linux-ext4 - Re: [jlayton:ctime] [ext4] ff9aaf58e8: ltp.statx06.fail

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Date:   Tue, 2 May 2023 10:39:29 +1000
From:   Dave Chinner <david@...morbit.com>
To:     Jeff Layton <jlayton@...nel.org>
Cc:     kernel test robot <oliver.sang@...el.com>, oe-lkp@...ts.linux.dev,
        lkp@...el.com, linux-ext4@...r.kernel.org, ltp@...ts.linux.it,
        Christian Brauner <brauner@...nel.org>,
        Amir Goldstein <amir73il@...il.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: [jlayton:ctime] [ext4]  ff9aaf58e8: ltp.statx06.fail

On Mon, May 01, 2023 at 12:05:17PM -0400, Jeff Layton wrote:
> On Mon, 2023-05-01 at 22:09 +0800, kernel test robot wrote:
> The test does this:
> 
>         SAFE_CLOCK_GETTIME(CLOCK_REALTIME_COARSE, &before_time);
>         clock_wait_tick();
>         tc->operation();
>         clock_wait_tick();
>         SAFE_CLOCK_GETTIME(CLOCK_REALTIME_COARSE, &after_time);
> 
> ...and with that, I usually end up with before/after_times that are 1ns
> apart, since my machine is reporting a 1ns granularity.
> 
> The first problem is that the coarse grained timestamps represent the
> lower bound of what time could end up in the inode. With multigrain
> ctimes, we can end up grabbing a fine-grained timestamp to store in the
> inode that will be later than either coarse grained time that was
> fetched.
> 
> That's easy enough to fix -- grab a coarse time for "before" and a fine-
> grained time for "after".
> 
> The clock_getres function though returns that it has a 1ns granularity
> (since it does). With multigrain ctimes, we no longer have that at the
> filesystem level. It's a 2ns granularity now (as we need the lowest bit
> for the flag).

Why are you even using the low bit for this? Nanosecond resolution
only uses 30 bits, leaving the upper two bits of a 32 bit tv_nsec
field available for internal status bits. As long as we mask out the
internal bits when reading the VFS timestamp tv_nsec field, then
we don't need to change the timestamp resolution, right?

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com