[PATCH 13/17] btrfs: introduce BTRFS_NESTING_SPLIT for split blocks

Josef Bacik
2020-08-10T15:43:15Z

If we are splitting a leaf/node, we could do something like the
following

lock(leaf) BTRFS_NESTING_NORMAL
lock(left) BTRFS_NESTING_LEFT + BTRFS_NESTING_COW
push from leaf -> left
reset path to point to left
split left
allocate new block, lock block BTRFS_NESTING_SPLIT

at the new block point we need to have a different nesting level,
because we have already used either BTRFS_NESTING_LEFT or
BTRFS_NESTING_RIGHT when pushing items from the original leaf into the
adjacent leaves.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
---
fs/btrfs/ctree.c | 4 ++--
fs/btrfs/locking.h | 9 +++++++++
2 files changed, 11 insertions(+), 2 deletions(-)

diff --git a/fs/btrfs/ctree.c b/fs/btrfs/ctree.c
index 6b63b3bcacd4..82dac6510a86 100644
--- a/fs/btrfs/ctree.c
+++ b/fs/btrfs/ctree.c
@@ -3473,7 +3473,7 @@ static noinline int split_node(struct btrfs_trans_handle *trans,
btrfs_node_key(c, &disk_key, mid);

split = alloc_tree_block_no_bg_flush(trans, root, 0, &disk_key, level,
- c->start, 0, BTRFS_NESTING_NORMAL);
+ c->start, 0, BTRFS_NESTING_SPLIT);
if (IS_ERR(split))
return PTR_ERR(split);

@@ -4250,7 +4250,7 @@ static noinline int split_leaf(struct btrfs_trans_handle *trans,
btrfs_item_key(l, &disk_key, mid);

right = alloc_tree_block_no_bg_flush(trans, root, 0, &disk_key, 0,
- l->start, 0, BTRFS_NESTING_NORMAL);
+ l->start, 0, BTRFS_NESTING_SPLIT);
if (IS_ERR(right))
return PTR_ERR(right);

diff --git a/fs/btrfs/locking.h b/fs/btrfs/locking.h
index 31a87477b889..4f5586fed25a 100644
--- a/fs/btrfs/locking.h
+++ b/fs/btrfs/locking.h
@@ -46,6 +46,15 @@ enum btrfs_lock_nesting {
*/
BTRFS_NESTING_LEFT_COW,
BTRFS_NESTING_RIGHT_COW,
+
+ /*
+ * When splitting we may push nodes to the left or right, but still use
+ * the subsequent nodes in our path, keeping our locks on those adjacent
+ * blocks. Thus when we go to allocate a new split block we've already
+ * used up all of our available subclasses, so this subclass exists to
+ * handle this case where we need to allocate a new split block.
+ */
+ BTRFS_NESTING_SPLIT,
};

struct btrfs_path;
--
2.24.1