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Peter Anvin" Cc: Andrey Ryabinin , Arnd Bergmann , Dennis Zhou , Tejun Heo , Christoph Lameter , Alexander Potapenko , Andrey Konovalov , Dmitry Vyukov , Vincenzo Frascino , Juergen Gross , Kevin Brodsky , Oscar Salvador , Joao Martins , Lorenzo Sccakes , Jane Chu , Alistair Popple , Mike Rapoport , David Hildenbrand , Gwan-gyeong Mun , "Aneesh Kumar K . V" , Uladzislau Rezki , "Liam R . 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Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: 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@vger.kernel.org > Suggested-by: Dave Hansen > Signed-off-by: Harry Yoo > --- > 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 > + > #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