Re: [PATCH 01/33] s390: Use _pt_s390_gaddr for gmap address tracking

[Vishal Moola <vishal.moola@xxxxxxxxx>]

On Tue, Apr 18, 2023 at 8:45 AM David Hildenbrand <david@xxxxxxxxxx> wrote:
>
> On 17.04.23 22:50, Vishal Moola (Oracle) wrote:
> > s390 uses page->index to keep track of page tables for the guest address
> > space. In an attempt to consolidate the usage of page fields in s390,
> > replace _pt_pad_2 with _pt_s390_gaddr to replace page->index in gmap.
> >
> > This will help with the splitting of struct ptdesc from struct page, as
> > well as allow s390 to use _pt_frag_refcount for fragmented page table
> > tracking.
> >
> > Since page->_pt_s390_gaddr aliases with mapping, ensure its set to NULL
> > before freeing the pages as well.
> >
> > Signed-off-by: Vishal Moola (Oracle) <vishal.moola@xxxxxxxxx>
> > ---
>
> [...]
>
> > diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
> > index 3fc9e680f174..2616d64c0e8c 100644
> > --- a/include/linux/mm_types.h
> > +++ b/include/linux/mm_types.h
> > @@ -144,7 +144,7 @@ struct page {
> >               struct {        /* Page table pages */
> >                       unsigned long _pt_pad_1;        /* compound_head */
> >                       pgtable_t pmd_huge_pte; /* protected by page->ptl */
> > -                     unsigned long _pt_pad_2;        /* mapping */
> > +                     unsigned long _pt_s390_gaddr;   /* mapping */
> >                       union {
> >                               struct mm_struct *pt_mm; /* x86 pgds only */
> >                               atomic_t pt_frag_refcount; /* powerpc */
>
> The confusing part is, that these gmap page tables are not ordinary
> process page tables that we would ordinarily place into this section
> here. That's why they are also not allocated/freed using the typical
> page table constructor/destructor ...

I initially thought the same, so I was quite confused when I saw
__gmap_segment_gaddr was using pmd_pgtable_page().

Although they are not ordinary process page tables, since we
eventually want to move them out of struct page, I think shifting them
to be in ptdescs, being a memory descriptor for page tables, makes
the most sense.

Another option is to leave pmd_pgtable_page() as is just for this case.
Or we can revert commit 7e25de77bc5ea which uses the function here
then figure out where these gmap pages table pages will go later.