Re: [PATCH V2 6.12.y] mm: introduce and use {pgd,p4d}_populate_kernel()

[Greg KH <gregkh@linuxfoundation.org>]

On Tue, Sep 09, 2025 at 02:54:32PM +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")
> Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
> Acked-by: Kiryl Shutsemau <kas@kernel.org>
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
> Acked-by: David Hildenbrand <david@redhat.com>
> Cc: Alexander Potapenko <glider@google.com>
> Cc: Alistair Popple <apopple@nvidia.com>
> Cc: Andrey Konovalov <andreyknvl@gmail.com>
> Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
> Cc: Anshuman Khandual <anshuman.khandual@arm.com>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Cc: bibo mao <maobibo@loongson.cn>
> Cc: Borislav Betkov <bp@alien8.de>
> Cc: Christoph Lameter (Ampere) <cl@gentwo.org>
> Cc: Dennis Zhou <dennis@kernel.org>
> Cc: Dev Jain <dev.jain@arm.com>
> Cc: Dmitriy Vyukov <dvyukov@google.com>
> Cc: Gwan-gyeong Mun <gwan-gyeong.mun@intel.com>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Jane Chu <jane.chu@oracle.com>
> Cc: Joao Martins <joao.m.martins@oracle.com>
> Cc: Joerg Roedel <joro@8bytes.org>
> Cc: John Hubbard <jhubbard@nvidia.com>
> Cc: Kevin Brodsky <kevin.brodsky@arm.com>
> Cc: Liam Howlett <liam.howlett@oracle.com>
> Cc: Michal Hocko <mhocko@suse.com>
> Cc: Oscar Salvador <osalvador@suse.de>
> Cc: Peter Xu <peterx@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Qi Zheng <zhengqi.arch@bytedance.com>
> Cc: Ryan Roberts <ryan.roberts@arm.com>
> Cc: Suren Baghdasaryan <surenb@google.com>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Thomas Gleinxer <tglx@linutronix.de>
> Cc: Thomas Huth <thuth@redhat.com>
> Cc: Uladzislau Rezki (Sony) <urezki@gmail.com>
> Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
> Cc: Vlastimil Babka <vbabka@suse.cz>
> Cc: Pedro Falcato <pfalcato@suse.de>
> Cc: stable@vger.kernel.org
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> [ Adjust context ]
> Signed-off-by: Harry Yoo <harry.yoo@oracle.com>
> ---
>  include/linux/pgalloc.h | 29 +++++++++++++++++++++++++++++
>  include/linux/pgtable.h | 13 +++++++------
>  mm/kasan/init.c         | 12 ++++++------
>  mm/percpu.c             |  6 +++---
>  mm/sparse-vmemmap.c     |  6 +++---
>  5 files changed, 48 insertions(+), 18 deletions(-)
>  create mode 100644 include/linux/pgalloc.h

Can you resend these with the upstream git id from Linus's tree in it,
so we know how to compare it with the original?

thanks,

greg k-h