From: Theo Buehler <tb@openbsd.org>
Subject: Re: Introduce uvmfault_promote()
To: tech@openbsd.org
Date: Sat, 30 Nov 2024 14:26:02 +0100

On Sat, Nov 30, 2024 at 01:17:49PM +0100, Martin Pieuchot wrote:
> Diff below introduces a new function to merge common code used to
> promote data to a new anon.
> 
> It is used for 3 different cases:
> 
> - CoW of an anon, `uobjpage' is set to oanon->an_page
> - Zero fill of a new page, `uobjpage' is set to PGO_DONTCARE
> - CoW of a uobj page, `uobjpage` is set to the center fetched by pgo_get()
> 
> This diff doesn't introduce any change in behavior.   More merging will
> come afterwards.

One tiny change of behavior is that uvm_wait() is no longer called with
"flt_noram5" in the second caller.

More importantly, on error in the lower fault case, the order of the
"unlock everything" step in uvmfault_promote() and the "arg!" step of
unbusying the uobjpage is reversed.

It is not obvious to me why this is entirely benign, but assuming that
it is, this change seems fine.