linux-kernel - Re: [PATCH V4 mm-hotfixes 2/3] mm: introduce and use {pgd,p4d}_populate

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Message-ID: <26796993-5a17-487e-a32e-d9f7577216c3@csgroup.eu>
Date: Mon, 25 Aug 2025 13:27:20 +0200
From: Christophe Leroy <christophe.leroy@...roup.eu>
To: Harry Yoo <harry.yoo@...cle.com>, Dennis Zhou <dennis@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 Andrey Ryabinin <ryabinin.a.a@...il.com>, x86@...nel.org,
 Borislav Petkov <bp@...en8.de>, Peter Zijlstra <peterz@...radead.org>,
 Andy Lutomirski <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>,
 Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
 Uladzislau Rezki <urezki@...il.com>,
 Dave Hansen <dave.hansen@...ux.intel.com>, Christoph Lameter
 <cl@...two.org>, David Hildenbrand <david@...hat.com>
Cc: Andrey Konovalov <andreyknvl@...il.com>,
 Vincenzo Frascino <vincenzo.frascino@....com>, "H. Peter Anvin"
 <hpa@...or.com>, kasan-dev@...glegroups.com, Mike Rapoport
 <rppt@...nel.org>, Ard Biesheuvel <ardb@...nel.org>,
 linux-kernel@...r.kernel.org, Dmitry Vyukov <dvyukov@...gle.com>,
 Alexander Potapenko <glider@...gle.com>, Vlastimil Babka <vbabka@...e.cz>,
 Suren Baghdasaryan <surenb@...gle.com>, Thomas Huth <thuth@...hat.com>,
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 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>, Michal Hocko
 <mhocko@...e.com>, "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 linux-mm@...ck.org, "Kirill A. Shutemov" <kas@...nel.org>,
 Oscar Salvador <osalvador@...e.de>, Jane Chu <jane.chu@...cle.com>,
 Gwan-gyeong Mun <gwan-gyeong.mun@...el.com>,
 "Aneesh Kumar K . V" <aneesh.kumar@...ux.ibm.com>,
 Joerg Roedel <joro@...tes.org>, Alistair Popple <apopple@...dia.com>,
 Joao Martins <joao.m.martins@...cle.com>, linux-arch@...r.kernel.org,
 stable@...r.kernel.org
Subject: Re: [PATCH V4 mm-hotfixes 2/3] mm: introduce and use
 {pgd,p4d}_populate_kernel()

Le 11/08/2025 à 07:34, Harry Yoo a écrit :
> Introduce and use {pgd,p4d}_populate_kernel() in core MM code when
> populating PGD and P4D entries for the kernel address space.
> These helpers ensure proper synchronization of page tables when
> updating the kernel portion of top-level page tables.
> 
> 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 for following reasons:
> 
>    1) It is easy to forget to perform the necessary page table
>       synchronization when introducing new changes.
>       For instance, commit 4917f55b4ef9 ("mm/sparse-vmemmap: improve memory
>       savings for compound devmaps") overlooked the need to synchronize
>       page tables for the vmemmap area.
> 
>    2) It is also easy to overlook that the vmemmap and direct mapping areas
>       must not be accessed before explicit page table synchronization.
>       For example, commit 8d400913c231 ("x86/vmemmap: handle unpopulated
>       sub-pmd ranges")) caused crashes by accessing the vmemmap area
>       before calling sync_global_pgds().
> 
> To address this, as suggested by Dave Hansen, introduce _kernel() variants
> of the page table population helpers, which invoke architecture-specific
> hooks to properly synchronize page tables. These are introduced in a new
> header file, include/linux/pgalloc.h, so they can be called from common code.
> 
> They reuse existing infrastructure for vmalloc and ioremap.
> Synchronization requirements are determined by ARCH_PAGE_TABLE_SYNC_MASK,
> and the actual synchronization is performed by arch_sync_kernel_mappings().
> 
> This change currently targets only x86_64, so only PGD and P4D level
> helpers are introduced. In theory, PUD and PMD level helpers can be added
> later if needed by other architectures.

AFAIK pmd_populate_kernel() already exist on all architectures, and I'm 
not sure it does what you expect. Or am I missing something ?

Christophe