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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 15:59:12 +0200
User-agent: Mutt/1.10.1 (2018-07-13)

On Mon, Aug 23, 2021 at 01:56:52PM +0200, Samuel Thibault wrote:
> Do you know if it is on purpose that all vm_page_grab() calls use the
> directmem segment, and not the highmem segment?  Notably vm_page_fault()
> ends up filling the directmem segment first, I even wonder how the
> highmem segment ends up getting filled :)
> Is that perhaps to rather try allocating in directmem that has faster
> access from kernel than highmem? (although for e.g. all kmsg purpose the
> kernel just uses the userland page table so it's not faster, right?)
> I was thinking about adding a vm_page_grab_high() that can be used when
> there are no directmap constraints, that will probably solve the
> (relatively rare) OOM issues on the buildds.


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.

Richard Braun


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