[PATCH] ceph: Fix a memory leak in ci->i_head_snapc

[Luis Henriques]

I'm occasionally seeing a kmemleak warning in xfstest generic/013:

unreferenced object 0xffff8881fccca940 (size 32):
  comm "kworker/0:1", pid 12, jiffies 4295005883 (age 130.648s)
  hex dump (first 32 bytes):
    01 00 00 00 00 00 00 00 01 00 00 00 00 00 00 00  ................
    00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00  ................
  backtrace:
    [<00000000d741a1ea>] build_snap_context+0x5b/0x2a0
    [<0000000021a00533>] rebuild_snap_realms+0x27/0x90
    [<00000000ac538600>] rebuild_snap_realms+0x42/0x90
    [<000000000e955fac>] ceph_update_snap_trace+0x2ee/0x610
    [<00000000a9550416>] ceph_handle_snap+0x317/0x5f3
    [<00000000fc287b83>] dispatch+0x362/0x176c
    [<00000000a312c741>] ceph_con_workfn+0x9ce/0x2cf0
    [<000000004168e3a9>] process_one_work+0x1d4/0x400
    [<000000002188e9e7>] worker_thread+0x2d/0x3c0
    [<00000000b593e4b3>] kthread+0x112/0x130
    [<00000000a8587dca>] ret_from_fork+0x35/0x40
    [<00000000ba1c9c1d>] 0xffffffffffffffff

It looks like it is possible that we miss a flush_ack from the MDS when,
for example, umounting the filesystem.  In that case, we can simply drop
the reference to the ceph_snap_context obtained in ceph_queue_cap_snap().

Link: https://tracker.ceph.com/issues/38224
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Luis Henriques <lhenriques@xxxxxxxx>
---
 fs/ceph/caps.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c
index 36a8dc699448..208f4dc6f574 100644
--- a/fs/ceph/caps.c
+++ b/fs/ceph/caps.c
@@ -1054,6 +1054,7 @@ int ceph_is_any_caps(struct inode *inode)
 static void drop_inode_snap_realm(struct ceph_inode_info *ci)
 {
 	struct ceph_snap_realm *realm = ci->i_snap_realm;
+
 	spin_lock(&realm->inodes_with_caps_lock);
 	list_del_init(&ci->i_snap_realm_item);
 	ci->i_snap_realm_counter++;
@@ -1063,6 +1064,12 @@ static void drop_inode_snap_realm(struct ceph_inode_info *ci)
 	spin_unlock(&realm->inodes_with_caps_lock);
 	ceph_put_snap_realm(ceph_sb_to_client(ci->vfs_inode.i_sb)->mdsc,
 			    realm);
+	/*
+	 * ci->i_head_snapc should be NULL, but we may still be waiting for a
+	 * flush_ack from the MDS.  In that case, we still hold a ref for the
+	 * ceph_snap_context and we need to drop it.
+	 */
+	ceph_put_snap_context(ci->i_head_snapc);
 }
 
 /*