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Message-ID: <aIh_Vtqp-bBDGgO9@hyeyoo>
Date: Tue, 29 Jul 2025 16:59:18 +0900
From: Harry Yoo <harry.yoo@...cle.com>
To: 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>, David Hildenbrand <david@...hat.com>,
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
Subject: Re: [PATCH v3 mm-hotfixes 2/5] mm: introduce and use
{pgd,p4d}_populate_kernel()
Adding some comment after looking at a kernel test robot report [1]
that seems to be rejected by linux-mm.
[1] https://lore.kernel.org/oe-kbuild-all/202507290917.T24WIcvt-lkp@intel.com
I will post the next version with it fixed and including only first
three patches that will be backported to -stable. (and post last 2
patches as a follow-up after that)
On Fri, Jul 25, 2025 at 10:21:03AM +0900, 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.
>
> 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.
>
> Currently this is a no-op, since no architecture sets
> PGTBL_{PGD,P4D}_MODIFIED in ARCH_PAGE_TABLE_SYNC_MASK.
>
> Cc: stable@...r.kernel.org
> Suggested-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Signed-off-by: Harry Yoo <harry.yoo@...cle.com>
> ---
> include/asm-generic/pgalloc.h | 16 ++++++++++++++++
> include/linux/pgtable.h | 4 ++--
> mm/kasan/init.c | 10 +++++-----
> mm/percpu.c | 4 ++--
> mm/sparse-vmemmap.c | 4 ++--
> 5 files changed, 27 insertions(+), 11 deletions(-)
>
> diff --git a/include/asm-generic/pgalloc.h b/include/asm-generic/pgalloc.h
> index 3c8ec3bfea44..fc0ab8eed5a6 100644
> --- a/include/asm-generic/pgalloc.h
> +++ b/include/asm-generic/pgalloc.h
> @@ -4,6 +4,8 @@
>
> #ifdef CONFIG_MMU
>
> +#include <linux/pgtable.h>
> +
> #define GFP_PGTABLE_KERNEL (GFP_KERNEL | __GFP_ZERO)
> #define GFP_PGTABLE_USER (GFP_PGTABLE_KERNEL | __GFP_ACCOUNT)
>
> @@ -296,6 +298,20 @@ static inline void pgd_free(struct mm_struct *mm, pgd_t *pgd)
> }
> #endif
>
> +#define pgd_populate_kernel(addr, pgd, p4d) \
> +do { \
> + pgd_populate(&init_mm, pgd, p4d); \
> + if (ARCH_PAGE_TABLE_SYNC_MASK & PGTBL_PGD_MODIFIED) \
> + arch_sync_kernel_mappings(addr, addr); \
> +} while (0)
> +
> +#define p4d_populate_kernel(addr, p4d, pud) \
> +do { \
> + p4d_populate(&init_mm, p4d, pud); \
> + if (ARCH_PAGE_TABLE_SYNC_MASK & PGTBL_P4D_MODIFIED) \
> + arch_sync_kernel_mappings(addr, addr); \
> +} while (0)
> +
> #endif /* CONFIG_MMU */
The report [1] complains that p*d_populate_kernel() is not defined:
mm/percpu.c: In function 'pcpu_populate_pte':
>> mm/percpu.c:3137:17: error: implicit declaration of function 'pgd_populate_kernel'; did you mean 'pmd_populate_kernel'? [-Wimplicit-function-declaration]
3137 | pgd_populate_kernel(addr, pgd, p4d);
| ^~~~~~~~~~~~~~~~~~~
| pmd_populate_kernel
>> mm/percpu.c:3143:17: error: implicit declaration of function 'p4d_populate_kernel'; did you mean 'pmd_populate_kernel'? [-Wimplicit-function-declaration]
3143 | p4d_populate_kernel(addr, p4d, pud);
| ^~~~~~~~~~~~~~~~~~~
| pmd_populate_kernel
--
mm/sparse-vmemmap.c: In function 'vmemmap_p4d_populate':
>> mm/sparse-vmemmap.c:232:17: error: implicit declaration of function 'p4d_populate_kernel'; did you mean 'pmd_populate_kernel'? [-Wimplicit-function-declaration]
232 | p4d_populate_kernel(addr, p4d, p);
| ^~~~~~~~~~~~~~~~~~~
| pmd_populate_kernel
mm/sparse-vmemmap.c: In function 'vmemmap_pgd_populate':
>> mm/sparse-vmemmap.c:244:17: error: implicit declaration of function 'pgd_populate_kernel'; did you mean 'pmd_populate_kernel'? [-Wimplicit-function-declaration]
244 | pgd_populate_kernel(addr, pgd, p);
| ^~~~~~~~~~~~~~~~~~~
| pmd_populate_kernel
I had incorrectly assumed that asm/pgalloc.h in all architecture would
include asm-generic/pgalloc.h. That's true for most architectures,
but a few architectures (sparc, powerpc, s390) don't do that.
As it turns out the assumption isn't valid on all arches, I think the
right thing to do now is to introduce include/linux/pgalloc.h and put
these helpers there, and include it from common code.
--
Cheers,
Harry / Hyeonggon
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