Re: directmap vs highmem usage in gnu mach?

[Richard Braun]

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.

Hello,

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

[1] 
https://git.savannah.gnu.org/cgit/hurd/gnumach.git/tree/vm/vm_resident.c?id=4dbaa718038fadc51d9b95d2383868a229d91457#n776