Re: [PATCH 1/3] vfs: track count of child mounts

Ian Kent <raven@xxxxxxxxxx> · Wed, 20 Jul 2022 10:17:58 +0800

On 20/7/22 09:50, Al Viro wrote:
On Mon, Jul 11, 2022 at 11:37:40AM +0800, Ian Kent wrote:
While the total reference count of a mount is mostly all that's needed
the reference count corresponding to the mounts only is occassionally
also needed (for example, autofs checking if a tree of mounts can be
expired).

To make this reference count avaialble with minimal changes add a
counter to track the number of child mounts under a given mount. This
count can then be used to calculate the mounts only reference count.
No.  This is a wrong approach - instead of keeping track of number of
children, we should just stop having them contribute to refcount of
the parent.  Here's what I've got in my local tree; life gets simpler
that way.

Right, I'll grab this and run some tests.


Ian


commit e99f1f9cc864103f326a5352e6ce1e377613437f
Author: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Date:   Sat Jul 9 14:45:39 2022 -0400

     namespace: don't keep ->mnt_parent pinned
     
     makes refcounting more consistent
     
     Signed-off-by: Al Viro <viro@xxxxxxxxxxxxxxxxxx>

diff --git a/fs/namespace.c b/fs/namespace.c
index 68789f896f08..53c29110a0cd 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -906,7 +906,6 @@ void mnt_set_mountpoint(struct mount *mnt,
  			struct mount *child_mnt)
  {
  	mp->m_count++;
-	mnt_add_count(mnt, 1);	/* essentially, that's mntget */
  	child_mnt->mnt_mountpoint = mp->m_dentry;
  	child_mnt->mnt_parent = mnt;
  	child_mnt->mnt_mp = mp;
@@ -1429,22 +1428,18 @@ void mnt_cursor_del(struct mnt_namespace *ns, struct mount *cursor)
  int may_umount_tree(struct vfsmount *m)
  {
  	struct mount *mnt = real_mount(m);
-	int actual_refs = 0;
-	int minimum_refs = 0;
-	struct mount *p;
  	BUG_ON(!m);
  
  	/* write lock needed for mnt_get_count */
  	lock_mount_hash();
-	for (p = mnt; p; p = next_mnt(p, mnt)) {
-		actual_refs += mnt_get_count(p);
-		minimum_refs += 2;
+	for (struct mount *p = mnt; p; p = next_mnt(p, mnt)) {
+		int allowed = p == mnt ? 2 : 1;
+		if (mnt_get_count(p) > allowed) {
+			unlock_mount_hash();
+			return 0;
+		}
  	}
  	unlock_mount_hash();
-
-	if (actual_refs > minimum_refs)
-		return 0;
-
  	return 1;
  }
  
@@ -1586,7 +1581,6 @@ static void umount_tree(struct mount *mnt, enum umount_tree_flags how)
  
  		disconnect = disconnect_mount(p, how);
  		if (mnt_has_parent(p)) {
-			mnt_add_count(p->mnt_parent, -1);
  			if (!disconnect) {
  				/* Don't forget about p */
  				list_add_tail(&p->mnt_child, &p->mnt_parent->mnt_mounts);
@@ -2892,12 +2886,8 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
  		put_mountpoint(old_mp);
  out:
  	unlock_mount(mp);
-	if (!err) {
-		if (attached)
-			mntput_no_expire(parent);
-		else
-			free_mnt_ns(ns);
-	}
+	if (!err && !attached)
+		free_mnt_ns(ns);
  	return err;
  }
  
@@ -3869,7 +3859,7 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
  		const char __user *, put_old)
  {
  	struct path new, old, root;
-	struct mount *new_mnt, *root_mnt, *old_mnt, *root_parent, *ex_parent;
+	struct mount *new_mnt, *root_mnt, *old_mnt, *root_parent;
  	struct mountpoint *old_mp, *root_mp;
  	int error;
  
@@ -3900,10 +3890,9 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
  	new_mnt = real_mount(new.mnt);
  	root_mnt = real_mount(root.mnt);
  	old_mnt = real_mount(old.mnt);
-	ex_parent = new_mnt->mnt_parent;
  	root_parent = root_mnt->mnt_parent;
  	if (IS_MNT_SHARED(old_mnt) ||
-		IS_MNT_SHARED(ex_parent) ||
+		IS_MNT_SHARED(new_mnt->mnt_parent) ||
  		IS_MNT_SHARED(root_parent))
  		goto out4;
  	if (!check_mnt(root_mnt) || !check_mnt(new_mnt))
@@ -3942,7 +3931,6 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
  	attach_mnt(root_mnt, old_mnt, old_mp);
  	/* mount new_root on / */
  	attach_mnt(new_mnt, root_parent, root_mp);
-	mnt_add_count(root_parent, -1);
  	touch_mnt_namespace(current->nsproxy->mnt_ns);
  	/* A moved mount should not expire automatically */
  	list_del_init(&new_mnt->mnt_expire);
@@ -3952,8 +3940,6 @@ SYSCALL_DEFINE2(pivot_root, const char __user *, new_root,
  	error = 0;
  out4:
  	unlock_mount(old_mp);
-	if (!error)
-		mntput_no_expire(ex_parent);
  out3:
  	path_put(&root);
  out2:
diff --git a/fs/pnode.c b/fs/pnode.c
index 1106137c747a..e2c8a4b18857 100644
--- a/fs/pnode.c
+++ b/fs/pnode.c
@@ -368,7 +368,7 @@ static inline int do_refcount_check(struct mount *mnt, int count)
   */
  int propagate_mount_busy(struct mount *mnt, int refcnt)
  {
-	struct mount *m, *child, *topper;
+	struct mount *m, *child;
  	struct mount *parent = mnt->mnt_parent;
  
  	if (mnt == parent)
@@ -384,7 +384,6 @@ int propagate_mount_busy(struct mount *mnt, int refcnt)
  
  	for (m = propagation_next(parent, parent); m;
  	     		m = propagation_next(m, parent)) {
-		int count = 1;
  		child = __lookup_mnt(&m->mnt, mnt->mnt_mountpoint);
  		if (!child)
  			continue;
@@ -392,13 +391,10 @@ int propagate_mount_busy(struct mount *mnt, int refcnt)
  		/* Is there exactly one mount on the child that covers
  		 * it completely whose reference should be ignored?
  		 */
-		topper = find_topper(child);
-		if (topper)
-			count += 1;
-		else if (!list_empty(&child->mnt_mounts))
+		if (!find_topper(child) && !list_empty(&child->mnt_mounts))
  			continue;
  
-		if (do_refcount_check(child, count))
+		if (do_refcount_check(child, 1))
  			return 1;
  	}
  	return 0;






