[PATCH] D39464: Define fs::allocate_file which preallocates disk blocks.

[Rui Ueyama via Phabricator via llvm-commits]

ruiu created this revision.
Herald added subscribers: hiraditya, mgorny.

Unlike fs::resize_file, the new function guarantees that disk blocks
for a given size are actually allocated on a file system. In other
words, it doesn't create a sparse file.

If you want to do mmap IO, you generally want to preallocate disk
blocks if you can, because if you mmap a sparse file for writing and
the disk becomes full, your process will be killed by SIGBUS. There's
no way to gracefully handle that error condition. If you use fallocate(2)
or equivalent, you can handle that error before calling mmap.

My plan is to use this function in FileOutputBuffer so that we do mmap
IO only when we can preallocate disk blocks. That prevents LLVM tools
from crashing with a mysterious bus error when a disk is almost full.


https://reviews.llvm.org/D39464

Files:
  llvm/cmake/config-ix.cmake
  llvm/include/llvm/Config/config.h.cmake
  llvm/include/llvm/Support/FileSystem.h
  llvm/lib/Support/Unix/Path.inc
  llvm/lib/Support/Windows/Path.inc


Index: llvm/lib/Support/Windows/Path.inc
===================================================================
--- llvm/lib/Support/Windows/Path.inc
+++ llvm/lib/Support/Windows/Path.inc
@@ -509,6 +509,8 @@
   return std::error_code(error, std::generic_category());
 }
 
+std::error_code allocate_file(int FD, uint64_t Size) { resize_file(FD, Size); }
+
 std::error_code access(const Twine &Path, AccessMode Mode) {
   SmallVector<wchar_t, 128> PathUtf16;
 
Index: llvm/lib/Support/Unix/Path.inc
===================================================================
--- llvm/lib/Support/Unix/Path.inc
+++ llvm/lib/Support/Unix/Path.inc
@@ -438,6 +438,23 @@
   return std::error_code();
 }
 
+std::error_code resize_file(int FD, uint64_t Size) {
+#ifdef HAVE_FALLOCATE
+  if (fallocate(FD, 0, 0, Size) == -1)
+    return std::error_code(errno, std::generic_category());
+  return std::error_code();
+#endif HAVE_FALLOCATE
+
+#ifdef __APPLE__
+  fstore_t Store = {F_ALLOCATEALL, F_PEOFPOSMODE, 0, Size};
+  if (fcntl(fd, F_PREALLOCATE, &Store) == -1)
+    return std::error_code(errno, std::generic_category());
+  return std::error_code();
+#endif
+
+  return make_error_code(errc::function_not_supported);
+}
+
 static int convertAccessMode(AccessMode Mode) {
   switch (Mode) {
   case AccessMode::Exist:
Index: llvm/include/llvm/Support/FileSystem.h
===================================================================
--- llvm/include/llvm/Support/FileSystem.h
+++ llvm/include/llvm/Support/FileSystem.h
@@ -391,6 +391,14 @@
 ///          platform-specific error_code.
 std::error_code resize_file(int FD, uint64_t Size);
 
+/// @brief Preallocate disk blocks to a given file.
+///
+/// @param FD Input file descriptor.
+/// @param Size Size to resize to.
+/// @returns errc::success if \a path has been resized to \a size, otherwise a
+///          platform-specific error_code.
+std::error_code allocate_file(int FD, uint64_t Size);
+
 /// @brief Compute an MD5 hash of a file's contents.
 ///
 /// @param FD Input file descriptor.
Index: llvm/include/llvm/Config/config.h.cmake
===================================================================
--- llvm/include/llvm/Config/config.h.cmake
+++ llvm/include/llvm/Config/config.h.cmake
@@ -59,6 +59,9 @@
 /* Define to 1 if you have the <errno.h> header file. */
 #cmakedefine HAVE_ERRNO_H ${HAVE_ERRNO_H}
 
+/* Define to 1 if you have the `fallocate' function. */
+#cmakedefine HAVE_FALLOCATE ${HAVE_FALLOCATE}
+
 /* Define to 1 if you have the <fcntl.h> header file. */
 #cmakedefine HAVE_FCNTL_H ${HAVE_FCNTL_H}
 
Index: llvm/cmake/config-ix.cmake
===================================================================
--- llvm/cmake/config-ix.cmake
+++ llvm/cmake/config-ix.cmake
@@ -197,6 +197,11 @@
 check_symbol_exists(futimens sys/stat.h HAVE_FUTIMENS)
 check_symbol_exists(futimes sys/time.h HAVE_FUTIMES)
 check_symbol_exists(posix_fallocate fcntl.h HAVE_POSIX_FALLOCATE)
+
+set(CMAKE_REQUIRED_DEFINITIONS "-D_GNU_SOURCE")
+check_symbol_exists(fallocate fcntl.h HAVE_FALLOCATE)
+set(CMAKE_REQUIRED_DEFINITIONS "")
+
 # AddressSanitizer conflicts with lib/Support/Unix/Signals.inc
 # Avoid sigaltstack on Apple platforms, where backtrace() cannot handle it
 # (rdar://7089625) and _Unwind_Backtrace is unusable because it cannot unwind


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