Patch "Btrfs: do not create empty block group if we have allocated data" has been added to the 4.5-stable tree

This is a note to let you know that I've just added the patch titled

Btrfs: do not create empty block group if we have allocated data

to the 4.5-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
btrfs-do-not-create-empty-block-group-if-we-have-allocated-data.patch
and it can be found in the queue-4.5 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Liu Bo <[email protected]>
Date: Mon, 14 Dec 2015 18:29:32 -0800
Subject: Btrfs: do not create empty block group if we have allocated data

From: Liu Bo <[email protected]>

commit cf25ce518e8ef9d59b292e51193bed2b023a32da upstream.

Now we force to create empty block group to keep data profile alive, however, in the below example, we eventually get an empty block group while we're trying to get more space for other types (metadata/system),

- Before,
block group "A": size=2G, used=1.2G
block group "B": size=2G, used=512M

- After "btrfs balance start -dusage=50 mount_point", block group "A": size=2G, used=(1.2+0.5)G block group "C": size=2G, used=0

Since there is no data in block group C, it won't be deleted automatically and we have to get the unused 2G until the next mount.

Balance itself just moves data and doesn't remove data, so it's safe to not create such a empty block group if we already have data
allocated in other block groups.

---
 fs/btrfs/volumes.c |    9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

@@ -3401,6 +3401,7 @@ static int __btrfs_balance(struct btrfs_
 	u32 count_meta = 0;
 	u32 count_sys = 0;
 	int chunk_reserved = 0;
+	u64 bytes_used = 0;
 
 	/* step one make some room on all the devices */
 	devices = &fs_info->fs_devices->devices;

@@ -3539,7 +3540,13 @@ again:
 			goto loop;
 		}
 
-		if ((chunk_type & BTRFS_BLOCK_GROUP_DATA) && !chunk_reserved) {
+		ASSERT(fs_info->data_sinfo);
+		spin_lock(&fs_info->data_sinfo->lock);
+		bytes_used = fs_info->data_sinfo->bytes_used;
+		spin_unlock(&fs_info->data_sinfo->lock);
+
+		if ((chunk_type & BTRFS_BLOCK_GROUP_DATA) &&
+		    !chunk_reserved && !bytes_used) {
 			trans = btrfs_start_transaction(chunk_root, 0);
 			if (IS_ERR(trans)) {
 				mutex_unlock(&fs_info->delete_unused_bgs_mutex);

Patches currently in stable-queue which might be from [email protected] are

queue-4.5/btrfs-fix-unexpected-return-value-of-fiemap.patch queue-4.5/btrfs-do-not-create-empty-block-group-if-we-have-allocated-data.patch queue-4.5/btrfs-fix-divide-error-upon-chunk-s-stripe_len.patch