[BUG] overflow in net/ipv4/route.c rt_check_expire()

[Eric Dumazet <dada1@cosmosbay.com>]

After having hard times tuning /proc/sys/net/ipv4/route/* values, with various crashes of production machines,
I did some investigations...

The rt_check_expire() has a serious problem on machines with large route caches, and a standard HZ value of 1000.

With default values, ie ip_rt_gc_interval = 60*HZ = 60000 ;

the loop count :

     for (t = ip_rt_gc_interval << rt_hash_log; t >= 0;


overflows (t is a 31 bit value) as soon rt_hash_log is >= 16  (65536 slots in route cache hash table)

Another problem is the fact that this function has close to 0 effect, because even if ip_rt_gc_interval is changed to 1 HZ,
only 1/300 of the table is scanned every second. And the loop breaks as soon a jiffie is consumed.

We should adapt the loop count based on the actual number of entries in the route cache,
and eventually give more 'jiffies' in some pressure cases.

I am experimenting some changes that I will share when ready.


Thank you
Eric Dumazet