From: Eric Dumazet <redacted>
Date: Thu, 30 Oct 2008 08:04:42 +0100

If we design something that could be reused, say for TCP sockets, we need
to be able to handle very large number of 'NULL' pointers, say, up to 64*1024*1024

So lets use the low order bit, set to one for "NULL" pointers, and 0 for regular pointers.

This gives us 31 bits (or 63 bits) to store any valuable info :)

and all ...._nulls() macros would not need to know the max value (UDP_HTABLE_SIZE in UDP case),
since all they have to do is a test ((unsigned long)pos & 1)

At iterator exit, pos would contain the 'index' value, (pos >> 1), to hide this
implementation detail.

This sound fine to me.  Quite an improvement in fact :)