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3l6sd6i6lw7GA4p/9XhThRTxcwBpfRpTYCiJfWCwFcZ4qPnPTbwfEEIzi2hcftAw+Z7BjKhQCTG BSGw3zlts09/ikFcp5OmofdWzetAbDHjxj8ugyVpfXqxuXV5vqvzCDshotLA8jUXrxsTHA0zMGF Y04kP1Rl Please don't apply this, I made a mistake while backporting. (Thanks to Pedro for catching it!) I'll resend the backport for v6.12, v6.6, v6.1, and v5.15 tomorrow. Leaving an inline comment below on what I got wrong below... On Mon, Sep 08, 2025 at 10:09:31AM +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. 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. > > [harry.yoo@oracle.com: fix KASAN build error due to p*d_populate_kernel()] > Link: https://lkml.kernel.org/r/20250822020727.202749-1-harry.yoo@oracle.com > Link: https://lkml.kernel.org/r/20250818020206.4517-3-harry.yoo@oracle.com > Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges") > Signed-off-by: Harry Yoo > Suggested-by: Dave Hansen > Acked-by: Kiryl Shutsemau > Reviewed-by: Mike Rapoport (Microsoft) > Reviewed-by: Lorenzo Stoakes > Acked-by: David Hildenbrand > Cc: Alexander Potapenko > Cc: Alistair Popple > Cc: Andrey Konovalov > Cc: Andrey Ryabinin > Cc: Andy Lutomirski > Cc: "Aneesh Kumar K.V" > Cc: Anshuman Khandual > Cc: Ard Biesheuvel > Cc: Arnd Bergmann > Cc: bibo mao > Cc: Borislav Betkov > Cc: Christoph Lameter (Ampere) > Cc: Dennis Zhou > Cc: Dev Jain > Cc: Dmitriy Vyukov > Cc: Gwan-gyeong Mun > Cc: Ingo Molnar > Cc: Jane Chu > Cc: Joao Martins > Cc: Joerg Roedel > Cc: John Hubbard > Cc: Kevin Brodsky > Cc: Liam Howlett > Cc: Michal Hocko > Cc: Oscar Salvador > Cc: Peter Xu > Cc: Peter Zijlstra > Cc: Qi Zheng > Cc: Ryan Roberts > Cc: Suren Baghdasaryan > Cc: Tejun Heo > Cc: Thomas Gleinxer > Cc: Thomas Huth > Cc: "Uladzislau Rezki (Sony)" > Cc: Vincenzo Frascino > Cc: Vlastimil Babka > Cc: > Signed-off-by: Andrew Morton > --- > include/linux/pgalloc.h | 29 +++++++++++++++++++++++++++++ > include/linux/pgtable.h | 26 ++++++++++++++++++++++---- > mm/kasan/init.c | 12 ++++++------ > mm/percpu.c | 6 +++--- > mm/sparse-vmemmap.c | 6 +++--- > 5 files changed, 63 insertions(+), 16 deletions(-) > create mode 100644 include/linux/pgalloc.h > > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h > index 1ba6e32909f8..e7d5c02ac0fb 100644 > --- a/include/linux/pgtable.h > +++ b/include/linux/pgtable.h > @@ -1343,6 +1343,23 @@ static inline void ptep_modify_prot_commit(struct vm_area_struct *vma, > __ptep_modify_prot_commit(vma, addr, ptep, pte); > } > #endif /* __HAVE_ARCH_PTEP_MODIFY_PROT_TRANSACTION */ > + > +/* > + * Architectures can set this mask to a combination of PGTBL_P?D_MODIFIED values > + * and let generic vmalloc, ioremap and page table update code know when > + * arch_sync_kernel_mappings() needs to be called. > + */ > +#ifndef ARCH_PAGE_TABLE_SYNC_MASK > +#define ARCH_PAGE_TABLE_SYNC_MASK 0 > +#endif > + > +/* > + * There is no default implementation for arch_sync_kernel_mappings(). It is > + * relied upon the compiler to optimize calls out if ARCH_PAGE_TABLE_SYNC_MASK > + * is 0. > + */ > +void arch_sync_kernel_mappings(unsigned long start, unsigned long end); > + > #endif /* CONFIG_MMU */ This is already present in include/linux/pgtable.h (due to "mm: move page table sync declarations to linux/pgtable.h"), and my backport mistakenly duplicated this hunk. -- Cheers, Harry / Hyeonggon