linux-kernel - [PATCH 4.5 109/128] Btrfs: do not create empty block group if we have allocated data

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Date:	Sun,  5 Jun 2016 15:24:24 -0700
From:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:	linux-kernel@...r.kernel.org
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	stable@...r.kernel.org, Liu Bo <bo.li.liu@...cle.com>,
	David Sterba <dsterba@...e.com>
Subject: [PATCH 4.5 109/128] Btrfs: do not create empty block group if we have allocated data

4.5-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Liu Bo <bo.li.liu@...cle.com>

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.

Signed-off-by: Liu Bo <bo.li.liu@...cle.com>
Signed-off-by: David Sterba <dsterba@...e.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>

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

--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -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);