Re: [PATCH 26/29] xfs: return -EAGAIN when nowait meets sync in transaction commit

Dave Chinner
2023-08-25T21:58:19Z

On Fri, Aug 25, 2023 at 09:54:28PM +0800, Hao Xu wrote:
> From: Hao Xu <howeyxu@tencent.com>
>
> if the log transaction is a sync one, let's fail the nowait try and
> return -EAGAIN directly since sync transaction means blocked by IO.
>
> Signed-off-by: Hao Xu <howeyxu@tencent.com>
> ---
> fs/xfs/xfs_trans.c | 14 +++++++++++++-
> 1 file changed, 13 insertions(+), 1 deletion(-)
>
> diff --git a/fs/xfs/xfs_trans.c b/fs/xfs/xfs_trans.c
> index 7988b4c7f36e..f1f84a3dd456 100644
> --- a/fs/xfs/xfs_trans.c
> +++ b/fs/xfs/xfs_trans.c
> @@ -968,12 +968,24 @@ __xfs_trans_commit(
> xfs_csn_t commit_seq = 0;
> int error = 0;
> int sync = tp->t_flags & XFS_TRANS_SYNC;
> + bool nowait = tp->t_flags & XFS_TRANS_NOWAIT;
> + bool perm_log = tp->t_flags & XFS_TRANS_PERM_LOG_RES;
>
> trace_xfs_trans_commit(tp, _RET_IP_);
>
> + if (nowait && sync) {
> + /*
> + * Currently nowait is only from xfs_vn_update_time()
> + * so perm_log is always false here, but let's make
> + * code general.
> + */
> + if (perm_log)
> + xfs_defer_cancel(tp);
> + goto out_unreserve;
> + }

This is fundamentally broken. We cannot about a transaction commit
with dirty items at this point with shutting down the filesystem.

This points to XFS_TRANS_NOWAIT being completely broken, too,
because we don't call xfs_trans_set_sync() until just before we
commit the transaction. At this point, it is -too late- for
nowait+sync to be handled gracefully, and it will *always* go bad.

IOWs, the whole transaction "nowait" semantics as designed and
implemented is not a workable solution....

-Dave.
--
Dave Chinner
david@fromorbit.com