Re: idea: user to user pipe copy

[Jamie Lokier <jamie@shareable.org>]

Mark Borgerding wrote:
> Why not keep track of blocked read()s on a pipe fd?
> 
> When the writer writes something to the pipe, data could be copied 
> directly from one user process to another, rather than
> calling copy_from_user then copy_to_user. 
> 
> This alleged speed increase would benefit all blocking pipes & fifos, 
> roughly half the time (i.e. whenever the read happens before the write).

You only get page faults from one of the mm contexts, so
copy_from_user_to_other_user would have to do explicit page table
operations to find the pages of the other mm.  There may also be
a little data cache flushing required with SMP.

It seems feasible.

-- Jamie