[PATCH v2] btrfs: transaction: Commit transaction more frequently for BPF

[Qu Wenruo <wqu@suse.com>]

Btrfs has btrfs_end_transaction_throttle() which could try to commit
transaction when needed.

However under most cases btrfs_end_transaction_throttle() won't really
commit transaction, due to the hard timing requirement.

Now introduce a new error injection point, btrfs_need_trans_pressure(),
to allow btrfs_should_end_transaction() to return 1 and
btrfs_end_transaction_throttle() to fallback to
btrfs_commit_transaction().

With such more aggressive transaction commit, we can dig deeper into
cases like snapshot drop.
Now each reference drop of btrfs_drop_snapshot() will lead to a
transaction commit, allowing dm-logwrites to catch more details, other
than one big transaction dropping everything.

Signed-off-by: Qu Wenruo <wqu@suse.com>
---
Changelog:
v2:
- Add comment to explain why this function is needed
---
 fs/btrfs/transaction.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 3f6811cdf803..8c5471b01d03 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -10,6 +10,7 @@
 #include <linux/pagemap.h>
 #include <linux/blkdev.h>
 #include <linux/uuid.h>
+#include <linux/error-injection.h>
 #include "ctree.h"
 #include "disk-io.h"
 #include "transaction.h"
@@ -749,10 +750,25 @@ void btrfs_throttle(struct btrfs_fs_info *fs_info)
 	wait_current_trans(fs_info);
 }
 
+/*
+ * This function is to allow BPF to override the return value so that we can
+ * make btrfs to commit transaction more aggressively.
+ *
+ * It's a debug only feature, mainly used with dm-log-writes to catch more details
+ * of transient operations like balance and subvolume drop.
+ */
+static noinline bool btrfs_need_trans_pressure(struct btrfs_trans_handle *trans)
+{
+	return false;
+}
+ALLOW_ERROR_INJECTION(btrfs_need_trans_pressure, TRUE);
+
 static int should_end_transaction(struct btrfs_trans_handle *trans)
 {
 	struct btrfs_fs_info *fs_info = trans->fs_info;
 
+	if (btrfs_need_trans_pressure(trans))
+		return 1;
 	if (btrfs_check_space_for_delayed_refs(fs_info))
 		return 1;
 
@@ -813,6 +829,8 @@ static int __btrfs_end_transaction(struct btrfs_trans_handle *trans,
 
 	btrfs_trans_release_chunk_metadata(trans);
 
+	if (throttle && btrfs_need_trans_pressure(trans))
+		return btrfs_commit_transaction(trans);
 	if (lock && READ_ONCE(cur_trans->state) == TRANS_STATE_BLOCKED) {
 		if (throttle)
 			return btrfs_commit_transaction(trans);
-- 
2.22.0