+ *
+ * These happen to all be powers of 2, which is not strictly
+ * necessary but helps enforce the real limitation, which is
+ * that they should be multiples of PAGE_CACHE_SIZE.
+ *
+ * For UDP transports, a block plus NFS,RPC, and UDP headers
+ * has to fit into the IP datagram limit of 64K. The largest
+ * feasible number for all known page sizes is probably 48K,
+ * but we choose 32K here. This is the same as the historical
+ * Linux limit; someone who cares more about NFS/UDP performance
+ * can test a larger number.
+ *
+ * For TCP transports we have more freedom. A size of 1MB is
+ * chosen to match the client limit. Other OSes are known to
+ * have larger limits, but those numbers are probably beyond
+ * the point of diminishing returns.