linux-kernel - Re: [PATCH] docs/mm: add VMA locks documentation

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Message-ID: <b81e4cc2-fc86-4ed2-996a-ad78a3524f31@lucifer.local>
Date: Fri, 8 Nov 2024 12:29:15 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: Bagas Sanjaya <bagasdotme@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
        Jonathan Corbet <corbet@....net>,
        "Liam R . Howlett" <Liam.Howlett@...cle.com>,
        Vlastimil Babka <vbabka@...e.cz>, Jann Horn <jannh@...gle.com>,
        Alice Ryhl <aliceryhl@...gle.com>, Boqun Feng <boqun.feng@...il.com>,
        Matthew Wilcox <willy@...radead.org>, Mike Rapoport <rppt@...nel.org>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, Suren Baghdasaryan <surenb@...gle.com>,
        Hillf Danton <hdanton@...a.com>, Qi Zheng <zhengqi.arch@...edance.com>,
        SeongJae Park <sj@...nel.org>
Subject: Re: [PATCH] docs/mm: add VMA locks documentation

On Fri, Nov 08, 2024 at 07:25:38PM +0700, Bagas Sanjaya wrote:
> On Thu, Nov 07, 2024 at 07:01:37PM +0000, Lorenzo Stoakes wrote:
> > +.. note:: In instances where the architecture supports fewer page tables than
> > +   five the kernel cleverly 'folds' page table levels, that is skips them within
> > +   the logic, regardless we can act as if there were always five.
>
> What are being skipped if e.g. we only have 3 or 4 page tables?
>
> Confused...

Page table levels, see [0].

Typically achieved through stubbing functions out, etc. So in the code you
actually write and _conceptually_ there are five levels, only in the final
compile you might have things like p4d_present(), p4d_clear() etc. squashed by
the compiler into nothing.

I don't want to get too bogged down into the details of this in the doc, more of
an aside!

[0]: https://elixir.bootlin.com/linux/v6.11.6/source/include/asm-generic/pgtable-nopud.h#L9

>
> --
> An old man doll... just what I always wanted! - Clara