[PATCH] mm: powerpc ptlock comments

Update comments (only) on page_table_lock and mmap_sem in arch/powerpc.
Removed the comment on page_table_lock from hash_huge_page: since it's no
longer taking page_table_lock itself, it's irrelevant whether others are; but
how it is safe (even against huge file truncation?) I can't say.

Signed-off-by: Hugh Dickins <[e-mail]>
Cc: Paul Mackerras <[e-mail]>
Cc: Benjamin Herrenschmidt <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

arch/powerpc/mm/hugetlbpage.c

···
 	}
 
 	/*
-	 * No need to use ldarx/stdcx here because all who
-	 * might be updating the pte will hold the
-	 * page_table_lock
+	 * No need to use ldarx/stdcx here
 	 */
 	*ptep = __pte(new_pte & ~_PAGE_BUSY);
 

arch/powerpc/mm/mem.c

···
  * We use it to preload an HPTE into the hash table corresponding to
  * the updated linux PTE.
  * 
- * This must always be called with the mm->page_table_lock held
+ * This must always be called with the pte lock held.
  */
 void update_mmu_cache(struct vm_area_struct *vma, unsigned long address,
 		      pte_t pte)

arch/powerpc/mm/tlb_32.c

···
 		return;
 	}
 
+	/*
+	 * It is safe to go down the mm's list of vmas when called
+	 * from dup_mmap, holding mmap_sem.  It would also be safe from
+	 * unmap_region or exit_mmap, but not from vmtruncate on SMP -
+	 * but it seems dup_mmap is the only SMP case which gets here.
+	 */
 	for (mp = mm->mmap; mp != NULL; mp = mp->vm_next)
 		flush_range(mp->vm_mm, mp->vm_start, mp->vm_end);
 	FINISH_FLUSH;

arch/powerpc/mm/tlb_64.c

···
 
 void pgtable_free_tlb(struct mmu_gather *tlb, pgtable_free_t pgf)
 {
-	/* This is safe as we are holding page_table_lock */
+	/* This is safe since tlb_gather_mmu has disabled preemption */
         cpumask_t local_cpumask = cpumask_of_cpu(smp_processor_id());
 	struct pte_freelist_batch **batchp = &__get_cpu_var(pte_freelist_cur);
 
···
 
 void pte_free_finish(void)
 {
-	/* This is safe as we are holding page_table_lock */
+	/* This is safe since tlb_gather_mmu has disabled preemption */
 	struct pte_freelist_batch **batchp = &__get_cpu_var(pte_freelist_cur);
 
 	if (*batchp == NULL)