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From:   David Howells <dhowells@redhat.com>
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To:     Alan Jenkins <alan.christopher.jenkins@gmail.com>,
        viro@zeniv.linux.org.uk
Cc:     dhowells@redhat.com, torvalds@linux-foundation.org,
        ebiederm@xmission.com, linux-fsdevel@vger.kernel.org,
        linux-kernel@vger.kernel.org, mszeredi@redhat.com
Subject: Re: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE_CLONE [ver #12]
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Date:   Fri, 19 Oct 2018 14:37:57 +0100
Message-ID: <6518.1539956277@warthog.procyon.org.uk>
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Alan Jenkins <alan.christopher.jenkins@gmail.com> wrote:

> If I close() the mount FD "mfd", and then do "mount --move . /mnt", my
> printk() shows MNT_UMOUNT has been set. ( I guess fchdir() works more like
> openat(... , O_PATH) than dup() ). Then unmounting /mnt hangs, as I would
> expect from my previous test.

Okay, I think the attached should fix it.

The issue being that do_move_mount() calls attach_recursive_mnt() with a NULL
parent_path, which means that the moved-mount doesn't get its refcount
incremented.

David
---
diff --git a/fs/namespace.c b/fs/namespace.c
index 6370bfabec99..ce9fff980549 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -1935,7 +1935,8 @@ int count_mounts(struct mnt_namespace *ns, struct mount *mnt)
 static int attach_recursive_mnt(struct mount *source_mnt,
 			struct mount *dest_mnt,
 			struct mountpoint *dest_mp,
-			struct path *parent_path)
+			struct path *parent_path,
+			bool moving)
 {
 	HLIST_HEAD(tree_list);
 	struct mnt_namespace *ns = dest_mnt->mnt_ns;
@@ -1976,6 +1977,8 @@ static int attach_recursive_mnt(struct mount *source_mnt,
 		attach_mnt(source_mnt, dest_mnt, dest_mp);
 		touch_mnt_namespace(source_mnt->mnt_ns);
 	} else {
+		if (moving)
+			mnt_add_count(source_mnt, 1);
 		mnt_set_mountpoint(dest_mnt, dest_mp, source_mnt);
 		commit_tree(source_mnt);
 	}
@@ -2062,7 +2065,7 @@ static int graft_tree(struct mount *mnt, struct mount *p, struct mountpoint *mp)
 	      d_is_dir(mnt->mnt.mnt_root))
 		return -ENOTDIR;
 
-	return attach_recursive_mnt(mnt, p, mp, NULL);
+	return attach_recursive_mnt(mnt, p, mp, NULL, false);
 }
 
 /*
@@ -2522,7 +2525,7 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
 			goto out1;
 
 	err = attach_recursive_mnt(old, real_mount(new_path->mnt), mp,
-				   attached ? &parent_path : NULL);
+				   attached ? &parent_path : NULL, true);
 	if (err)
 		goto out1;