linux-kernel - [PATCH 3.16.y-ckt 006/216] btrfs: don't go readonly on existing qgroup items

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Date:	Mon, 12 Jan 2015 18:02:03 +0000
From:	Luis Henriques <luis.henriques@...onical.com>
To:	linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	kernel-team@...ts.ubuntu.com
Cc:	Mark Fasheh <mfasheh@...e.de>, Chris Mason <clm@...com>,
	"Sebastiaan L. Zoutendijk" <baszoutendijk@...il.com>,
	Luis Henriques <luis.henriques@...onical.com>
Subject: [PATCH 3.16.y-ckt 006/216] btrfs: don't go readonly on existing qgroup items

3.16.7-ckt4 -stable review patch.  If anyone has any objections, please let me know.

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

From: Mark Fasheh <mfasheh@...e.de>

commit 0b4699dcb65c2cff793210b07f40b98c2d423a43 upstream.

btrfs_drop_snapshot() leaves subvolume qgroup items on disk after
completion. This can cause problems with snapshot creation. If a new
snapshot tries to claim the deleted subvolumes id, btrfs will get -EEXIST
from add_qgroup_item() and go read-only. The following commands will
reproduce this problem (assume btrfs is on /dev/sda and is mounted at
/btrfs)

mkfs.btrfs -f /dev/sda
mount -t btrfs /dev/sda /btrfs/
btrfs quota enable /btrfs/
btrfs su sna /btrfs/ /btrfs/snap
btrfs su de /btrfs/snap
sleep 45
umount /btrfs/
mount -t btrfs /dev/sda /btrfs/

We can fix this by catching -EEXIST in add_qgroup_item() and
initializing the existing items. We have the problem of orphaned
relation items being on disk from an old snapshot but that is outside
the scope of this patch.

Signed-off-by: Mark Fasheh <mfasheh@...e.de>
Signed-off-by: Chris Mason <clm@...com>
Cc: Sebastiaan L. Zoutendijk <baszoutendijk@...il.com>
Signed-off-by: Luis Henriques <luis.henriques@...onical.com>
---
 fs/btrfs/qgroup.c | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
index 3eec914710b2..0e32f21e0868 100644
--- a/fs/btrfs/qgroup.c
+++ b/fs/btrfs/qgroup.c
@@ -551,9 +551,15 @@ static int add_qgroup_item(struct btrfs_trans_handle *trans,
 	key.type = BTRFS_QGROUP_INFO_KEY;
 	key.offset = qgroupid;
 
+	/*
+	 * Avoid a transaction abort by catching -EEXIST here. In that
+	 * case, we proceed by re-initializing the existing structure
+	 * on disk.
+	 */
+
 	ret = btrfs_insert_empty_item(trans, quota_root, path, &key,
 				      sizeof(*qgroup_info));
-	if (ret)
+	if (ret && ret != -EEXIST)
 		goto out;
 
 	leaf = path->nodes[0];
@@ -572,7 +578,7 @@ static int add_qgroup_item(struct btrfs_trans_handle *trans,
 	key.type = BTRFS_QGROUP_LIMIT_KEY;
 	ret = btrfs_insert_empty_item(trans, quota_root, path, &key,
 				      sizeof(*qgroup_limit));
-	if (ret)
+	if (ret && ret != -EEXIST)
 		goto out;
 
 	leaf = path->nodes[0];
-- 
2.1.4

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