mm: add mm_pxd_folded checks to pgtable_bytes accounting functions

The common mm code calls mm_dec_nr_pmds() and mm_dec_nr_puds()
in free_pgtables() if the address range spans a full pud or pmd.
If mm_dec_nr_puds/mm_dec_nr_pmds are non-empty due to configuration
settings they blindly subtract the size of the pmd or pud table from
pgtable_bytes even if the pud or pmd page table layer is folded.

Add explicit mm_[pmd|pud]_folded checks to the four pgtable_bytes
accounting functions mm_inc_nr_puds, mm_inc_nr_pmds, mm_dec_nr_puds
and mm_dec_nr_pmds. As the check for folded page tables can be
overwritten by the architecture, this allows to keep a correct
pgtable_bytes value for platforms that use a dynamic number of
page table levels.

Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

include/linux/mm.h

@@ -1742,11 +1742,15 @@ int __pud_alloc(struct mm_struct *mm, p4d_t *p4d, unsigned long address);
 
 static inline void mm_inc_nr_puds(struct mm_struct *mm)
 {
+	if (mm_pud_folded(mm))
+		return;
 	atomic_long_add(PTRS_PER_PUD * sizeof(pud_t), &mm->pgtables_bytes);
 }
 
 static inline void mm_dec_nr_puds(struct mm_struct *mm)
 {
+	if (mm_pud_folded(mm))
+		return;
 	atomic_long_sub(PTRS_PER_PUD * sizeof(pud_t), &mm->pgtables_bytes);
 }
 #endif
@@ -1766,11 +1770,15 @@ int __pmd_alloc(struct mm_struct *mm, pud_t *pud, unsigned long address);
 
 static inline void mm_inc_nr_pmds(struct mm_struct *mm)
 {
+	if (mm_pmd_folded(mm))
+		return;
 	atomic_long_add(PTRS_PER_PMD * sizeof(pmd_t), &mm->pgtables_bytes);
 }
 
 static inline void mm_dec_nr_pmds(struct mm_struct *mm)
 {
+	if (mm_pmd_folded(mm))
+		return;
 	atomic_long_sub(PTRS_PER_PMD * sizeof(pmd_t), &mm->pgtables_bytes);
 }
 #endif