Re: REGRESSION: Performance regressions from switching anon_vma->lock to mutex

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2011-06-17 at 13:28 +0200, Peter Zijlstra wrote:
> +static int anon_vma_unlink(struct anon_vma_chain *anon_vma_chain,
> struct anon_vma *anon_vma)
>  {
>         list_del(&anon_vma_chain->same_anon_vma);
>  
>         /* We must garbage collect the anon_vma if it's empty */
> +       if (list_empty(&anon_vma->head))
> +               return 1;
>  
> +       return 0;
>  } 

Is of course a little pathetic, so lets kill it..

---
Index: linux-2.6/mm/rmap.c
===================================================================
--- linux-2.6.orig/mm/rmap.c
+++ linux-2.6/mm/rmap.c
@@ -324,36 +324,42 @@ int anon_vma_fork(struct vm_area_struct
 	return -ENOMEM;
 }
 
-static void anon_vma_unlink(struct anon_vma_chain *anon_vma_chain)
-{
-	struct anon_vma *anon_vma = anon_vma_chain->anon_vma;
-	int empty;
-
-	/* If anon_vma_fork fails, we can get an empty anon_vma_chain. */
-	if (!anon_vma)
-		return;
-
-	anon_vma_lock(anon_vma);
-	list_del(&anon_vma_chain->same_anon_vma);
-
-	/* We must garbage collect the anon_vma if it's empty */
-	empty = list_empty(&anon_vma->head);
-	anon_vma_unlock(anon_vma);
-
-	if (empty)
-		put_anon_vma(anon_vma);
-}
-
 void unlink_anon_vmas(struct vm_area_struct *vma)
 {
 	struct anon_vma_chain *avc, *next;
+	struct anon_vma *root = NULL;
 
 	/*
 	 * Unlink each anon_vma chained to the VMA.  This list is ordered
 	 * from newest to oldest, ensuring the root anon_vma gets freed last.
 	 */
 	list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {
-		anon_vma_unlink(avc);
+		struct anon_vma *anon_vma = avc->anon_vma;
+
+		/* If anon_vma_fork fails, we can get an empty anon_vma_chain. */
+		if (anon_vma) {
+			root = lock_anon_vma_root(root, anon_vma);
+			list_del(&avc->same_anon_vma);
+			/* Leave empty anon_vmas on the list. */
+			if (list_empty(&anon_vma->head))
+				continue;
+		}
+		list_del(&avc->same_vma);
+		anon_vma_chain_free(avc);
+	}
+	unlock_anon_vma_root(root);
+
+	/*
+	 * Iterate the list once more, it now only contains empty and unlinked
+	 * anon_vmas, destroy them. Could not do before due to __put_anon_vma()
+	 * needing to acquire the anon_vma->root->mutex.
+	 */
+	list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {
+		struct anon_vma *anon_vma = avc->anon_vma;
+
+		if (anon_vma)
+			put_anon_vma(anon_vma);
+
 		list_del(&avc->same_vma);
 		anon_vma_chain_free(avc);
 	}