linux-kernel - Re: [PATCH v2] time.c::timespec_trunc: fix nanosecond file time rounding

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Message-ID: <CALAqxLWf4CFzHwMvkAWq0yj4hhhry9D1LKiKXodU_ATs4kXOHA@mail.gmail.com>
Date:	Wed, 1 Jul 2015 11:07:47 -0700
From:	John Stultz <john.stultz@...aro.org>
To:	Karsten Blees <karsten.blees@...il.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] time.c::timespec_trunc: fix nanosecond file time rounding

On Thu, Jun 25, 2015 at 5:13 AM, Karsten Blees <karsten.blees@...il.com> wrote:
> 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.
>
> Signed-off-by: Karsten Blees <blees@...n.de>


Ok. Looks good. I'm queuing it for testing, targeting the 4.3 merge window.

thanks
-john
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