time: Fix nanosecond file time rounding in timespec_trunc()

timespec_trunc() avoids rounding if granularity <= nanoseconds-per-jiffie
(or TICK_NSEC). This optimization assumes that:

 1. current_kernel_time().tv_nsec is already rounded to TICK_NSEC (i.e.
    with HZ=1000 you'd get 1000000, 2000000, 3000000... but never 1000001).
    This is no longer true (probably since hrtimers introduced in 2.6.16).

 2. TICK_NSEC is evenly divisible by all possible granularities. This may
    be true for HZ=100, 250, 1000, but obviously not for HZ=300 /
    TICK_NSEC=3333333 (introduced in 2.6.20).

Thus, sub-second portions of in-core file times are not rounded to on-disk
granularity. I.e. file times may change when the inode is re-read from disk
or when the file system is remounted.

This affects all file systems with file time granularities > 1 ns and < 1s,
e.g. CEPH (1000 ns), UDF (1000 ns), CIFS (100 ns), NTFS (100 ns) and FUSE
(configurable from user mode via struct fuse_init_out.time_gran).

Steps to reproduce with e.g. UDF:

  $ dd if=/dev/zero of=udfdisk count=10000 && mkudffs udfdisk
  $ mkdir udf && mount udfdisk udf
  $ touch udf/test && stat -c %y udf/test
  2015-06-09 10:22:56.130006767 +0200
  $ umount udf && mount udfdisk udf
  $ stat -c %y udf/test
  2015-06-09 10:22:56.130006000 +0200

Remounting truncates the mtime to 1 µs.

Fix the rounding in timespec_trunc() and update the documentation.

timespec_trunc() is exclusively used to calculate inode's [acm]time (mostly
via current_fs_time()), and always with super_block.s_time_gran as second
argument. So this can safely be changed without side effects.

Note: This does _not_ fix the issue for FAT's 2 second mtime resolution,
as super_block.s_time_gran isn't prepared to handle different ctime /
mtime / atime resolutions nor resolutions > 1 second.

Cc: Prarit Bhargava <prarit@redhat.com>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: John Stultz <john.stultz@linaro.org>

kernel/time/time.c

@@ -287,26 +287,20 @@ EXPORT_SYMBOL(jiffies_to_usecs);
  * @t: Timespec
  * @gran: Granularity in ns.
  *
- * Truncate a timespec to a granularity. gran must be smaller than a second.
- * Always rounds down.
- *
- * This function should be only used for timestamps returned by
- * current_kernel_time() or CURRENT_TIME, not with do_gettimeofday() because
- * it doesn't handle the better resolution of the latter.
+ * Truncate a timespec to a granularity. Always rounds down. gran must
+ * not be 0 nor greater than a second (NSEC_PER_SEC, or 10^9 ns).
  */
 struct timespec timespec_trunc(struct timespec t, unsigned gran)
 {
-	/*
-	 * Division is pretty slow so avoid it for common cases.
-	 * Currently current_kernel_time() never returns better than
-	 * jiffies resolution. Exploit that.
-	 */
-	if (gran <= jiffies_to_usecs(1) * 1000) {
+	/* Avoid division in the common cases 1 ns and 1 s. */
+	if (gran == 1) {
 		/* nothing */
-	} else if (gran == 1000000000) {
+	} else if (gran == NSEC_PER_SEC) {
 		t.tv_nsec = 0;
-	} else {
+	} else if (gran > 1 && gran < NSEC_PER_SEC) {
 		t.tv_nsec -= t.tv_nsec % gran;
+	} else {
+		WARN(1, "illegal file time granularity: %u", gran);
 	}
 	return t;
 }