Re: [PATCH 1/2] xfs: log new intent items created as part of finishing recovered intent items

Dave Chinner
2020-09-17T04:59:06Z

On Wed, Sep 16, 2020 at 08:28:49PM -0700, Darrick J. Wong wrote:
> From: Darrick J. Wong <darrick.wong@oracle.com>
>
> During a code inspection, I found a serious bug in the log intent item
> recovery code when an intent item cannot complete all the work and
> decides to requeue itself to get that done. When this happens, the
> item recovery creates a new incore deferred op representing the
> remaining work and attaches it to the transaction that it allocated. At
> the end of _item_recover, it moves the entire chain of deferred ops to
> the dummy parent_tp that xlog_recover_process_intents passed to it, but
> fail to log a new intent item for the remaining work before committing
> the transaction for the single unit of work.
>
> xlog_finish_defer_ops logs those new intent items once recovery has
> finished dealing with the intent items that it recovered, but this isn't
> sufficient. If the log is forced to disk after a recovered log item
> decides to requeue itself and the system goes down before we call
> xlog_finish_defer_ops, the second log recovery will never see the new
> intent item and therefore has no idea that there was more work to do.
> It will finish recovery leaving the filesystem in a corrupted state.
>
> The same logic applies to /any/ deferred ops added during intent item
> recovery, not just the one handling the remaining work.

Yup, that looks like a problem.

> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
> ---
> fs/xfs/libxfs/xfs_defer.c | 26 ++++++++++++++++++++++++--
> fs/xfs/libxfs/xfs_defer.h | 6 ++++++
> fs/xfs/xfs_bmap_item.c | 2 +-
> fs/xfs/xfs_refcount_item.c | 2 +-
> 4 files changed, 32 insertions(+), 4 deletions(-)
>
>
> diff --git a/fs/xfs/libxfs/xfs_defer.c b/fs/xfs/libxfs/xfs_defer.c
> index d8f586256add..29e9762f3b77 100644
> --- a/fs/xfs/libxfs/xfs_defer.c
> +++ b/fs/xfs/libxfs/xfs_defer.c
> @@ -186,8 +186,9 @@ xfs_defer_create_intent(
> {
> const struct xfs_defer_op_type *ops = defer_op_types[dfp->dfp_type];
>
> - dfp->dfp_intent = ops->create_intent(tp, &dfp->dfp_work,
> - dfp->dfp_count, sort);
> + if (!dfp->dfp_intent)
> + dfp->dfp_intent = ops->create_intent(tp, &dfp->dfp_work,
> + dfp->dfp_count, sort);
> }
>
> /*
> @@ -390,6 +391,7 @@ xfs_defer_finish_one(
> list_add(li, &dfp->dfp_work);
> dfp->dfp_count++;
> dfp->dfp_done = NULL;
> + dfp->dfp_intent = NULL;
> xfs_defer_create_intent(tp, dfp, false);
> }
>
> @@ -552,3 +554,23 @@ xfs_defer_move(
>
> xfs_defer_reset(stp);
> }
> +
> +/*
> + * Prepare a chain of fresh deferred ops work items to be completed later. Log
> + * recovery requires the ability to put off until later the actual finishing
> + * work so that it can process unfinished items recovered from the log in
> + * correct order.
> + *
> + * Create and log intent items for all the work that we're capturing so that we
> + * can be assured that the items will get replayed if the system goes down
> + * before log recovery gets a chance to finish the work it put off. Then we
> + * move the chain from stp to dtp.
> + */
> +void
> +xfs_defer_capture(
> + struct xfs_trans *dtp,
> + struct xfs_trans *stp)
> +{
> + xfs_defer_create_intents(stp);
> + xfs_defer_move(dtp, stp);
> +}

Not sold on the "capture" name, but it'll do for now.

Reviewed-by: Dave Chinner <dchinner@redhat.com>

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com