At issue is really application capture and customer adoption.  There are
tons of apps and lots of engineers that know socket operations and 
write/writev.  Asking all ISVs to recode for linux would leave them with 
two separate APIs to deal with. They would have send/sendto or 
write/writev on Solaris, HPUX and whatever else, and linux would have 
sendfile.

aio_write / lio_listio exists on Solaris and HP/UX too.

(and even Windows; their completion port interfaces are very similar) 

We really want to do this in such a way that it doesn't create a huge 
footprint (and we think we can) and we want to make sure we don't impact 
systems that can't take advantage of fast TLB updates.

So how do you avoid the two TLB flush IPIs to all CPUs that have the current mm
mapped ?

-Andi