linux-kernel - Re: [PATCH v3 2/3] fs/proc: apply the time namespace offset to /proc/stat btime

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Message-ID: <20201015075319.GA271498@gmail.com>
Date:   Thu, 15 Oct 2020 00:53:19 -0700
From:   Andrei Vagin <avagin@...il.com>
To:     Michael Weiß <michael.weiss@...ec.fraunhofer.de>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Dmitry Safonov <0x7f454c46@...il.com>,
        linux-kernel@...r.kernel.org,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Chuck Lever <chuck.lever@...cle.com>,
        Trond Myklebust <trond.myklebust@...merspace.com>,
        Anna Schumaker <anna.schumaker@...app.com>
Subject: Re: [PATCH v3 2/3] fs/proc: apply the time namespace offset to
 /proc/stat btime

On Sun, Oct 11, 2020 at 04:59:23PM +0200, Michael Weiß wrote:
> @@ -79,6 +80,20 @@ static u64 get_iowait_time(struct kernel_cpustat *kcs, int cpu)
>  
>  #endif
>  
> +static void get_boottime(struct timespec64 *ts)
> +{

> +	ktime_t boottime;
> +
> +	/* get kernel internal system boot timestamp */
> +	getboottime64(ts);
> +
> +	/* shift boot timestamp according to the timens offset */
> +	boottime = timespec64_to_ktime(*ts);
> +	boottime = timens_ktime_to_host(CLOCK_BOOTTIME, boottime);

timens_ktime_to_host is used to convert timens' time to host's time.
Here it looks like we are using it in the opposite direction. I spent
some time to figure out what is going on here. I think it worth to add a
comment here.

> +
> +	*ts = ktime_to_timespec64(boottime);

I don't like all these conversions back and forth. Maybe something like
this will look better:

#ifdef CONFIG_TIME_NS
	if (current->nsproxy->time_ns != init_time_ns) {
		struct timens_offsets *ns_offsets;
		ns_offsets = &current->nsproxy->time_ns->offsets;
		ts = timespec64_sub(ts, timens_offsets->boottime);
	}
#endif


Thanks,
Andrei