ARM: 7985/1: mm: implement pte_accessible for faulting mappings

The pte_accessible macro can be used to identify page table entries
capable of being cached by a TLB. In principle, this differs from
pte_present, since PROT_NONE mappings are mapped using invalid entries
identified as present and ptes designated as `old' can use either
invalid entries or those with the access flag cleared (guaranteed not to
be in the TLB). However, there is a race to take care of, as described
in 20841405940e ("mm: fix TLB flush race between migration, and
change_protection_range"), between a page being migrated and mprotected
at the same time. In this case, we can check whether a TLB invalidation
is pending for the mm and if so, temporarily consider PROT_NONE mappings
as valid.

This patch implements a quick pte_accessible macro for ARM by simply
checking if the pte is valid/present depending on the mm. For classic
MMU, these checks are identical and will generate some false positives
for PROT_NONE mappings, but this is better than the current asm-generic
definition of ((void)(pte),1).

Finally, pte_present_user is moved to use pte_valid (and renamed
appropriately) since we don't care about cache flushing for faulting
mappings.

Acked-by: Steve Capper <steve.capper@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>

arch/arm/include/asm/pgtable.h

@@ -216,13 +216,16 @@ static inline pte_t *pmd_page_vaddr(pmd_t pmd)
 
 #define pte_none(pte)		(!pte_val(pte))
 #define pte_present(pte)	(pte_val(pte) & L_PTE_PRESENT)
+#define pte_valid(pte)		(pte_val(pte) & L_PTE_VALID)
+#define pte_accessible(mm, pte)	(mm_tlb_flush_pending(mm) ? pte_present(pte) : pte_valid(pte))
 #define pte_write(pte)		(!(pte_val(pte) & L_PTE_RDONLY))
 #define pte_dirty(pte)		(pte_val(pte) & L_PTE_DIRTY)
 #define pte_young(pte)		(pte_val(pte) & L_PTE_YOUNG)
 #define pte_exec(pte)		(!(pte_val(pte) & L_PTE_XN))
 #define pte_special(pte)	(0)
 
-#define pte_present_user(pte)  (pte_present(pte) && (pte_val(pte) & L_PTE_USER))
+#define pte_valid_user(pte)	\
+	(pte_valid(pte) && (pte_val(pte) & L_PTE_USER) && pte_young(pte))
 
 #if __LINUX_ARM_ARCH__ < 6
 static inline void __sync_icache_dcache(pte_t pteval)
@@ -237,7 +240,7 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr,
 {
 	unsigned long ext = 0;
 
-	if (addr < TASK_SIZE && pte_present_user(pteval)) {
+	if (addr < TASK_SIZE && pte_valid_user(pteval)) {
 		__sync_icache_dcache(pteval);
 		ext |= PTE_EXT_NG;
 	}