Re: [PATCH 03/45] xfs: separate CIL commit record IO

Brian Foster
2021-03-15T14:42:56Z

On Fri, Mar 05, 2021 at 04:11:01PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> To allow for iclog IO device cache flush behaviour to be optimised,
> we first need to separate out the commit record iclog IO from the
> rest of the checkpoint so we can wait for the checkpoint IO to
> complete before we issue the commit record.
>
> This separation is only necessary if the commit record is being
> written into a different iclog to the start of the checkpoint as the
> upcoming cache flushing changes requires completion ordering against
> the other iclogs submitted by the checkpoint.
>
> If the entire checkpoint and commit is in the one iclog, then they
> are both covered by the one set of cache flush primitives on the
> iclog and hence there is no need to separate them for ordering.
>
> Otherwise, we need to wait for all the previous iclogs to complete
> so they are ordered correctly and made stable by the REQ_PREFLUSH
> that the commit record iclog IO issues. This guarantees that if a
> reader sees the commit record in the journal, they will also see the
> entire checkpoint that commit record closes off.
>
> This also provides the guarantee that when the commit record IO
> completes, we can safely unpin all the log items in the checkpoint
> so they can be written back because the entire checkpoint is stable
> in the journal.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> Reviewed-by: Darrick J. Wong <djwong@kernel.org>
> ---

I still think the patch could be titled much more descriptively. For
example:

xfs: checkpoint completion to commit record submission ordering

Otherwise (and despite my slight unease over now blocking async log
forces on iclog callback completion) looks Ok:

Reviewed-by: Brian Foster <bfoster@redhat.com>

> fs/xfs/xfs_log.c | 8 +++++---
> fs/xfs/xfs_log_cil.c | 9 +++++++++
> fs/xfs/xfs_log_priv.h | 2 ++
> 3 files changed, 16 insertions(+), 3 deletions(-)
>
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index fa284f26d10e..317c466232d4 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -784,10 +784,12 @@ xfs_log_mount_cancel(
> }
>
> /*
> - * Wait for the iclog to be written disk, or return an error if the log has been
> - * shut down.
> + * Wait for the iclog and all prior iclogs to be written disk as required by the
> + * log force state machine. Waiting on ic_force_wait ensures iclog completions
> + * have been ordered and callbacks run before we are woken here, hence
> + * guaranteeing that all the iclogs up to this one are on stable storage.
> */
> -static int
> +int
> xlog_wait_on_iclog(
> struct xlog_in_core *iclog)
> __releases(iclog->ic_log->l_icloglock)
> diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> index b0ef071b3cb5..1e5fd6f268c2 100644
> --- a/fs/xfs/xfs_log_cil.c
> +++ b/fs/xfs/xfs_log_cil.c
> @@ -870,6 +870,15 @@ xlog_cil_push_work(
> wake_up_all(&cil->xc_commit_wait);
> spin_unlock(&cil->xc_push_lock);
>
> + /*
> + * If the checkpoint spans multiple iclogs, wait for all previous
> + * iclogs to complete before we submit the commit_iclog.
> + */
> + if (ctx->start_lsn != commit_lsn) {
> + spin_lock(&log->l_icloglock);
> + xlog_wait_on_iclog(commit_iclog->ic_prev);
> + }
> +
> /* release the hounds! */
> xfs_log_release_iclog(commit_iclog);
> return;
> diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
> index 037950cf1061..ee7786b33da9 100644
> --- a/fs/xfs/xfs_log_priv.h
> +++ b/fs/xfs/xfs_log_priv.h
> @@ -584,6 +584,8 @@ xlog_wait(
> remove_wait_queue(wq, &wait);
> }
>
> +int xlog_wait_on_iclog(struct xlog_in_core *iclog);
> +
> /*
> * The LSN is valid so long as it is behind the current LSN. If it isn't, this
> * means that the next log record that includes this metadata could have a
> --
> 2.28.0
>