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

[Harry Yoo <harry.yoo@oracle.com>]

On Tue, Sep 09, 2025 at 03:52:35PM +0200, Greg KH wrote:
> On Tue, Sep 09, 2025 at 03:52:14PM +0200, Greg KH wrote:
> > 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?
>  
> Same for the other backports, sorry I forgot to say that here.

It's

commit f2d2f9598ebb0158a3fe17cda0106d7752e654a2 upstream.

Sorry for the inconvenience, I'll resend it with the upstream git id.

Thanks!

-- 
Cheers,
Harry / Hyeonggon