Re: [PATCH v6 03/33] pgtable: Create struct ptdesc

[Vishal Moola <vishal.moola@gmail.com>]

On Tue, Jun 27, 2023 at 9:07 AM Peter Xu <peterx@redhat.com> wrote:
>
> On Mon, Jun 26, 2023 at 08:14:01PM -0700, Vishal Moola (Oracle) wrote:
> > Currently, page table information is stored within struct page. As part
> > of simplifying struct page, create struct ptdesc for page table
> > information.
> >
> > Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
> > Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
> > ---
> >  include/linux/pgtable.h | 68 +++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 68 insertions(+)
> >
> > diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> > index 5063b482e34f..d46cb709ce08 100644
> > --- a/include/linux/pgtable.h
> > +++ b/include/linux/pgtable.h
> > @@ -987,6 +987,74 @@ static inline void ptep_modify_prot_commit(struct vm_area_struct *vma,
> >  #endif /* __HAVE_ARCH_PTEP_MODIFY_PROT_TRANSACTION */
> >  #endif /* CONFIG_MMU */
> >
> > +
> > +/**
> > + * struct ptdesc -    Memory descriptor for page tables.
> > + * @__page_flags:     Same as page flags. Unused for page tables.
> > + * @pt_rcu_head:      For freeing page table pages.
> > + * @pt_list:          List of used page tables. Used for s390 and x86.
> > + * @_pt_pad_1:        Padding that aliases with page's compound head.
> > + * @pmd_huge_pte:     Protected by ptdesc->ptl, used for THPs.
> > + * @_pt_s390_gaddr:   Aliases with page's mapping. Used for s390 gmap only.
>
> Should some arch-specific bits (and a few others) always under some
> #ifdefs, so it shouldn't appear on other archs?

Right now this struct completely overlays struct page, so the padding as
well as any arch-specific fields have to stay. Whenever we get ptdescs
independent of struct page we can cleanup any unnecessary fields, as
well as omit unnecessary fields from unrelated architectures.