[3.16.y-ckt stable] Patch "btrfs: don't go readonly on existing qgroup items" has been added to staging queue

[Luis Henriques]

This is a note to let you know that I have just added a patch titled

    btrfs: don't go readonly on existing qgroup items

to the linux-3.16.y-queue branch of the 3.16.y-ckt extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.16.y-queue

This patch is scheduled to be released in version 3.16.7-ckt4.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.16.y-ckt tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Luis

------

>From a3871f584e03957c1c5abd9d1442ebec8d24fe8f Mon Sep 17 00:00:00 2001
From: Mark Fasheh <mfasheh at suse.de>
Date: Mon, 18 Aug 2014 14:01:17 -0700
Subject: btrfs: don't go readonly on existing qgroup items

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 at suse.de>
Signed-off-by: Chris Mason <clm at fb.com>
Cc: Sebastiaan L. Zoutendijk <baszoutendijk at gmail.com>
Signed-off-by: Luis Henriques <luis.henriques at canonical.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