Re: [PATCH V5 mm-hotfixes 2/3] mm: introduce and use {pgd,p4d}_populate_kernel()

[David Hildenbrand]

On 18.08.25 04:02, 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.
>
> Cc: <stable@xxxxxxxxxxxxxxx>
> Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
> Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> Acked-by: Kiryl Shutsemau <kas@xxxxxxxxxx>
> Reviewed-by: Mike Rapoport (Microsoft) <rppt@xxxxxxxxxx>
> Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
> Signed-off-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
> ---


Acked-by: David Hildenbrand <david@xxxxxxxxxx>

--
Cheers

David / dhildenb