Re: [PATCH v2 1/2] mm: Allow lockless kernel pagetable walking

[Dev Jain]

On 10/06/25 6:57 pm, Lorenzo Stoakes wrote:
> On Tue, Jun 10, 2025 at 03:24:16PM +0200, David Hildenbrand wrote:
> > On 10.06.25 14:07, Lorenzo Stoakes wrote:
> > > OK so I think the best solution here is to just update check_ops_valid(), which
> > > was kind of sucky anyway (we check everywhere but walk_page_range_mm() to
> > > enforce the install pte thing).
> > >
> > > Let's do something like:
> > >
> > > #define OPS_MAY_INSTALL_PTE	(1<<0)
> > > #define OPS_MAY_AVOID_LOCK	(1<<1)
> > >
> > > and update check_ops_valid() to take a flags or maybe 'capabilities' field.
> > >
> > > Then check based on this e.g.:
> > >
> > > if (ops->install_pte && !(capabilities & OPS_MAY_INSTALL_PTE))
> > > 	return false;
> > >
> > > if (ops->walk_lock == PGWALK_NOLOCK && !(capabilities & OPS_MAY_AVOID_LOCK))
> > > 	return false;
> > Hm. I mean, we really only want to allow this lockless check for
> > walk_kernel_page_table_range(), right?
> >
> > Having a walk_kernel_page_table_range_lockeless() might (or might not) be
> > better, to really only special-case this specific path.
> Agree completely, Dev - let's definitely do this.

Makes sense.

> > So, I am wondering if we should further start splitting the
> > kernel-page-table walker up from the mm walker, at least on the "entry"
> > function for now.
> How do you mean?
>
> > --
> > Cheers,
> >
> > David / dhildenb