Re: [PATCH] cpufreq:stats: Handle the case when trans_table goes beyond PAGE_SIZE

[Viresh Kumar <viresh.kumar@linaro.org>]

On 07-11-17, 10:52, Gautham R. Shenoy wrote:
> From: "Gautham R. Shenoy" <ego@linux.vnet.ibm.com>
> 
> On platforms with large number of Pstates, the transition table, which
> is a NxN matrix, can overflow beyond the PAGE_SIZE boundary.
> 
> This can be seen on POWER9 which has 100+ Pstates.
> 
> As a result, each time the trans_table is read for any of the CPUs, we
> will get the following error.
> 
> ---------------------------------------------------
> fill_read_buffer: show+0x0/0xa0 returned bad count
> ---------------------------------------------------
> 
> This patch detect the overflow the first time the trans_table is read
> and print a warning in the dmesg and return return the FILE TOO LARGE

s/return return/return/

> error.  The subsequent reads also return FILE TOO LARGE error.
> 
> Signed-off-by: Gautham R. Shenoy <ego@linux.vnet.ibm.com>
> ---
>  Documentation/cpu-freq/cpufreq-stats.txt |  3 +++
>  drivers/cpufreq/cpufreq_stats.c          | 11 +++++++++--
>  2 files changed, 12 insertions(+), 2 deletions(-)
> 
> diff --git a/Documentation/cpu-freq/cpufreq-stats.txt b/Documentation/cpu-freq/cpufreq-stats.txt
> index 2bbe207..a873855 100644
> --- a/Documentation/cpu-freq/cpufreq-stats.txt
> +++ b/Documentation/cpu-freq/cpufreq-stats.txt
> @@ -90,6 +90,9 @@ Freq_i to Freq_j. Freq_i is in descending order with increasing rows and
>  Freq_j is in descending order with increasing columns. The output here also 
>  contains the actual freq values for each row and column for better readability.
>  
> +If the transition table is bigger than PAGE_SIZE, reading this will
> +return an -EFBIG error.
> +
>  --------------------------------------------------------------------------------
>  <mysystem>:/sys/devices/system/cpu/cpu0/cpufreq/stats # cat trans_table
>     From  :    To
> diff --git a/drivers/cpufreq/cpufreq_stats.c b/drivers/cpufreq/cpufreq_stats.c
> index e75880e..3cb717c 100644
> --- a/drivers/cpufreq/cpufreq_stats.c
> +++ b/drivers/cpufreq/cpufreq_stats.c
> @@ -79,6 +79,7 @@ static ssize_t store_reset(struct cpufreq_policy *policy, const char *buf,
>  	return count;
>  }
>  
> +bool trans_table_overflow;

This should be marked static and would be better to move it as local to below
function, but ...

>  static ssize_t show_trans_table(struct cpufreq_policy *policy, char *buf)
>  {
>  	struct cpufreq_stats *stats = policy->stats;
> @@ -88,6 +89,9 @@ static ssize_t show_trans_table(struct cpufreq_policy *policy, char *buf)
>  	if (policy->fast_switch_enabled)
>  		return 0;
>  
> +	if (unlikely(trans_table_overflow))
> +		return -EFBIG;
> +

Do we really need to optimize this path at all? This is debug stuff really. Over
that a person can theoretically rmmod the driver, insmod another driver and the
problem may go away then. And so above would be problematic then.

>  	len += snprintf(buf + len, PAGE_SIZE - len, "   From  :    To\n");
>  	len += snprintf(buf + len, PAGE_SIZE - len, "         : ");
>  	for (i = 0; i < stats->state_num; i++) {
> @@ -118,8 +122,11 @@ static ssize_t show_trans_table(struct cpufreq_policy *policy, char *buf)
>  			break;
>  		len += snprintf(buf + len, PAGE_SIZE - len, "\n");
>  	}
> -	if (len >= PAGE_SIZE)
> -		return PAGE_SIZE;
> +	if (len >= PAGE_SIZE) {
> +		trans_table_overflow = true;
> +		pr_warn("cpufreq transition table exceeds PAGE_SIZE. Disabling\n");
> +		return -EFBIG;
> +	}

I would suggest making only this change and removing other optimizations
completely.

-- 
viresh