Re: [RFC V1 PATCH mm-hotfixes 1/3] mm: introduce and use {pgd,p4d}_populate_kernel()

Harry Yoo
2025-07-13T11:41:17Z

On Fri, Jul 11, 2025 at 06:18:44PM +0200, David Hildenbrand wrote:
> On 09.07.25 15:16, Harry Yoo wrote:
> > Intrdocue and use {pgd,p4d}_pouplate_kernel() in core MM code when
> > populating PGD and P4D entries corresponding to the kernel address
> > space. The main purpose of these helpers is to ensure synchronization of
> > the kernel portion of the top-level page tables whenever such an entry
> > is populated.
> >
> > 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, as it's easy to forget to
> > perform the necessary synchronization when introducing new changes.
> >
> > To address this, introduce _kernel() varients of the page table
>
> s/varients/variants/

Will fix. Thanks.

> > population helpers that invoke architecture-specific hooks to properly
> > synchronize the page tables.
>
> I was expecting to see the sync be done in common code -- such that it
> cannot be missed :)

You mean something like an arch-independent implementation of
sync_global_pgds()?

That would be a "much more robust" approach ;)

To do that, the kernel would need to maintain a list of page tables that
have kernel portion mapped and perform the sync in the common code.

But determining which page tables to add to the list would be highly
architecture-specific. For example, I think some architectures use separate
page tables for kernel space, unlike x86 (e.g., arm64 TTBR1, SPARC) and
user page tables should not be affected.

While doing the sync in common code might be a more robust option
in the long term, I'm afraid that making it work correctly across
all architectures would be challenging, due to differences in how each
architecture manages the kernel address space.

> But it's really just rerouting to the arch code where the sync can be done,
> correct?

Yes, that's correct.

Thanks for taking a look!