linux-kernel - Re: [PATCH V5 mm-hotfixes 2/3] mm: introduce and use {pgd,p4d}_populate

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Message-ID: <0e613cf3-7ff1-49d9-9fff-fcf824f2df72@redhat.com>
Date: Mon, 18 Aug 2025 09:48:54 +0200
From: David Hildenbrand <david@...hat.com>
To: Harry Yoo <harry.yoo@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>,
 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>,
 "H . Peter Anvin" <hpa@...cr.com>
Cc: Andrey Ryabinin <ryabinin.a.a@...il.com>, Arnd Bergmann <arnd@...db.de>,
 Dennis Zhou <dennis@...nel.org>, Tejun Heo <tj@...nel.org>,
 Christoph Lameter <cl@...two.org>, Alexander Potapenko <glider@...gle.com>,
 Andrey Konovalov <andreyknvl@...il.com>, Dmitry Vyukov <dvyukov@...gle.com>,
 Vincenzo Frascino <vincenzo.frascino@....com>, Juergen Gross
 <jgross@...e.de>, Kevin Brodsky <kevin.brodsky@....com>,
 Oscar Salvador <osalvador@...e.de>, Joao Martins
 <joao.m.martins@...cle.com>, Lorenzo Sccakes <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>, Uladzislau Rezki <urezki@...il.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>,
 Vlastimil Babka <vbabka@...e.cz>, Suren Baghdasaryan <surenb@...gle.com>,
 Michal Hocko <mhocko@...e.com>, Qi Zheng <zhengqi.arch@...edance.com>,
 Ard Biesheuvel <ardb@...nel.org>, Thomas Huth <thuth@...hat.com>,
 John Hubbard <jhubbard@...dia.com>, Ryan Roberts <ryan.roberts@....com>,
 Peter Xu <peterx@...hat.com>, Dev Jain <dev.jain@....com>,
 Bibo Mao <maobibo@...ngson.cn>, Anshuman Khandual
 <anshuman.khandual@....com>, Joerg Roedel <joro@...tes.org>, x86@...nel.org,
 linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
 linux-mm@...ck.org, stable@...r.kernel.org, Kiryl Shutsemau <kas@...nel.org>
Subject: Re: [PATCH V5 mm-hotfixes 2/3] mm: introduce and use
 {pgd,p4d}_populate_kernel()

On 18.08.25 04:02, Harry Yoo wrote:
> 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. Currently, these helpers are no-ops since no
> architecture sets PGTBL_{PGD,P4D}_MODIFIED in ARCH_PAGE_TABLE_SYNC_MASK.
> 
> In theory, PUD and PMD level helpers can be added later if needed by
> other architectures. For now, 32-bit architectures (x86-32 and arm) only
> handle PGTBL_PMD_MODIFIED, so p*d_populate_kernel() will never affect
> them unless we introduce a PMD level helper.
> 
> Cc: <stable@...r.kernel.org>
> Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
> Suggested-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Acked-by: Kiryl Shutsemau <kas@...nel.org>
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
> Signed-off-by: Harry Yoo <harry.yoo@...cle.com>
> ---

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers

David / dhildenb