smb: Log an error when close_all_cached_dirs fails

BugLink: https://bugs.launchpad.net/bugs/2120812

commit a2182743a8b4969481f64aec4908ff162e8a206c upstream.

Under low-memory conditions, close_all_cached_dirs() can't move the
dentries to a separate list to dput() them once the locks are dropped.
This will result in a "Dentry still in use" error, so add an error
message that makes it clear this is what happened:

[  495.281119] CIFS: VFS: \\otters.example.com\share Out of memory while dropping dentries
[  495.281595] ------------[ cut here ]------------
[  495.281887] BUG: Dentry ffff888115531138{i=78,n=/}  still in use (2) [unmount of cifs cifs]
[  495.282391] WARNING: CPU: 1 PID: 2329 at fs/dcache.c:1536 umount_check+0xc8/0xf0

Also, bail out of looping through all tcons as soon as a single
allocation fails, since we're already in trouble, and kmalloc() attempts
for subseqeuent tcons are likely to fail just like the first one did.

Signed-off-by: Paul Aurich <paul@darkrain42.org>
Acked-by: Bharath SM <bharathsm@microsoft.com>
Suggested-by: Ruben Devos <rdevos@oxya.com>
Cc: stable@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Noah Wager <noah.wager@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>

Author: darkrain42

fs/smb/client/cached_dir.c

@@ -486,8 +486,17 @@ void close_all_cached_dirs(struct cifs_sb_info *cifs_sb)
 		spin_lock(&cfids->cfid_list_lock);
 		list_for_each_entry(cfid, &cfids->entries, entry) {
 			tmp_list = kmalloc(sizeof(*tmp_list), GFP_ATOMIC);
-			if (tmp_list == NULL)
-				break;
+			if (tmp_list == NULL) {
+				/*
+				 * If the malloc() fails, we won't drop all
+				 * dentries, and unmounting is likely to trigger
+				 * a 'Dentry still in use' error.
+				 */
+				cifs_tcon_dbg(VFS, "Out of memory while dropping dentries\n");
+				spin_unlock(&cfids->cfid_list_lock);
+				spin_unlock(&cifs_sb->tlink_tree_lock);
+				goto done;
+			}
 			spin_lock(&cfid->fid_lock);
 			tmp_list->dentry = cfid->dentry;
 			cfid->dentry = NULL;
@@ -499,6 +508,7 @@ void close_all_cached_dirs(struct cifs_sb_info *cifs_sb)
 	}
 	spin_unlock(&cifs_sb->tlink_tree_lock);
 
+done:
 	list_for_each_entry_safe(tmp_list, q, &entry, entry) {
 		list_del(&tmp_list->entry);
 		dput(tmp_list->dentry);