btrfs: qgroup: check generation when recording simple quota delta

Simple quotas count extents only from the moment the feature is enabled.
Therefore, if we do something like:

1. create subvol S
2. write F in S
3. enable quotas
4. remove F
5. write G in S

then after 3. and 4. we would expect the simple quota usage of S to be 0
(putting aside some metadata extents that might be written) and after
5., it should be the size of G plus metadata. Therefore, we need to be
able to determine whether a particular quota delta we are processing
predates simple quota enablement.

To do this, store the transaction id when quotas were enabled. In
fs_info for immediate use and in the quota status item to make it
recoverable on mount. When we see a delta, check if the generation of
the extent item is less than that of quota enablement. If so, we should
ignore the delta from this extent.

Signed-off-by: Boris Burkov <boris@bur.io>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/include/uapi/linux/btrfs_tree.h b/include/uapi/linux/btrfs_tree.h
index 8b4def6aee52..c25fc9614594 100644
--- a/include/uapi/linux/btrfs_tree.h
+++ b/include/uapi/linux/btrfs_tree.h
@@ -1277,6 +1277,15 @@ struct btrfs_qgroup_status_item {
 	 * of the scan. It contains a logical address
 	 */
 	__le64 rescan;
+
+	/*
+	 * The generation when quotas were last enabled. Used by simple quotas to
+	 * avoid decrementing when freeing an extent that was written before
+	 * enable.
+	 *
+	 * Set only if flags contain BTRFS_QGROUP_STATUS_FLAG_SIMPLE_MODE.
+	 */
+	__le64 enable_gen;
 } __attribute__ ((__packed__));
 
 struct btrfs_qgroup_info_item {