btrfs: qgroup: Introduce per-root swapped blocks infrastructure

To allow delayed subtree swap rescan, btrfs needs to record per-root
information about which tree blocks get swapped.  This patch introduces
the required infrastructure.

The designed workflow will be:

1) Record the subtree root block that gets swapped.

   During subtree swap:
   O = Old tree blocks
   N = New tree blocks
         reloc tree                         subvolume tree X
            Root                               Root
           /    \                             /    \
         NA     OB                          OA      OB
       /  |     |  \                      /  |      |  \
     NC  ND     OE  OF                   OC  OD     OE  OF

  In this case, NA and OA are going to be swapped, record (NA, OA) into
  subvolume tree X.

2) After subtree swap.
         reloc tree                         subvolume tree X
            Root                               Root
           /    \                             /    \
         OA     OB                          NA      OB
       /  |     |  \                      /  |      |  \
     OC  OD     OE  OF                   NC  ND     OE  OF

3a) COW happens for OB
    If we are going to COW tree block OB, we check OB's bytenr against
    tree X's swapped_blocks structure.
    If it doesn't fit any, nothing will happen.

3b) COW happens for NA
    Check NA's bytenr against tree X's swapped_blocks, and get a hit.
    Then we do subtree scan on both subtrees OA and NA.
    Resulting 6 tree blocks to be scanned (OA, OC, OD, NA, NC, ND).

    Then no matter what we do to subvolume tree X, qgroup numbers will
    still be correct.
    Then NA's record gets removed from X's swapped_blocks.

4)  Transaction commit
    Any record in X's swapped_blocks gets removed, since there is no
    modification to swapped subtrees, no need to trigger heavy qgroup
    subtree rescan for them.

This will introduce 128 bytes overhead for each btrfs_root even qgroup
is not enabled. This is to reduce memory allocations and potential
failures.
Signed-off-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -1208,6 +1208,17 @@ enum {
 	BTRFS_ROOT_DEAD_RELOC_TREE,
 };
 
+/*
+ * Record swapped tree blocks of a subvolume tree for delayed subtree trace
+ * code. For detail check comment in fs/btrfs/qgroup.c.
+ */
+struct btrfs_qgroup_swapped_blocks {
+	spinlock_t lock;
+	/* RM_EMPTY_ROOT() of above blocks[] */
+	bool swapped;
+	struct rb_root blocks[BTRFS_MAX_LEVEL];
+};
+
 /*
  * in ram representation of the tree.  extent_root is used for all allocations
  * and for the extent tree extent_root root.
@@ -1343,6 +1354,9 @@ struct btrfs_root {
 	/* Number of active swapfiles */
 	atomic_t nr_swapfiles;
 
+	/* Record pairs of swapped blocks for qgroup */
+	struct btrfs_qgroup_swapped_blocks swapped_blocks;
+
 #ifdef CONFIG_BTRFS_FS_RUN_SANITY_TESTS
 	u64 alloc_bytenr;
 #endif

fs/btrfs/disk-io.c

@@ -1220,6 +1220,7 @@ static void __setup_root(struct btrfs_root *root, struct btrfs_fs_info *fs_info,
 	root->anon_dev = 0;
 
 	spin_lock_init(&root->root_item_lock);
+	btrfs_qgroup_init_swapped_blocks(&root->swapped_blocks);
 }
 
 static struct btrfs_root *btrfs_alloc_root(struct btrfs_fs_info *fs_info,

fs/btrfs/qgroup.c

@@ -3818,3 +3818,153 @@ void btrfs_qgroup_check_reserved_leak(struct inode *inode)
 	}
 	extent_changeset_release(&changeset);
 }
+
+void btrfs_qgroup_init_swapped_blocks(
+	struct btrfs_qgroup_swapped_blocks *swapped_blocks)
+{
+	int i;
+
+	spin_lock_init(&swapped_blocks->lock);
+	for (i = 0; i < BTRFS_MAX_LEVEL; i++)
+		swapped_blocks->blocks[i] = RB_ROOT;
+	swapped_blocks->swapped = false;
+}
+
+/*
+ * Delete all swapped blocks record of @root.
+ * Every record here means we skipped a full subtree scan for qgroup.
+ *
+ * Gets called when committing one transaction.
+ */
+void btrfs_qgroup_clean_swapped_blocks(struct btrfs_root *root)
+{
+	struct btrfs_qgroup_swapped_blocks *swapped_blocks;
+	int i;
+
+	swapped_blocks = &root->swapped_blocks;
+
+	spin_lock(&swapped_blocks->lock);
+	if (!swapped_blocks->swapped)
+		goto out;
+	for (i = 0; i < BTRFS_MAX_LEVEL; i++) {
+		struct rb_root *cur_root = &swapped_blocks->blocks[i];
+		struct btrfs_qgroup_swapped_block *entry;
+		struct btrfs_qgroup_swapped_block *next;
+
+		rbtree_postorder_for_each_entry_safe(entry, next, cur_root,
+						     node)
+			kfree(entry);
+		swapped_blocks->blocks[i] = RB_ROOT;
+	}
+	swapped_blocks->swapped = false;
+out:
+	spin_unlock(&swapped_blocks->lock);
+}
+
+/*
+ * Add subtree roots record into @subvol_root.
+ *
+ * @subvol_root:	tree root of the subvolume tree get swapped
+ * @bg:			block group under balance
+ * @subvol_parent/slot:	pointer to the subtree root in subvolume tree
+ * @reloc_parent/slot:	pointer to the subtree root in reloc tree
+ *			BOTH POINTERS ARE BEFORE TREE SWAP
+ * @last_snapshot:	last snapshot generation of the subvolume tree
+ */
+int btrfs_qgroup_add_swapped_blocks(struct btrfs_trans_handle *trans,
+		struct btrfs_root *subvol_root,
+		struct btrfs_block_group_cache *bg,
+		struct extent_buffer *subvol_parent, int subvol_slot,
+		struct extent_buffer *reloc_parent, int reloc_slot,
+		u64 last_snapshot)
+{
+	struct btrfs_fs_info *fs_info = subvol_root->fs_info;
+	struct btrfs_qgroup_swapped_blocks *blocks = &subvol_root->swapped_blocks;
+	struct btrfs_qgroup_swapped_block *block;
+	struct rb_node **cur;
+	struct rb_node *parent = NULL;
+	int level = btrfs_header_level(subvol_parent) - 1;
+	int ret = 0;
+
+	if (!test_bit(BTRFS_FS_QUOTA_ENABLED, &fs_info->flags))
+		return 0;
+
+	if (btrfs_node_ptr_generation(subvol_parent, subvol_slot) >
+	    btrfs_node_ptr_generation(reloc_parent, reloc_slot)) {
+		btrfs_err_rl(fs_info,
+		"%s: bad parameter order, subvol_gen=%llu reloc_gen=%llu",
+			__func__,
+			btrfs_node_ptr_generation(subvol_parent, subvol_slot),
+			btrfs_node_ptr_generation(reloc_parent, reloc_slot));
+		return -EUCLEAN;
+	}
+
+	block = kmalloc(sizeof(*block), GFP_NOFS);
+	if (!block) {
+		ret = -ENOMEM;
+		goto out;
+	}
+
+	/*
+	 * @reloc_parent/slot is still before swap, while @block is going to
+	 * record the bytenr after swap, so we do the swap here.
+	 */
+	block->subvol_bytenr = btrfs_node_blockptr(reloc_parent, reloc_slot);
+	block->subvol_generation = btrfs_node_ptr_generation(reloc_parent,
+							     reloc_slot);
+	block->reloc_bytenr = btrfs_node_blockptr(subvol_parent, subvol_slot);
+	block->reloc_generation = btrfs_node_ptr_generation(subvol_parent,
+							    subvol_slot);
+	block->last_snapshot = last_snapshot;
+	block->level = level;
+	if (bg->flags & BTRFS_BLOCK_GROUP_DATA)
+		block->trace_leaf = true;
+	else
+		block->trace_leaf = false;
+	btrfs_node_key_to_cpu(reloc_parent, &block->first_key, reloc_slot);
+
+	/* Insert @block into @blocks */
+	spin_lock(&blocks->lock);
+	cur = &blocks->blocks[level].rb_node;
+	while (*cur) {
+		struct btrfs_qgroup_swapped_block *entry;
+
+		parent = *cur;
+		entry = rb_entry(parent, struct btrfs_qgroup_swapped_block,
+				 node);
+
+		if (entry->subvol_bytenr < block->subvol_bytenr) {
+			cur = &(*cur)->rb_left;
+		} else if (entry->subvol_bytenr > block->subvol_bytenr) {
+			cur = &(*cur)->rb_right;
+		} else {
+			if (entry->subvol_generation !=
+					block->subvol_generation ||
+			    entry->reloc_bytenr != block->reloc_bytenr ||
+			    entry->reloc_generation !=
+					block->reloc_generation) {
+				/*
+				 * Duplicated but mismatch entry found.
+				 * Shouldn't happen.
+				 *
+				 * Marking qgroup inconsistent should be enough
+				 * for end users.
+				 */
+				WARN_ON(IS_ENABLED(CONFIG_BTRFS_DEBUG));
+				ret = -EEXIST;
+			}
+			kfree(block);
+			goto out_unlock;
+		}
+	}
+	rb_link_node(&block->node, parent, cur);
+	rb_insert_color(&block->node, &blocks->blocks[level]);
+	blocks->swapped = true;
+out_unlock:
+	spin_unlock(&blocks->lock);
+out:
+	if (ret < 0)
+		fs_info->qgroup_flags |=
+			BTRFS_QGROUP_STATUS_FLAG_INCONSISTENT;
+	return ret;
+}

fs/btrfs/qgroup.h

@@ -6,6 +6,8 @@
 #ifndef BTRFS_QGROUP_H
 #define BTRFS_QGROUP_H
 
+#include <linux/spinlock.h>
+#include <linux/rbtree.h>
 #include "ulist.h"
 #include "delayed-ref.h"
 
@@ -37,6 +39,66 @@
  *    Normally at qgroup rescan and transaction commit time.
  */
 
+/*
+ * Special performance optimization for balance.
+ *
+ * For balance, we need to swap subtree of subvolume and reloc trees.
+ * In theory, we need to trace all subtree blocks of both subvolume and reloc
+ * trees, since their owner has changed during such swap.
+ *
+ * However since balance has ensured that both subtrees are containing the
+ * same contents and have the same tree structures, such swap won't cause
+ * qgroup number change.
+ *
+ * But there is a race window between subtree swap and transaction commit,
+ * during that window, if we increase/decrease tree level or merge/split tree
+ * blocks, we still need to trace the original subtrees.
+ *
+ * So for balance, we use a delayed subtree tracing, whose workflow is:
+ *
+ * 1) Record the subtree root block get swapped.
+ *
+ *    During subtree swap:
+ *    O = Old tree blocks
+ *    N = New tree blocks
+ *          reloc tree                     subvolume tree X
+ *             Root                               Root
+ *            /    \                             /    \
+ *          NA     OB                          OA      OB
+ *        /  |     |  \                      /  |      |  \
+ *      NC  ND     OE  OF                   OC  OD     OE  OF
+ *
+ *   In this case, NA and OA are going to be swapped, record (NA, OA) into
+ *   subvolume tree X.
+ *
+ * 2) After subtree swap.
+ *          reloc tree                     subvolume tree X
+ *             Root                               Root
+ *            /    \                             /    \
+ *          OA     OB                          NA      OB
+ *        /  |     |  \                      /  |      |  \
+ *      OC  OD     OE  OF                   NC  ND     OE  OF
+ *
+ * 3a) COW happens for OB
+ *     If we are going to COW tree block OB, we check OB's bytenr against
+ *     tree X's swapped_blocks structure.
+ *     If it doesn't fit any, nothing will happen.
+ *
+ * 3b) COW happens for NA
+ *     Check NA's bytenr against tree X's swapped_blocks, and get a hit.
+ *     Then we do subtree scan on both subtrees OA and NA.
+ *     Resulting 6 tree blocks to be scanned (OA, OC, OD, NA, NC, ND).
+ *
+ *     Then no matter what we do to subvolume tree X, qgroup numbers will
+ *     still be correct.
+ *     Then NA's record gets removed from X's swapped_blocks.
+ *
+ * 4)  Transaction commit
+ *     Any record in X's swapped_blocks gets removed, since there is no
+ *     modification to the swapped subtrees, no need to trigger heavy qgroup
+ *     subtree rescan for them.
+ */
+
 /*
  * Record a dirty extent, and info qgroup to update quota on it
  * TODO: Use kmem cache to alloc it.
@@ -48,6 +110,24 @@ struct btrfs_qgroup_extent_record {
 	struct ulist *old_roots;
 };
 
+struct btrfs_qgroup_swapped_block {
+	struct rb_node node;
+
+	int level;
+	bool trace_leaf;
+
+	/* bytenr/generation of the tree block in subvolume tree after swap */
+	u64 subvol_bytenr;
+	u64 subvol_generation;
+
+	/* bytenr/generation of the tree block in reloc tree after swap */
+	u64 reloc_bytenr;
+	u64 reloc_generation;
+
+	u64 last_snapshot;
+	struct btrfs_key first_key;
+};
+
 /*
  * Qgroup reservation types:
  *
@@ -325,4 +405,16 @@ void btrfs_qgroup_convert_reserved_meta(struct btrfs_root *root, int num_bytes);
 
 void btrfs_qgroup_check_reserved_leak(struct inode *inode);
 
+/* btrfs_qgroup_swapped_blocks related functions */
+void btrfs_qgroup_init_swapped_blocks(
+	struct btrfs_qgroup_swapped_blocks *swapped_blocks);
+
+void btrfs_qgroup_clean_swapped_blocks(struct btrfs_root *root);
+int btrfs_qgroup_add_swapped_blocks(struct btrfs_trans_handle *trans,
+		struct btrfs_root *subvol_root,
+		struct btrfs_block_group_cache *bg,
+		struct extent_buffer *subvol_parent, int subvol_slot,
+		struct extent_buffer *reloc_parent, int reloc_slot,
+		u64 last_snapshot);
+
 #endif

fs/btrfs/relocation.c

@@ -1898,6 +1898,13 @@ int replace_path(struct btrfs_trans_handle *trans, struct reloc_control *rc,
 		if (ret < 0)
 			break;
 
+		btrfs_node_key_to_cpu(parent, &first_key, slot);
+		ret = btrfs_qgroup_add_swapped_blocks(trans, dest,
+				rc->block_group, parent, slot,
+				path->nodes[level], path->slots[level],
+				last_snapshot);
+		if (ret < 0)
+			break;
 		/*
 		 * swap blocks in fs tree and reloc tree.
 		 */

fs/btrfs/transaction.c

@@ -122,6 +122,7 @@ static noinline void switch_commit_roots(struct btrfs_transaction *trans)
 		if (is_fstree(root->root_key.objectid))
 			btrfs_unpin_free_ino(root);
 		clear_btree_io_tree(&root->dirty_log_pages);
+		btrfs_qgroup_clean_swapped_blocks(root);
 	}
 
 	/* We can free old roots now. */