On Wed 26-06-24 15:38:42, Zhang Yi wrote:
On 2024/6/25 1:01, Jan Kara wrote:
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.
I understand that the incorrect max transaction limit in start_this_handle() could lead to infinite loop in start_this_handle()-> add_transaction_credits() with large enough userspace credits (from j_max_transaction_buffers - overheads to j_max_transaction_buffers), but I don't get how could it lead to ran out of space in the journal commit traction? IIUC, below codes in add_transaction_credits() could make sure that we have enough space when committing traction:
static int add_transaction_credits() { ... if (jbd2_log_space_left(journal) < journal->j_max_transaction_buffers) { ... return 1; ... } ... }
I can't open and download the image Alexander gave, so I can't get to the bottom of this issue, please let me know what happened with jbd2_journal_next_log_block().
Sure. So what was exactly happening is a loop like this:
start_this_handle() blocks = 252 (handle->h_total_credits) - starts a new transaction - t_outstanding_credits set to 6 to account for the commit block and descriptor blocks add_transaction_credits(journal, 252, 0) needed = atomic_add_return(252, &t->t_outstanding_credits); if (needed > journal->j_max_transaction_buffers) { /* Yes, this is exceeded due to descriptor blocks being in * t_outstanding_credits */ ... wait_transaction_locked(journal); - this commits an empty transaction - contains only the commit block return 1 goto repeat
So we commit single block transactions in a loop until we exhaust all the journal space. The condition in add_transaction_credits() whether there's enough space in the journal is never reached so we don't ever push the journal tail to make space in the journal.
Honza