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

Harry Yoo
2025-07-14T08:11:29Z

On Sun, Jul 13, 2025 at 08:56:10PM +0300, Mike Rapoport wrote:
> On Sun, Jul 13, 2025 at 08:39:53PM +0900, Harry Yoo wrote:
> > On Fri, Jul 11, 2025 at 06:18:44PM +0200, David Hildenbrand wrote:
> > > > 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.
>
> sync_global_pgds() can be still implemented per architecture, but it can be
> called from the common code.

A good point, and that can be done!

Actually, that was the initial plan and I somehow thought that
you can't determine if the architecture is using 5-level or 4-level paging
and decide whether to call arch_sync_kernel_pagetables(). But looking at
how it's done in vmalloc, I think it can be done in a similar way.

> We already have something like that for vmalloc that calls
> arch_sync_kernel_mappings(). It's implemented only by x86-32 and arm, other
> architectures do not define it.

It is indeed a good example and was helpful.
Thank you for the comment, Mike!

> --
> Sincerely yours,
> Mike.