[PATCH 03/33] pgtable: Create struct ptdesc

["Vishal Moola (Oracle)" <vishal.moola@gmail.com>]

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>
---
 include/linux/pgtable.h | 50 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 50 insertions(+)

diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
index 023918666dd4..7cc6ea057ee9 100644
--- a/include/linux/pgtable.h
+++ b/include/linux/pgtable.h
@@ -47,6 +47,56 @@
 #define pmd_pgtable(pmd) pmd_page(pmd)
 #endif
 
+/**
+ * struct ptdesc - Memory descriptor for page tables.
+ * @__page_flags: Same as page flags. Unused for page tables.
+ * @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.
+ * @pt_mm: Used for x86 pgds.
+ * @pt_frag_refcount: For fragmented page table tracking. Powerpc and s390 only.
+ * @ptl: Lock for the page table.
+ *
+ * This struct overlays struct page for now. Do not modify without a good
+ * understanding of the issues.
+ */
+struct ptdesc {
+	unsigned long __page_flags;
+
+	union {
+		struct list_head pt_list;
+		struct {
+			unsigned long _pt_pad_1;
+			pgtable_t pmd_huge_pte;
+		};
+	};
+	unsigned long _pt_s390_gaddr;
+
+	union {
+		struct mm_struct *pt_mm;
+		atomic_t pt_frag_refcount;
+	};
+
+#if ALLOC_SPLIT_PTLOCKS
+	spinlock_t *ptl;
+#else
+	spinlock_t ptl;
+#endif
+};
+
+#define TABLE_MATCH(pg, pt)						\
+	static_assert(offsetof(struct page, pg) == offsetof(struct ptdesc, pt))
+TABLE_MATCH(flags, __page_flags);
+TABLE_MATCH(compound_head, pt_list);
+TABLE_MATCH(compound_head, _pt_pad_1);
+TABLE_MATCH(mapping, _pt_s390_gaddr);
+TABLE_MATCH(pmd_huge_pte, pmd_huge_pte);
+TABLE_MATCH(pt_mm, pt_mm);
+TABLE_MATCH(ptl, ptl);
+#undef TABLE_MATCH
+static_assert(sizeof(struct ptdesc) <= sizeof(struct page));
+
 /*
  * A page table page can be thought of an array like this: pXd_t[PTRS_PER_PxD]
  *
-- 
2.39.2