arm64: mm: Make flush_tlb_fix_spurious_fault() a no-op

Our use of broadcast TLB maintenance means that spurious page-faults
that have been handled already by another CPU do not require additional
TLB maintenance.

Make flush_tlb_fix_spurious_fault() a no-op and rely on the existing TLB
invalidation instead. Add an explicit flush_tlb_page() when making a page
dirty, as the TLB is permitted to cache the old read-only entry.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20200728092220.GA21800@willie-the-truck
Signed-off-by: Will Deacon <will@kernel.org>

arch/arm64/mm/fault.c

@@ -218,7 +218,9 @@ int ptep_set_access_flags(struct vm_area_struct *vma,
 		pteval = cmpxchg_relaxed(&pte_val(*ptep), old_pteval, pteval);
 	} while (pteval != old_pteval);
 
-	flush_tlb_fix_spurious_fault(vma, address);
+	/* Invalidate a stale read-only entry */
+	if (dirty)
+		flush_tlb_page(vma, address);
 	return 1;
 }