[PATCH v2 3/8] mm/rmap: remove unnecessary root lock dance in anon_vma clone, unmap

[Lorenzo Stoakes <lorenzo.stoakes@oracle.com>]

The root anon_vma of all anon_vma's linked to a VMA must by definition be
the same - a VMA and all of its descendants/ancestors must exist in the
same CoW chain.

Commit bb4aa39676f7 ("mm: avoid repeated anon_vma lock/unlock sequences in
anon_vma_clone()") introduced paranoid checking of the root anon_vma
remaining the same throughout all AVC's in 2011.

I think 15 years later we can safely assume that this is always the case.

Additionally, since unfaulted VMAs being cloned from or unlinked are
no-op's, we can simply lock the anon_vma's associated with this rather than
doing any specific dance around this.

This removes unnecessary checks and makes it clear that the root anon_vma
is shared between all anon_vma's in a given VMA's anon_vma_chain.

Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
---
 mm/rmap.c | 51 +++++++++++++++------------------------------------
 1 file changed, 15 insertions(+), 36 deletions(-)

diff --git a/mm/rmap.c b/mm/rmap.c
index de2cbe860566..6ac42671bedd 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -232,32 +232,6 @@ int __anon_vma_prepare(struct vm_area_struct *vma)
 	return -ENOMEM;
 }
 
-/*
- * This is a useful helper function for locking the anon_vma root as
- * we traverse the vma->anon_vma_chain, looping over anon_vma's that
- * have the same vma.
- *
- * Such anon_vma's should have the same root, so you'd expect to see
- * just a single mutex_lock for the whole traversal.
- */
-static inline struct anon_vma *lock_anon_vma_root(struct anon_vma *root, struct anon_vma *anon_vma)
-{
-	struct anon_vma *new_root = anon_vma->root;
-	if (new_root != root) {
-		if (WARN_ON_ONCE(root))
-			up_write(&root->rwsem);
-		root = new_root;
-		down_write(&root->rwsem);
-	}
-	return root;
-}
-
-static inline void unlock_anon_vma_root(struct anon_vma *root)
-{
-	if (root)
-		up_write(&root->rwsem);
-}
-
 static void check_anon_vma_clone(struct vm_area_struct *dst,
 				 struct vm_area_struct *src)
 {
@@ -310,26 +284,28 @@ static void cleanup_partial_anon_vmas(struct vm_area_struct *vma);
 int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
 {
 	struct anon_vma_chain *avc, *pavc;
-	struct anon_vma *root = NULL;
 
 	check_anon_vma_clone(dst, src);
 
 	if (!src->anon_vma)
 		return 0;
 
+	check_anon_vma_clone(dst, src);
+
+	/* 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;
 
 		avc = anon_vma_chain_alloc(GFP_NOWAIT);
 		if (unlikely(!avc)) {
-			unlock_anon_vma_root(root);
-			root = NULL;
+			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;
-		root = lock_anon_vma_root(root, anon_vma);
 		anon_vma_chain_link(dst, avc, anon_vma);
 
 		/*
@@ -346,7 +322,8 @@ int anon_vma_clone(struct vm_area_struct *dst, struct vm_area_struct *src)
 	}
 	if (dst->anon_vma)
 		dst->anon_vma->num_active_vmas++;
-	unlock_anon_vma_root(root);
+
+	anon_vma_unlock_write(src->anon_vma);
 	return 0;
 
  enomem_failure:
@@ -471,17 +448,19 @@ static void cleanup_partial_anon_vmas(struct vm_area_struct *vma)
 void unlink_anon_vmas(struct vm_area_struct *vma)
 {
 	struct anon_vma_chain *avc, *next;
-	struct anon_vma *root = NULL;
+	struct anon_vma *active_anon_vma = vma->anon_vma;
 
 	/* Always hold mmap lock, read-lock on unmap possibly. */
 	mmap_assert_locked(vma->vm_mm);
 
 	/* Unfaulted is a no-op. */
-	if (!vma->anon_vma) {
+	if (!active_anon_vma) {
 		VM_WARN_ON_ONCE(!list_empty(&vma->anon_vma_chain));
 		return;
 	}
 
+	anon_vma_lock_write(active_anon_vma);
+
 	/*
 	 * Unlink each anon_vma chained to the VMA.  This list is ordered
 	 * from newest to oldest, ensuring the root anon_vma gets freed last.
@@ -489,7 +468,6 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
 	list_for_each_entry_safe(avc, next, &vma->anon_vma_chain, same_vma) {
 		struct anon_vma *anon_vma = avc->anon_vma;
 
-		root = lock_anon_vma_root(root, anon_vma);
 		anon_vma_interval_tree_remove(avc, &anon_vma->rb_root);
 
 		/*
@@ -505,13 +483,14 @@ void unlink_anon_vmas(struct vm_area_struct *vma)
 		anon_vma_chain_free(avc);
 	}
 
-	vma->anon_vma->num_active_vmas--;
+	active_anon_vma->num_active_vmas--;
 	/*
 	 * vma would still be needed after unlink, and anon_vma will be prepared
 	 * when handle fault.
 	 */
 	vma->anon_vma = NULL;
-	unlock_anon_vma_root(root);
+	anon_vma_unlock_write(active_anon_vma);
+
 
 	/*
 	 * Iterate the list once more, it now only contains empty and unlinked
-- 
2.52.0