Re: [PATCH v2 1/8] mm/rmap: improve anon_vma_clone(), unlink_anon_vmas() comments, add asserts

[Andrew Morton]

On Wed, 14 Jan 2026 19:02:20 +0000 Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx> wrote:

> Can you apply the below fix-patch to this to fix up a rather silly
> failure-to-unlock mistake that Suren picked up on?
> 
> Luckily this partial unmap function is unlikely to ever be triggerable in real
> life, AND more to the point - a later patch completely eliminates the locking -
> but to avoid bisection hazard let's fix this.
> 
> Note that there is a conflict at 'mm/rmap: allocate anon_vma_chain objects
> unlocked when possible', please resolve it by just taking that patch and
> dropping _everything_ from this one _including_ the trailing 'if (root) ...'
> code.

No probs. 
mm-rmap-allocate-anon_vma_chain-objects-unlocked-when-possible.patch is
now

--- a/mm/rmap.c~mm-rmap-allocate-anon_vma_chain-objects-unlocked-when-possible
+++ a/mm/rmap.c
@@ -147,14 +147,13 @@ static void anon_vma_chain_free(struct a
 	kmem_cache_free(anon_vma_chain_cachep, anon_vma_chain);
 }
 
-static void anon_vma_chain_link(struct vm_area_struct *vma,
-				struct anon_vma_chain *avc,
-				struct anon_vma *anon_vma)
+static void anon_vma_chain_assign(struct vm_area_struct *vma,
+				  struct anon_vma_chain *avc,
+				  struct anon_vma *anon_vma)
 {
 	avc->vma = vma;
 	avc->anon_vma = anon_vma;
 	list_add(&avc->same_vma, &vma->anon_vma_chain);
-	anon_vma_interval_tree_insert(avc, &anon_vma->rb_root);
 }
 
 /**
@@ -211,7 +210,8 @@ int __anon_vma_prepare(struct vm_area_st
 	spin_lock(&mm->page_table_lock);
 	if (likely(!vma->anon_vma)) {
 		vma->anon_vma = anon_vma;
-		anon_vma_chain_link(vma, avc, anon_vma);
+		anon_vma_chain_assign(vma, avc, anon_vma);
+		anon_vma_interval_tree_insert(avc, &anon_vma->rb_root);
 		anon_vma->num_active_vmas++;
 		allocated = NULL;
 		avc = NULL;
@@ -292,21 +292,31 @@ int anon_vma_clone(struct vm_area_struct
 
 	check_anon_vma_clone(dst, src);
 
-	/* All anon_vma's share the same root. */
+	/*
+	 * Allocate AVCs. We don't need an anon_vma lock for this as we
+	 * are not updating the anon_vma rbtree nor are we changing
+	 * anon_vma statistics.
+	 *
+	 * We hold the exclusive mmap write lock so there's no possibliity of
+	 * the unlinked AVC's being observed yet.
+	 */
+	list_for_each_entry(pavc, &src->anon_vma_chain, same_vma) {
+		avc = anon_vma_chain_alloc(GFP_KERNEL);
+		if (!avc)
+			goto enomem_failure;
+
+		anon_vma_chain_assign(dst, avc, pavc->anon_vma);
+	}
+
+	/*
+	 * Now link the anon_vma's back to the newly inserted AVCs.
+	 * Note that all anon_vma's share the same root.
+	 */
 	anon_vma_lock_write(src->anon_vma);
-	list_for_each_entry_reverse(pavc, &src->anon_vma_chain, same_vma) {
-		struct anon_vma *anon_vma;
+	list_for_each_entry_reverse(avc, &dst->anon_vma_chain, same_vma) {
+		struct anon_vma *anon_vma = avc->anon_vma;
 
-		avc = anon_vma_chain_alloc(GFP_NOWAIT);
-		if (unlikely(!avc)) {
-			anon_vma_unlock_write(src->anon_vma);
-			avc = anon_vma_chain_alloc(GFP_KERNEL);
-			if (!avc)
-				goto enomem_failure;
-			anon_vma_lock_write(src->anon_vma);
-		}
-		anon_vma = pavc->anon_vma;
-		anon_vma_chain_link(dst, avc, anon_vma);
+		anon_vma_interval_tree_insert(avc, &anon_vma->rb_root);
 
 		/*
 		 * Reuse existing anon_vma if it has no vma and only one
@@ -322,7 +332,6 @@ int anon_vma_clone(struct vm_area_struct
 	}
 	if (dst->anon_vma)
 		dst->anon_vma->num_active_vmas++;
-
 	anon_vma_unlock_write(src->anon_vma);
 	return 0;
 
@@ -384,8 +393,10 @@ int anon_vma_fork(struct vm_area_struct
 	get_anon_vma(anon_vma->root);
 	/* Mark this anon_vma as the one where our new (COWed) pages go. */
 	vma->anon_vma = anon_vma;
+	anon_vma_chain_assign(vma, avc, anon_vma);
+	/* Now let rmap see it. */
 	anon_vma_lock_write(anon_vma);
-	anon_vma_chain_link(vma, avc, anon_vma);
+	anon_vma_interval_tree_insert(avc, &anon_vma->rb_root);
 	anon_vma->parent->num_children++;
 	anon_vma_unlock_write(anon_vma);
 
@@ -402,40 +413,18 @@ int anon_vma_fork(struct vm_area_struct
  * In the unfortunate case of anon_vma_clone() failing to allocate memory we
  * have to clean things up.
  *
- * On clone we hold the exclusive mmap write lock, so we can't race
- * unlink_anon_vmas(). Since we're cloning, we know we can't have empty
- * anon_vma's, since existing anon_vma's are what we're cloning from.
- *
- * So this function needs only traverse the anon_vma_chain and free each
- * allocated anon_vma_chain.
+ * Since we allocate anon_vma_chain's before we insert them into the interval
+ * trees, we simply have to free up the AVC's and remove the entries from the
+ * VMA's anon_vma_chain.
  */
 static void cleanup_partial_anon_vmas(struct vm_area_struct *vma)
 {
 	struct anon_vma_chain *avc, *next;
-	struct anon_vma *root = NULL;
-
-	/*
-	 * We exclude everybody else from being able to modify anon_vma's
-	 * underneath us.
-	 */
-	mmap_assert_locked(vma->vm_mm);
 
 	list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {
-		struct anon_vma *anon_vma = avc->anon_vma;
-
-		/* All anon_vma's share the same root. */
-		if (!root) {
-			root = anon_vma->root;
-			anon_vma_lock_write(root);
-		}
-
-		anon_vma_interval_tree_remove(avc, &anon_vma->rb_root);
 		list_del(&avc->same_vma);
 		anon_vma_chain_free(avc);
 	}
-
-	if (root)
-		anon_vma_unlock_write(root);
 }
 
 /**
_