powerpc/mm/books3s: Add new pte bit to mark pte temporarily invalid.

When splitting a huge pmd pte, we need to mark the pmd entry invalid. We
can do that by clearing _PAGE_PRESENT bit. But then that will be taken as a
swap pte. In order to differentiate between the two use a software pte bit
when invalidating.

For regular pte, due to bd5050e38aec ("powerpc/mm/radix: Change pte relax
sequence to handle nest MMU hang") we need to mark the pte entry invalid when
relaxing access permission. Instead of marking pte_none which can result in
different page table walk routines possibly skipping this pte entry, invalidate
it but still keep it marked present.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

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

@@ -44,6 +44,16 @@
 
 #define _PAGE_PTE		0x4000000000000000UL	/* distinguishes PTEs from pointers */
 #define _PAGE_PRESENT		0x8000000000000000UL	/* pte contains a translation */
+/*
+ * We need to mark a pmd pte invalid while splitting. We can do that by clearing
+ * the _PAGE_PRESENT bit. But then that will be taken as a swap pte. In order to
+ * differentiate between two use a SW field when invalidating.
+ *
+ * We do that temporary invalidate for regular pte entry in ptep_set_access_flags
+ *
+ * This is used only when _PAGE_PRESENT is cleared.
+ */
+#define _PAGE_INVALID		_RPAGE_SW0
 
 /*
  * Top and bottom bits of RPN which can be used by hash
@@ -568,7 +578,13 @@ static inline pte_t pte_clear_savedwrite(pte_t pte)
 
 static inline int pte_present(pte_t pte)
 {
-	return !!(pte_raw(pte) & cpu_to_be64(_PAGE_PRESENT));
+	/*
+	 * A pte is considerent present if _PAGE_PRESENT is set.
+	 * We also need to consider the pte present which is marked
+	 * 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));
 }
 
 #ifdef CONFIG_PPC_MEM_KEYS