btrfs: track original extent owner in head_ref

Simple quotas requires tracking the original creating root of any given
extent. This gets complicated when multiple subvolumes create
overlapping/contradictory refs in the same transaction. For example,
due to modifying or deleting an extent while also snapshotting it.

To resolve this in a general way, take advantage of the fact that we are
essentially already tracking this for handling releasing reservations.
The head ref coalesces the various refs and uses must_insert_reserved to
check if it needs to create an extent/free reservation. Store the ref
that set must_insert_reserved as the owning ref on the head ref.

Note that this can result in writing an extent for the very first time
with an owner different from its only ref, but it will look the same as
if you first created it with the original owning ref, then added the
other ref, then removed the owning ref.

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

1 files changed, 7 insertions, 0 deletions

diff --git a/fs/btrfs/delayed-ref.h b/fs/btrfs/delayed-ref.h
index 84b37888bff3..47e4bdf4fb30 100644
--- a/fs/btrfs/delayed-ref.h
+++ b/fs/btrfs/delayed-ref.h
@@ -111,6 +111,12 @@ struct btrfs_delayed_ref_head {
 	int ref_mod;
 
 	/*
+	 * The root that triggered the allocation when must_insert_reserved is
+	 * set to true.
+	 */
+	u64 owning_root;
+
+	/*
 	 * when a new extent is allocated, it is just reserved in memory
 	 * The actual extent isn't inserted into the extent allocation tree
 	 * until the delayed ref is processed.  must_insert_reserved is
@@ -123,6 +129,7 @@ struct btrfs_delayed_ref_head {
 	 * the free has happened.
 	 */
 	bool must_insert_reserved;
+
 	bool is_data;
 	bool is_system;
 	bool processing;