x86/mm: Introduce lookup_address_in_mm()

Add a helper, lookup_address_in_mm(), to traverse the page tables of a
given mm struct.  KVM will use the helper to retrieve the host mapping
level, e.g. 4k vs. 2mb vs. 1gb, of a compound (or DAX-backed) page
without having to resort to implementation specific metadata.  E.g. KVM
currently uses different logic for HugeTLB vs. THP, and would add a
third variant for DAX-backed files.

Cc: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/pgtable_types.h

@@ -561,6 +561,10 @@ static inline void update_page_count(int level, unsigned long pages) { }
 extern pte_t *lookup_address(unsigned long address, unsigned int *level);
 extern pte_t *lookup_address_in_pgd(pgd_t *pgd, unsigned long address,
 				    unsigned int *level);
+
+struct mm_struct;
+extern pte_t *lookup_address_in_mm(struct mm_struct *mm, unsigned long address,
+				   unsigned int *level);
 extern pmd_t *lookup_pmd_address(unsigned long address);
 extern phys_addr_t slow_virt_to_phys(void *__address);
 extern int __init kernel_map_pages_in_pgd(pgd_t *pgd, u64 pfn,

arch/x86/mm/pageattr.c

@@ -618,6 +618,17 @@ pte_t *lookup_address(unsigned long address, unsigned int *level)
 }
 EXPORT_SYMBOL_GPL(lookup_address);
 
+/*
+ * Lookup the page table entry for a virtual address in a given mm. Return a
+ * pointer to the entry and the level of the mapping.
+ */
+pte_t *lookup_address_in_mm(struct mm_struct *mm, unsigned long address,
+			    unsigned int *level)
+{
+	return lookup_address_in_pgd(pgd_offset(mm, address), address, level);
+}
+EXPORT_SYMBOL_GPL(lookup_address_in_mm);
+
 static pte_t *_lookup_address_cpa(struct cpa_data *cpa, unsigned long address,
 				  unsigned int *level)
 {