Re: [PATCH 1/8] xfs: don't reverse order of items in bulk AIL insertion

Chandan Babu R
2023-06-29T12:27:05Z

On Wed, Jun 28, 2023 at 08:44:05 AM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> XFS has strict metadata ordering requirements. One of the things it
> does is maintain the commit order of items from transaction commit
> through the CIL and into the AIL. That is, if a transaction logs
> item A before item B in a modification, then they will be inserted
> into the CIL in the order {A, B}. These items are then written into
> the iclog during checkpointing in the order {A, B}. When the
> checkpoint commits, they are supposed to be inserted into the AIL in
> the order {A, B}, and when they are pushed from the AIL, they are
> pushed in the order {A, B}.
>
> If we crash, log recovery then replays the two items from the
> checkpoint in the order {A, B}, resulting in the objects the items
> apply to being queued for writeback at the end of the checkpoint
> in the order {A, B}. This means recovery behaves the same way as the
> runtime code.
>
> In places, we have subtle dependencies on this ordering being
> maintained. One of this place is performing intent recovery from the
> log. It assumes that recovering an intent will result in a
> non-intent object being the first thing that is modified in the
> recovery transaction, and so when the transaction commits and the
> journal flushes, the first object inserted into the AIL beyond the
> intent recovery range will be a non-intent item. It uses the
> transistion from intent items to non-intent items to stop the
> recovery pass.
>
> A recent log recovery issue indicated that an intent was appearing
> as the first item in the AIL beyond the recovery range, hence
> breaking the end of recovery detection that exists.
>
> Tracing indicated insertion of the items into the AIL was apparently
> occurring in the right order (the intent was last in the commit item
> list), but the intent was appearing first in the AIL. IOWs, the
> order of items in the AIL was {D,C,B,A}, not {A,B,C,D}, and bulk
> insertion was reversing the order of the items in the batch of items
> being inserted.
>
> Lucky for us, all the items fed to bulk insertion have the same LSN,
> so the reversal of order does not affect the log head/tail tracking
> that is based on the contents of the AIL. It only impacts on code
> that has implicit, subtle dependencies on object order, and AFAICT
> only the intent recovery loop is impacted by it.
>
> Make sure bulk AIL insertion does not reorder items incorrectly.

Looks good to me.

Reviewed-by: Chandan Babu R <chandan.babu@oracle.com>

>
> Fixes: 0e57f6a36f9b ("xfs: bulk AIL insertion during transaction commit")
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> fs/xfs/xfs_trans_ail.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/xfs/xfs_trans_ail.c b/fs/xfs/xfs_trans_ail.c
> index 7d4109af193e..1098452e7f95 100644
> --- a/fs/xfs/xfs_trans_ail.c
> +++ b/fs/xfs/xfs_trans_ail.c
> @@ -823,7 +823,7 @@ xfs_trans_ail_update_bulk(
> trace_xfs_ail_insert(lip, 0, lsn);
> }
> lip->li_lsn = lsn;
> - list_add(&lip->li_ail, &tmp);
> + list_add_tail(&lip->li_ail, &tmp);
> }
>
> if (!list_empty(&tmp))

--
chandan