powerpc/64s: free page table caches at exit_mmap time

The kernel page table caches are tied to init_mm, so there is no
more need for them after userspace is finished.

destroy_context() gets called when we drop the last reference for an
mm, which can be much later than the task exit due to other lazy mm
references to it. We can free the page table cache pages on task exit
because they only cache the userspace page tables and kernel threads
should not access user space addresses.

The mapping for kernel threads itself is maintained in init_mm and
page table cache for that is attached to init_mm.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
[mpe: Merge change log additions from Aneesh]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/mm/mmu_context_book3s64.c

@@ -221,7 +221,7 @@ static void pmd_frag_destroy(void *pmd_frag)
 	}
 }
 
-static void destroy_pagetable_page(struct mm_struct *mm)
+static void destroy_pagetable_cache(struct mm_struct *mm)
 {
 	void *frag;
 
@@ -244,13 +244,14 @@ void destroy_context(struct mm_struct *mm)
 		WARN_ON(process_tb[mm->context.id].prtb0 != 0);
 	else
 		subpage_prot_free(mm);
-	destroy_pagetable_page(mm);
 	destroy_contexts(&mm->context);
 	mm->context.id = MMU_NO_CONTEXT;
 }
 
 void arch_exit_mmap(struct mm_struct *mm)
 {
+	destroy_pagetable_cache(mm);
+
 	if (radix_enabled()) {
 		/*
 		 * Radix doesn't have a valid bit in the process table