When open_by_handle_at() is used with iso9660 filesystems, exportfs
creates disconnected dentries during file handle resolution. If the
operation fails (e.g., with -ESTALE during reconnect_path()), these
dentries remain cached with their associated inodes.

During unmount, shrink_dcache_for_umount() does not fully evict these
disconnected dentries, leaving their inodes with non-zero reference
counts. This triggers the "VFS: Busy inodes after unmount" warning
and causes inode leaks that accumulate across mount/unmount cycles.

The issue occurs because:
1. open_by_handle_at() calls exportfs_decode_fh_raw() to resolve
   file handles
2. For iso9660 with Joliet extensions, this creates disconnected
   dentries for both primary (iso9660) and secondary (Joliet) root
   inodes
3. When path reconnection fails with -ESTALE, the dentries are left
   in DCACHE_DISCONNECTED state
4. shrink_dcache_for_umount() in generic_shutdown_super() does not
   aggressively evict these disconnected dentries
5. The associated inodes (typically root inodes 1792 and 1807)
   remain with i_count=1, triggering the busy inode check

Add explicit shrink_dcache_sb() call in isofs_put_super() to ensure
all cached dentries, including disconnected ones created by exportfs
operations, are released before the superblock is destroyed.

Reported-by: syzbot+1d79ebe5383fc016cf07@syzkaller.appspotmail.com
Tested-by: syzbot+1d79ebe5383fc016cf07@syzkaller.appspotmail.com
Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
---
 fs/isofs/inode.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/fs/isofs/inode.c b/fs/isofs/inode.c
index 6f0e6b19383c..bee410705442 100644
--- a/fs/isofs/inode.c
+++ b/fs/isofs/inode.c
@@ -52,6 +52,7 @@ static int isofs_dentry_cmp_ms(const struct dentry *dentry,
 static void isofs_put_super(struct super_block *sb)
 {
 	struct isofs_sb_info *sbi = ISOFS_SB(sb);
+	shrink_dcache_sb(sb);
 
 #ifdef CONFIG_JOLIET
 	unload_nls(sbi->s_nls_iocharset);
-- 
2.43.0

On Wed 01-10-25 15:13:10, Deepanshu Kartikey wrote:
> When open_by_handle_at() is used with iso9660 filesystems, exportfs
> creates disconnected dentries during file handle resolution. If the
> operation fails (e.g., with -ESTALE during reconnect_path()), these
> dentries remain cached with their associated inodes.
> 
> During unmount, shrink_dcache_for_umount() does not fully evict these
> disconnected dentries, leaving their inodes with non-zero reference
> counts. This triggers the "VFS: Busy inodes after unmount" warning
> and causes inode leaks that accumulate across mount/unmount cycles.
> 
> The issue occurs because:
> 1. open_by_handle_at() calls exportfs_decode_fh_raw() to resolve
>    file handles
> 2. For iso9660 with Joliet extensions, this creates disconnected
>    dentries for both primary (iso9660) and secondary (Joliet) root
>    inodes
> 3. When path reconnection fails with -ESTALE, the dentries are left
>    in DCACHE_DISCONNECTED state

True, but when reconnection fails, exportfs_decode_fh_raw() calls dput() on
the created dentry and dput() immediately destroys DCACHE_DISCONNECTED
dentries. So I'm not following how these dentries could still survive until
umount(). Can you please explain?

> 4. shrink_dcache_for_umount() in generic_shutdown_super() does not
>    aggressively evict these disconnected dentries
> 5. The associated inodes (typically root inodes 1792 and 1807)
>    remain with i_count=1, triggering the busy inode check
> 
> Add explicit shrink_dcache_sb() call in isofs_put_super() to ensure
> all cached dentries, including disconnected ones created by exportfs
> operations, are released before the superblock is destroyed.

This is almost certainly a wrong way of fixing the problem. First we need
to better understand why DCACHE_DISCONNECTED aren't getting properly
evicted...

								Honza

> 
> Reported-by: syzbot+1d79ebe5383fc016cf07@syzkaller.appspotmail.com
> Tested-by: syzbot+1d79ebe5383fc016cf07@syzkaller.appspotmail.com
> Signed-off-by: Deepanshu Kartikey <kartikey406@gmail.com>
> ---
>  fs/isofs/inode.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/fs/isofs/inode.c b/fs/isofs/inode.c
> index 6f0e6b19383c..bee410705442 100644
> --- a/fs/isofs/inode.c
> +++ b/fs/isofs/inode.c
> @@ -52,6 +52,7 @@ static int isofs_dentry_cmp_ms(const struct dentry *dentry,
>  static void isofs_put_super(struct super_block *sb)
>  {
>  	struct isofs_sb_info *sbi = ISOFS_SB(sb);
> +	shrink_dcache_sb(sb);
>  
>  #ifdef CONFIG_JOLIET
>  	unload_nls(sbi->s_nls_iocharset);
> -- 
> 2.43.0
> 
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR