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

[Harry Yoo]

On Mon, Aug 11, 2025 at 10:10:58AM +0100, Lorenzo Stoakes wrote:
> On Mon, Aug 11, 2025 at 02:34:19PM +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. In theory, PUD and PMD level helpers can be added
> > later if needed by other architectures.
> >
> > Currently this is a no-op, since no architecture sets
> > PGTBL_{PGD,P4D}_MODIFIED in ARCH_PAGE_TABLE_SYNC_MASK.
> >
> > Cc: <stable@xxxxxxxxxxxxxxx>
> > Fixes: 8d400913c231 ("x86/vmemmap: handle unpopulated sub-pmd ranges")
> > Suggested-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > Signed-off-by: Harry Yoo <harry.yoo@xxxxxxxxxx>
> > ---
> >  include/linux/pgalloc.h | 24 ++++++++++++++++++++++++
> 
> Could we put this in the correct place in MAINTAINERS please?

Definitely yes!

Since this series will be backported to about five -stable kernels
(v5.13.x and later), I will add that as part of a follow-up series
that is not intended for backporting.

Does that sound okay?

> I think MEMORY MANAGEMENT - CORE is correct, given the below file is there.

Thanks for confirming that!

-- 
Cheers,
Harry / Hyeonggon