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

["Gautham R. Shenoy" <ego@linux.vnet.ibm.com>]

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
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;
 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;
+
 	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;
+	}
 	return len;
 }
 cpufreq_freq_attr_ro(trans_table);
-- 
1.8.3.1