Re: [PATCH 02/39] xfs: separate CIL commit record IO

[Allison Henderson <allison.henderson@oracle.com>]

On 5/19/21 5:12 AM, 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>
> Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>
> Reviewed-by: Brian Foster <bfoster@redhat.com>
Ok, makes sense
Reviewed-by: Allison Henderson <allison.henderson@oracle.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 0e563ff8cd3b..4cd5840e953a 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -786,10 +786,12 @@ 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
>