splice() and the ghost of set

splice() and the ghost of set_fs()

Posted May 28, 2022 2:37 UTC (Sat) by wahern (subscriber, #37304)
In reply to: splice() and the ghost of set_fs() by Sesse
Parent article: splice() and the ghost of set_fs()

This was exactly Linus' original reasoning wrt vmsplice:

On Thu, 20 Apr 2006, Piet Delaney wrote:
> 
> What about marking the pages Read-Only while it's being used by the
> kernel

NO!

That's a huge mistake, and anybody that does it that way (FreeBSD) is totally incompetent.

[...]

That cost is _bigger_ than the cost of just copying the page in the first place.

Source: https://lkml.org/lkml/2006/4/20/310

See also Linus' justifications of splice and tee earlier in that thread. vmsplice is also briefly mentioned, and it's implicit from the context that much of the net value of vmsplice comes from combining with tee, as you mentioned earlier.