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directmap vs highmem usage in gnu mach?

From: Samuel Thibault
Subject: directmap vs highmem usage in gnu mach?
Date: Mon, 23 Aug 2021 13:56:52 +0200
User-agent: NeoMutt/20170609 (1.8.3)

Hello Richard,

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.


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