btrfs: qgroup: Avoid calling qgroup functions if qgroup is not enabled

Some qgroup trace events like btrfs_qgroup_release_data() and btrfs_qgroup_free_delayed_ref() can still be triggered even qgroup is not enabled.

This is caused by the lack of qgroup status check before really calling qgroup functions.
Thankfully related functions can handle quota disabled case well and just do nothing for qgroup disabled case.

This patch will do earlier check before triggering related trace events.

And for enabled <-> disabled race case:
1) For enabled->disabled case
Disable will wipe out all qgroups data including reservation and excl/rfer. Even we leaks some reserveration or numbers, it will still be wiped, so nothing can go wrong.

2) For disabled -> enabled case
Current btrfs_qgroup_release_data() will use extent_io tree to ensure we won't underflow reservation. And for delayed_ref we use head->qgroup_reserved to record reserved space, so in that case head->qgroup_reserved should be 0 and we won't underflow.

---
 fs/btrfs/qgroup.c | 4 ++++
 fs/btrfs/qgroup.h | 2 ++
 2 files changed, 6 insertions(+)

index 4353bb69bb86..0656f4e78db6 100644
@@ -3107,6 +3107,10 @@ static int __btrfs_qgroup_release_data(struct inode *inode,
 	int trace_op = QGROUP_RELEASE;
 	int ret;
 
+	if (!test_bit(BTRFS_FS_QUOTA_ENABLED,
+		      &BTRFS_I(inode)->root->fs_info->flags))
+		return 0;
+
 	/* In release case, we shouldn't have @reserved */
 	WARN_ON(!free && reserved);
 	if (free && reserved)

index 54b8bb282c0e..4bbcc1e92a93 100644
@@ -249,6 +249,8 @@ void btrfs_qgroup_free_refroot(struct btrfs_fs_info *fs_info,
 static inline void btrfs_qgroup_free_delayed_ref(struct btrfs_fs_info *fs_info,
 						 u64 ref_root, u64 num_bytes)
 {
+	if (!test_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags))
+		return;
 	trace_btrfs_qgroup_free_delayed_ref(fs_info, ref_root, num_bytes);
 	btrfs_qgroup_free_refroot(fs_info, ref_root, num_bytes,
 				  BTRFS_QGROUP_RSV_DATA);

2.19.1

I've added the link to the report.