Btrfs: fix fs mapping extent map leak

On chunk allocation error (label "error_del_extent"), after adding the
extent map to the tree and to the pending chunks list, we would leave
decrementing the extent map's refcount by 2 instead of 3 (our allocation
+ tree reference + list reference).

Also, on chunk/block group removal, if the block group was on the list
pending_chunks we weren't decrementing the respective list reference.

Detected by 'rmmod btrfs':

[20770.105881] kmem_cache_destroy btrfs_extent_map: Slab cache still has objects
[20770.106127] CPU: 2 PID: 11093 Comm: rmmod Tainted: G        W    L 3.17.0-rc5-btrfs-next-1+ #1
[20770.106128] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.7.5-0-ge51488c-20140602_164612-nilsson.home.kraxel.org 04/01/2014
[20770.106130]  0000000000000000 ffff8800ba867eb8 ffffffff813e7a13 ffff8800a2e11040
[20770.106132]  ffff8800ba867ed0 ffffffff81105d0c 0000000000000000 ffff8800ba867ee0
[20770.106134]  ffffffffa035d65e ffff8800ba867ef0 ffffffffa03b0654 ffff8800ba867f78
[20770.106136] Call Trace:
[20770.106142]  [<ffffffff813e7a13>] dump_stack+0x45/0x56
[20770.106145]  [<ffffffff81105d0c>] kmem_cache_destroy+0x4b/0x90
[20770.106164]  [<ffffffffa035d65e>] extent_map_exit+0x1a/0x1c [btrfs]
[20770.106176]  [<ffffffffa03b0654>] exit_btrfs_fs+0x27/0x9d3 [btrfs]
[20770.106179]  [<ffffffff8109dc97>] SyS_delete_module+0x153/0x1c4
[20770.106182]  [<ffffffff8121261b>] ? trace_hardirqs_on_thunk+0x3a/0x3c
[20770.106184]  [<ffffffff813ebf52>] system_call_fastpath+0x16/0x1b

This applies on top (depends on) of my previous patch titled:
"Btrfs: fix race between fs trimming and block group remove/allocation"

But the issue in fact was already present before that change, it only
became easier to hit after Josef's 3.18 patch that added automatic
removal of empty block groups.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>

2 files changed, 6 insertions, 0 deletions

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 1b85cf503aec..ba7042e2390b 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -9469,6 +9469,10 @@ int btrfs_remove_block_group(struct btrfs_trans_handle *trans,
         memcpy(&key, &block_group->key, sizeof(key));
 
         lock_chunks(root);
+        if (!list_empty(&em->list)) {
+                /* We're in the transaction->pending_chunks list. */
+                free_extent_map(em);
+        }
         spin_lock(&block_group->lock);
         block_group->removed = 1;
         /*
diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 588f37e0a564..ff2b35114972 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -4501,6 +4501,8 @@ error_del_extent:
         free_extent_map(em);
         /* One for the tree reference */
         free_extent_map(em);
+        /* One for the pending_chunks list reference */
+        free_extent_map(em);
 error:
         kfree(devices_info);
         return ret;