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

[David Hildenbrand <david@xxxxxxxxxx>]

On 18.04.23 23:33, Vishal Moola wrote:
> 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.
Seeing utilities like tlb_remove_page_ptdesc() that don't really apply 
to such page tables, I wonder if we should much rather treat such 
shadow/auxiliary/... page tables (just like other architectures like 
x86, arm, ... employ as well) as a distinct type.
And have ptdesc be the common type for all process page tables.

> 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.
I'm always confused when reading gmap code, so let me have another look :)

The confusing part is that s390x shares the lowest level page tables 
(PTE tables) between the process and gmap ("guest mapping", similar to 
EPT on x86-64). It maps these process PTE tables (covering 1 MiB) into 
gmap-specific PMD tables.
pmd_pgtable_page() should indeed always give us a gmap-specific 
PMD-table. In fact, something allocated via gmap_alloc_table().
Decoupling both concepts sounds like a good idea.

--
Thanks,

David / dhildenb