[PATCH v2 3/4] jbd2: Avoid infinite transaction commit loop

[Jan Kara <jack@suse.cz>]

Commit 9f356e5a4f12 ("jbd2: Account descriptor blocks into
t_outstanding_credits") started to account descriptor blocks into
transactions outstanding credits. However it didn't appropriately
decrease the maximum amount of credits available to userspace. Thus if
the filesystem requests a transaction smaller than
j_max_transaction_buffers but large enough that when descriptor blocks
are added the size exceeds j_max_transaction_buffers, we confuse
add_transaction_credits() into thinking previous handles have grown the
transaction too much and enter infinite journal commit loop in
start_this_handle() -> add_transaction_credits() trying to create
transaction with enough credits available.

Fix the problem by properly accounting for transaction space reserved
for descriptor blocks when verifying requested transaction handle size.

CC: stable@vger.kernel.org
Fixes: 9f356e5a4f12 ("jbd2: Account descriptor blocks into t_outstanding_credits")
Reported-by: Alexander Coffin <alex.coffin@maticrobots.com>
Link: https://lore.kernel.org/all/CA+hUFcuGs04JHZ_WzA1zGN57+ehL2qmHOt5a7RMpo+rv6Vyxtw@mail.gmail.com
Signed-off-by: Jan Kara <jack@suse.cz>
---
 fs/jbd2/transaction.c | 21 ++++++++++++++-------
 1 file changed, 14 insertions(+), 7 deletions(-)

diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
index a095f1a3114b..66513c18ca29 100644
--- a/fs/jbd2/transaction.c
+++ b/fs/jbd2/transaction.c
@@ -191,6 +191,13 @@ static void sub_reserved_credits(journal_t *journal, int blocks)
 	wake_up(&journal->j_wait_reserved);
 }
 
+/* Maximum number of blocks for user transaction payload */
+static int jbd2_max_user_trans_buffers(journal_t *journal)
+{
+	return journal->j_max_transaction_buffers -
+				journal->j_transaction_overhead_buffers;
+}
+
 /*
  * Wait until we can add credits for handle to the running transaction.  Called
  * with j_state_lock held for reading. Returns 0 if handle joined the running
@@ -240,12 +247,12 @@ __must_hold(&journal->j_state_lock)
 		 * big to fit this handle? Wait until reserved credits are freed.
 		 */
 		if (atomic_read(&journal->j_reserved_credits) + total >
-		    journal->j_max_transaction_buffers) {
+		    jbd2_max_user_trans_buffers(journal)) {
 			read_unlock(&journal->j_state_lock);
 			jbd2_might_wait_for_commit(journal);
 			wait_event(journal->j_wait_reserved,
 				   atomic_read(&journal->j_reserved_credits) + total <=
-				   journal->j_max_transaction_buffers);
+				   jbd2_max_user_trans_buffers(journal));
 			__acquire(&journal->j_state_lock); /* fake out sparse */
 			return 1;
 		}
@@ -285,14 +292,14 @@ __must_hold(&journal->j_state_lock)
 
 	needed = atomic_add_return(rsv_blocks, &journal->j_reserved_credits);
 	/* We allow at most half of a transaction to be reserved */
-	if (needed > journal->j_max_transaction_buffers / 2) {
+	if (needed > jbd2_max_user_trans_buffers(journal) / 2) {
 		sub_reserved_credits(journal, rsv_blocks);
 		atomic_sub(total, &t->t_outstanding_credits);
 		read_unlock(&journal->j_state_lock);
 		jbd2_might_wait_for_commit(journal);
 		wait_event(journal->j_wait_reserved,
 			 atomic_read(&journal->j_reserved_credits) + rsv_blocks
-			 <= journal->j_max_transaction_buffers / 2);
+			 <= jbd2_max_user_trans_buffers(journal) / 2);
 		__acquire(&journal->j_state_lock); /* fake out sparse */
 		return 1;
 	}
@@ -322,12 +329,12 @@ static int start_this_handle(journal_t *journal, handle_t *handle,
 	 * size and limit the number of total credits to not exceed maximum
 	 * transaction size per operation.
 	 */
-	if ((rsv_blocks > journal->j_max_transaction_buffers / 2) ||
-	    (rsv_blocks + blocks > journal->j_max_transaction_buffers)) {
+	if (rsv_blocks > jbd2_max_user_trans_buffers(journal) / 2 ||
+	    rsv_blocks + blocks > jbd2_max_user_trans_buffers(journal)) {
 		printk(KERN_ERR "JBD2: %s wants too many credits "
 		       "credits:%d rsv_credits:%d max:%d\n",
 		       current->comm, blocks, rsv_blocks,
-		       journal->j_max_transaction_buffers);
+		       jbd2_max_user_trans_buffers(journal));
 		WARN_ON(1);
 		return -ENOSPC;
 	}
-- 
2.35.3