[PATCH] support for large number of network devices.

[Stephen Hemminger <shemminger@xxxxxxxx>]

When using pseudo network devices, and really big machines; there is
sometimes a need to have a lot of network devices.  This replaces the
existing 2.6.1 limit of 100 entries an was O(n^2)
with a algorithm that will handle up to 32768 entries with O(n) behaviour.

Does need a temporary page, but that shouldn't be a big deal.
It has the same semantics, it will find the first empty name and use it.

diff -Nru a/net/core/dev.c b/net/core/dev.c
--- a/net/core/dev.c    Tue Jan 13 15:42:19 2004
+++ b/net/core/dev.c    Tue Jan 13 15:42:19 2004
@@ -650,8 +650,11 @@
 int dev_alloc_name(struct net_device *dev, const char *name)
 {
        int i;
-       char buf[32];
        char *p;
+       const int max_netdevices = 8*PAGE_SIZE;
+       long *inuse;
+       struct net_device *d;
+       char buf[IFNAMSIZ];
 
        /*
         * Verify the string as this thing may have come from
@@ -662,17 +665,34 @@
        if (p && (p[1] != 'd' || strchr(p + 2, '%')))
                return -EINVAL;
 
-       /*
-        * If you need over 100 please also fix the algorithm...
-        */
-       for (i = 0; i < 100; i++) {
-               snprintf(buf, sizeof(buf), name, i);
-               if (!__dev_get_by_name(buf)) {
-                       strcpy(dev->name, buf);
-                       return i;
+       /* Use one page as a bit array of possible slots */
+       inuse = (long *) get_zeroed_page(GFP_ATOMIC);
+       if (!inuse) 
+               return -ENOMEM;
+
+       for (d = dev_base; d; d = d->next) {
+               if (sscanf(d->name, name, &i)) {
+                       if (i < 0 || i >= max_netdevices)
+                               continue;
+
+                       set_bit(i, inuse);
                }
        }
-       return -ENFILE; /* Over 100 of the things .. bail out! */
+
+       i = find_first_zero_bit(inuse, max_netdevices);
+       free_page((unsigned long) inuse);
+       snprintf(buf, sizeof(buf), name, i);
+
+       if (!__dev_get_by_name(buf)) {
+               strcpy(dev->name, buf);
+               return i;
+       }
+
+       /* It is possible to run out of possible slots
+        * when the name is long and there isn't enough space left
+        * for the digits, or if all bits are used.
+        */
+       return -ENFILE;
 }