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Re: directmap vs highmem usage in gnu mach?

From: Richard Braun
Subject: Re: directmap vs highmem usage in gnu mach?
Date: Wed, 25 Aug 2021 17:12:44 +0200
User-agent: Mutt/1.10.1 (2018-07-13)

On Wed, Aug 25, 2021 at 04:23:23PM +0200, Samuel Thibault wrote:
> Richard Braun, le mer. 25 août 2021 15:59:12 +0200, a ecrit:
> > I can't recall exactly, but that's why we write comments, right ?
> > See [1].
> > 
> > Fixing this requires changing all callers to not assume that allocated
> > memory can always be directly mapped, in particular some kvtophys calls.
> Ok, I was not sure which way to interpret the comment: it says "Instead
> of updating all users" and talks about wiring, so I wasn't sure it was
> really talking about directmap vs highmem (which AIUI doesn't matter for
> wiring), or something else, and whether it was already done/optimized.
> I have a patch that makes the vm_fault_page, vm_map_copy_steal_pages,
> vm_page_convert, and vm_page_alloc use highmem. It boots fine, can build
> packages without problems, I guess that's a good-enough test?

For practical purposes, sure, but in the long run, there must be
some kind of very clear (preferrably explicit) policy about all that.

Personally, I'd do the same as Linux, with all callers explicitely
passing flags, instead of having various versions of vm_page_grab.
These flags may become useful on multiple layers of the "memory stack".

Richard Braun

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