source-id: 34e095350a9eeb9b07105bed6f3379f2fe472ae6

ReactCommon/cxxreact/JSBigString.cpp

@@ -48,6 +48,65 @@ 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 = getpagesize();
+    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_data) {
@@ -58,11 +117,19 @@ const char *JSBigFileString::c_str() const {
       << " 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
   }
   return m_data + m_pageOff;
 }
 
 size_t JSBigFileString::size() const {
+  // Ensure mapping has been initialized.
+  c_str();
   return m_size - m_pageOff;
 }