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

[David Hildenbrand <david@redhat.com>]

On 18.04.23 23:33, Vishal Moola wrote:
> On Tue, Apr 18, 2023 at 8:45 AM David Hildenbrand <david@redhat.com> 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@gmail.com>
>>> ---
>>
>> [...]
>>
>>> 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