[PATCH v2 2/3] btrfs-progs: csum-change: add leaf based threshold

[Qu Wenruo <wqu@suse.com>]

For "btrfstune --csum", currently we do the following operations in just
one transaction for each:

- Delete old data csums
- Change new data csums objectid

Both opeartion can modify up to GiB or even TiB level of metadata, doing
them in just one transaction is definitely going to cause problems.

This patch adds a leaf number based threshold (32 leaves), after
modifying/deleteing this many leaves, we commit a transaction to avoid
huge amount of dirty leaves piling up.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
 tune/change-csum.c | 68 +++++++++++++++++++++++++++++++++++-----------
 1 file changed, 52 insertions(+), 16 deletions(-)

diff --git a/tune/change-csum.c b/tune/change-csum.c
index 0f95cdb25533..66fdd207335c 100644
--- a/tune/change-csum.c
+++ b/tune/change-csum.c
@@ -371,29 +371,48 @@ static int generate_new_data_csums(struct btrfs_fs_info *fs_info, u16 new_csum_t
 	return generate_new_data_csums_range(fs_info, 0, new_csum_type);
 }
 
+/* After deleting/modifying this many leaves, commit a transaction. */
+#define CSUM_CHANGE_LEAVES_THRESHOLD	32
+
 static int delete_old_data_csums(struct btrfs_fs_info *fs_info)
 {
 	struct btrfs_root *csum_root = btrfs_csum_root(fs_info, 0);
-	struct btrfs_trans_handle *trans;
+	struct btrfs_trans_handle *trans = 0;
 	struct btrfs_path path = { 0 };
 	struct btrfs_key last_key;
+	unsigned int deleted_leaves = 0;
 	int ret;
 
 	last_key.objectid = BTRFS_EXTENT_CSUM_OBJECTID;
 	last_key.type = BTRFS_EXTENT_CSUM_KEY;
 	last_key.offset = (u64)-1;
 
-	trans = btrfs_start_transaction(csum_root, 1);
-	if (IS_ERR(trans)) {
-		ret = PTR_ERR(trans);
-		errno = -ret;
-		error("failed to start transaction to delete old data csums: %m");
-		return ret;
-	}
 	while (true) {
 		int start_slot;
 		int nr;
 
+		if (deleted_leaves >= CSUM_CHANGE_LEAVES_THRESHOLD) {
+			assert(trans);
+			ret = btrfs_commit_transaction(trans, csum_root);
+			if (ret < 0) {
+				errno = -ret;
+				error(
+		"failed to commit transaction to delete old data csums: %m");
+				return ret;
+			}
+			trans = NULL;
+			deleted_leaves = 0;
+		}
+		if (!trans) {
+			trans = btrfs_start_transaction(csum_root, 1);
+			if (IS_ERR(trans)) {
+				ret = PTR_ERR(trans);
+				errno = -ret;
+				error(
+		"failed to start transaction to delete old data csums: %m");
+				return ret;
+			}
+		}
 		ret = btrfs_search_slot(trans, csum_root, &last_key, &path, -1, 1);
 		if (ret < 0) {
 			errno = -ret;
@@ -428,6 +447,7 @@ static int delete_old_data_csums(struct btrfs_fs_info *fs_info)
 			break;
 		}
 		btrfs_release_path(&path);
+		deleted_leaves++;
 	}
 	btrfs_release_path(&path);
 	if (ret < 0)
@@ -445,9 +465,10 @@ static int delete_old_data_csums(struct btrfs_fs_info *fs_info)
 static int change_csum_objectids(struct btrfs_fs_info *fs_info)
 {
 	struct btrfs_root *csum_root = btrfs_csum_root(fs_info, 0);
-	struct btrfs_trans_handle *trans;
+	struct btrfs_trans_handle *trans = NULL;
 	struct btrfs_path path = { 0 };
 	struct btrfs_key last_key;
+	unsigned int changed_leaves = 0;
 	u64 super_flags;
 	int ret = 0;
 
@@ -455,17 +476,32 @@ static int change_csum_objectids(struct btrfs_fs_info *fs_info)
 	last_key.type = BTRFS_EXTENT_CSUM_KEY;
 	last_key.offset = (u64)-1;
 
-	trans = btrfs_start_transaction(csum_root, 1);
-	if (IS_ERR(trans)) {
-		ret = PTR_ERR(trans);
-		errno = -ret;
-		error("failed to start transaction to change csum objectids: %m");
-		return ret;
-	}
 	while (true) {
 		struct btrfs_key found_key;
 		int nr;
 
+		if (changed_leaves >= CSUM_CHANGE_LEAVES_THRESHOLD) {
+			assert(trans);
+			ret = btrfs_commit_transaction(trans, csum_root);
+			if (ret < 0) {
+				errno = -ret;
+				error(
+		"failed to commit transaction to change data csum objectid: %m");
+				return ret;
+			}
+			trans = NULL;
+			changed_leaves = 0;
+		}
+		if (!trans) {
+			trans = btrfs_start_transaction(csum_root, 1);
+			if (IS_ERR(trans)) {
+				ret = PTR_ERR(trans);
+				errno = -ret;
+				error(
+		"failed to start transaction to change csum objectids: %m");
+				return ret;
+			}
+		}
 		ret = btrfs_search_slot(trans, csum_root, &last_key, &path, 0, 1);
 		if (ret < 0)
 			goto out;
-- 
2.45.2