btrfs: qgroup: iterate qgroups without memory allocation for qgroup_reserve()

Qgroup heavily relies on ulist to go through all the involved
qgroups, but since we're using ulist inside fs_info->qgroup_lock
spinlock, this means we're doing a lot of GFP_ATOMIC allocations.

This patch reduces the GFP_ATOMIC usage for qgroup_reserve() by
eliminating the memory allocation completely.

This is done by moving the needed memory to btrfs_qgroup::iterator
list_head, so that we can put all the involved qgroup into a on-stack
list, thus eliminating the need to allocate memory while holding
spinlock.

The only cost is the slightly higher memory usage, but considering the
reduce GFP_ATOMIC during a hot path, it should still be acceptable.

Function qgroup_reserve() is the perfect start point for this
conversion.

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

1 files changed, 9 insertions, 0 deletions

diff --git a/fs/btrfs/qgroup.h b/fs/btrfs/qgroup.h
index 7bffa10589d6..6a43618580b6 100644
--- a/fs/btrfs/qgroup.h
+++ b/fs/btrfs/qgroup.h
@@ -220,6 +220,15 @@ struct btrfs_qgroup {
 	struct list_head groups;  /* groups this group is member of */
 	struct list_head members; /* groups that are members of this group */
 	struct list_head dirty;   /* dirty groups */
+
+	/*
+	 * For qgroup iteration usage.
+	 *
+	 * The iteration list should always be empty until qgroup_iterator_add()
+	 * is called.  And should be reset to empty after the iteration is
+	 * finished.
+	 */
+	struct list_head iterator;
 	struct rb_node node;	  /* tree of qgroups */
 
 	/*