mm: pgtable: add missing pt_index to struct ptdesc

In s390, the page->index field is used for gmap (see gmap_shadow_pgt()),
so add the corresponding pt_index to struct ptdesc and add a comment to
clarify this.

Link: https://lkml.kernel.org/r/283624c2af45fb2090b41a6b1b5481bb0a45bad7.1709541697.git.zhengqi.arch@bytedance.comSigned-off-by: Qi Zheng <zhengqi.arch@bytedance.com>
Cc: Christian Borntraeger <borntraeger@linux.ibm.com>
Cc: Claudio Imbrenda <imbrenda@linux.ibm.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Janosch Frank <frankja@linux.ibm.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Muchun Song <muchun.song@linux.dev>
Cc: Vishal Moola (Oracle) <vishal.moola@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/mm_types.h

@@ -425,6 +425,7 @@ FOLIO_MATCH(compound_head, _head_2a);
  * @_pt_pad_1:        Padding that aliases with page's compound head.
  * @pmd_huge_pte:     Protected by ptdesc->ptl, used for THPs.
  * @__page_mapping:   Aliases with page->mapping. Unused for page tables.
+ * @pt_index:         Used for s390 gmap.
  * @pt_mm:            Used for x86 pgds.
  * @pt_frag_refcount: For fragmented page table tracking. Powerpc only.
  * @_pt_pad_2:        Padding to ensure proper alignment.
@@ -450,6 +451,7 @@ struct ptdesc {
 	unsigned long __page_mapping;
 
 	union {
+		pgoff_t pt_index;
 		struct mm_struct *pt_mm;
 		atomic_t pt_frag_refcount;
 	};
@@ -475,6 +477,7 @@ TABLE_MATCH(flags, __page_flags);
 TABLE_MATCH(compound_head, pt_list);
 TABLE_MATCH(compound_head, _pt_pad_1);
 TABLE_MATCH(mapping, __page_mapping);
+TABLE_MATCH(index, pt_index);
 TABLE_MATCH(rcu_head, pt_rcu_head);
 TABLE_MATCH(page_type, __page_type);
 TABLE_MATCH(_refcount, __page_refcount);