Re: [PATCH V4 mm-hotfixes 2/3] mm: introduce and use {pgd,p4d}_populate_kernel()

[Christophe Leroy <christophe.leroy@csgroup.eu>]

Le 11/08/2025 à 07:34, Harry Yoo a écrit :
> 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. In theory, PUD and PMD level helpers can be added
> later if needed by other architectures.

AFAIK pmd_populate_kernel() already exist on all architectures, and I'm 
not sure it does what you expect. Or am I missing something ?

Christophe