powerpc/mm/hash64: use _PAGE_PTE when checking for pte_present

This makes the pte_present check stricter by checking for additional _PAGE_PTE
bit. A level 1 pte pointer (THP pte) can be switched to a pointer to level 0 pte
page table page by following two operations.

1) THP split.
2) madvise(MADV_DONTNEED) in parallel to page fault.

A lockless page table walk need to make sure we can handle such changes
gracefully.
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200505071729.54912-4-aneesh.kumar@linux.ibm.com

arch/powerpc/include/asm/book3s/64/pgtable.h

@@ -553,6 +553,12 @@ static inline pte_t pte_clear_savedwrite(pte_t pte)
 }
 #endif /* CONFIG_NUMA_BALANCING */
 
+static inline bool pte_hw_valid(pte_t pte)
+{
+	return (pte_raw(pte) & cpu_to_be64(_PAGE_PRESENT | _PAGE_PTE)) ==
+		cpu_to_be64(_PAGE_PRESENT | _PAGE_PTE);
+}
+
 static inline int pte_present(pte_t pte)
 {
 	/*
@@ -561,12 +567,11 @@ static inline int pte_present(pte_t pte)
 	 * invalid during ptep_set_access_flags. Hence we look for _PAGE_INVALID
 	 * if we find _PAGE_PRESENT cleared.
 	 */
-	return !!(pte_raw(pte) & cpu_to_be64(_PAGE_PRESENT | _PAGE_INVALID));
-}
 
-static inline bool pte_hw_valid(pte_t pte)
-{
-	return !!(pte_raw(pte) & cpu_to_be64(_PAGE_PRESENT));
+	if (pte_hw_valid(pte))
+		return true;
+	return (pte_raw(pte) & cpu_to_be64(_PAGE_INVALID | _PAGE_PTE)) ==
+		cpu_to_be64(_PAGE_INVALID | _PAGE_PTE);
 }
 
 #ifdef CONFIG_PPC_MEM_KEYS

arch/powerpc/mm/book3s64/hash_utils.c

@@ -1350,8 +1350,15 @@ int hash_page_mm(struct mm_struct *mm, unsigned long ea,
 		goto bail;
 	}
 
-	/* Add _PAGE_PRESENT to the required access perm */
-	access |= _PAGE_PRESENT;
+	/*
+	 * Add _PAGE_PRESENT to the required access perm. If there are parallel
+	 * updates to the pte that can possibly clear _PAGE_PTE, catch that too.
+	 *
+	 * We can safely use the return pte address in rest of the function
+	 * because we do set H_PAGE_BUSY which prevents further updates to pte
+	 * from generic code.
+	 */
+	access |= _PAGE_PRESENT | _PAGE_PTE;
 
 	/*
 	 * Pre-check access permissions (will be re-checked atomically