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