linux-kernel - Re: [RFC V1 PATCH mm-hotfixes 1/3] mm: introduce and use {pgd,p4d}_populate

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Message-ID: <02146c79-a4de-430f-8357-0608e796fa60@redhat.com>
Date: Fri, 11 Jul 2025 18:18:44 +0200
From: David Hildenbrand <david@...hat.com>
To: Harry Yoo <harry.yoo@...cle.com>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
 Dave Hansen <dave.hansen@...ux.intel.com>, Andy Lutomirski
 <luto@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
 Andrey Ryabinin <ryabinin.a.a@...il.com>, Arnd Bergmann <arnd@...db.de>,
 Andrew Morton <akpm@...ux-foundation.org>, Dennis Zhou <dennis@...nel.org>,
 Tejun Heo <tj@...nel.org>, Christoph Lameter <cl@...two.org>
Cc: "H . Peter Anvin" <hpa@...or.com>, Alexander Potapenko
 <glider@...gle.com>, Andrey Konovalov <andreyknvl@...il.com>,
 Dmitry Vyukov <dvyukov@...gle.com>,
 Vincenzo Frascino <vincenzo.frascino@....com>,
 Juergen Gross <jgross@...e.com>, Kevin Brodsky <kevin.brodsky@....com>,
 Muchun Song <muchun.song@...ux.dev>, Oscar Salvador <osalvador@...e.de>,
 Joao Martins <joao.m.martins@...cle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Jane Chu
 <jane.chu@...cle.com>, Alistair Popple <apopple@...dia.com>,
 Mike Rapoport <rppt@...nel.org>, Gwan-gyeong Mun
 <gwan-gyeong.mun@...el.com>, "Aneesh Kumar K . V"
 <aneesh.kumar@...ux.ibm.com>, x86@...nel.org, linux-kernel@...r.kernel.org,
 linux-arch@...r.kernel.org, linux-mm@...ck.org, stable@...r.kernel.org
Subject: Re: [RFC V1 PATCH mm-hotfixes 1/3] mm: introduce and use
 {pgd,p4d}_populate_kernel()

On 09.07.25 15:16, Harry Yoo wrote:
> Intrdocue and use {pgd,p4d}_pouplate_kernel() in core MM code when
> populating PGD and P4D entries corresponding to the kernel address
> space. The main purpose of these helpers is to ensure synchronization of
> the kernel portion of the top-level page tables whenever such an entry
> is populated.
> 
> Until now, the kernel has relied on each architecture to handle
> synchronization of top-level page tables in an ad-hoc manner.
> For example, see commit 9b861528a801 ("x86-64, mem: Update all PGDs for
> direct mapping and vmemmap mapping changes").
> 
> However, this approach has proven fragile, as it's easy to forget to
> perform the necessary synchronization when introducing new changes.
> 
> To address this, introduce _kernel() varients of the page table

s/varients/variants/

> population helpers that invoke architecture-specific hooks to properly
> synchronize the page tables.

I was expecting to see the sync be done in common code -- such that it 
cannot be missed :)

But it's really just rerouting to the arch code where the sync can be 
done, correct?

-- 
Cheers,

David / dhildenb