linux-kernel - Re: [PATCH v2 6/7] x86/mm: remove p4d

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Message-ID: <Z+6QrsAhA4nBz0sj@MiWiFi-R3L-srv>
Date: Thu, 3 Apr 2025 21:44:14 +0800
From: Baoquan He <bhe@...hat.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, yanjun.zhu@...ux.dev, x86@...nel.org
Subject: Re: [PATCH v2 6/7] x86/mm: remove p4d_leaf definition

On 04/01/25 at 09:20am, Ingo Molnar wrote:
> 
> * Baoquan He <bhe@...hat.com> wrote:
> 
> > > So the above comment should have said '256 TB' pages, unless 
> > > there's some naming weirdness I missed.
> > 
> > Hmm, there could be misunderstanding here. In 5-level paging, PGD is 
> > the highest level, P4D is the next level of PGD. You may have 
> > reversed their order.
> 
> Erm, yes indeed I flipped those two, so the correct table should be:
> 
>   - level 0:                 4K pages
>   - level 1: 512x    4K =   2MB 'large' pages
>   - level 2: 512x   2MB =   1GB 'huge' pages
>   - level 3: 512x   1GB = 512GB 'P4D' pages
>   - level 4: 512x 512GB = 256TB 'PGD' pages
> 
> I'm wondering whether 512GB pages will be called 'terapages'. ;-)

Forgot replying to this one.

With my understanding, MM usually don't name large page of different
size a specific name, just call them hugepage + size, e.g 2M huge page,
or 1G huge page. Sometime when we build mapping for them, we will call
them PMD hugepage or PUD hugepage.

I remember in x86 ARCH direct mapping is built with invocation of
init_mem_mapping(), and the mapping to 2M or 1G directly is called
large page mapping or PMD/PUD huge page mapping. They are similar.

Hope other MM people can correct me if I am wrong.