pipe/fifo code merged.

[Kostik Belousov]

On Tue, Jan 10, 2012 at 01:34:23AM +1100, Bruce Evans wrote:
> For the socket layer, there is the ZERO_COPY_SOCKETS options.  This
> gives optimizations related to the ones for pipes.  I have no experience
> with it.  It seems to be only for hardware sockets.  It is apparently
> not very popular or well maintained, since it isn't an any GENERIC.
It is known to be (very) broken with regard to the vnode-backed mappings.
AFAIR, after the COW kicks in, buffer code operates on the wrong page.
In the best case, it results in the kernel panic, in the worst, the user
data is corrupted.
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