Re: [PATCH 2/5] xfs: separate CIL commit record IO

[Chandan Babu R <chandanrlinux@gmail.com>]

On 28 Jan 2021 at 10:11, 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 separate is only necessary if the commit record is being
> written into a different iclog to the start of 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.
>
> 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.
>

W.r.t correctness of the changes in this patch,

Reviewed-by: Chandan Babu R <chandanrlinux@gmail.com>

> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
>  fs/xfs/xfs_log.c      | 47 +++++++++++++++++++++++++++++++++++++++++++
>  fs/xfs/xfs_log_cil.c  |  7 +++++++
>  fs/xfs/xfs_log_priv.h |  2 ++
>  3 files changed, 56 insertions(+)
>
> diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
> index c5f507c24577..c5e3da23961c 100644
> --- a/fs/xfs/xfs_log.c
> +++ b/fs/xfs/xfs_log.c
> @@ -808,6 +808,53 @@ xlog_wait_on_iclog(
>  	return 0;
>  }
>  
> +/*
> + * Wait on any iclogs that are still flushing in the range of start_lsn to
> + * the current iclog's lsn. The caller holds a reference to the iclog, but
> + * otherwise holds no log locks.
> + *
> + * We walk backwards through the iclogs to find the iclog with the highest lsn
> + * in the range that we need to wait for and then wait for it to complete.
> + * Completion ordering of iclog IOs ensures that all prior iclogs to this IO are
> + * complete by the time our candidate has completed.
> + */
> +int
> +xlog_wait_on_iclog_lsn(
> +	struct xlog_in_core	*iclog,
> +	xfs_lsn_t		start_lsn)
> +{
> +	struct xlog		*log = iclog->ic_log;
> +	struct xlog_in_core	*prev;
> +	int			error = -EIO;
> +
> +	spin_lock(&log->l_icloglock);
> +	if (XLOG_FORCED_SHUTDOWN(log))
> +		goto out_unlock;
> +
> +	error = 0;
> +	for (prev = iclog->ic_prev; prev != iclog; prev = prev->ic_prev) {
> +
> +		/* Done if the lsn is before our start lsn */
> +		if (XFS_LSN_CMP(be64_to_cpu(prev->ic_header.h_lsn),
> +				start_lsn) < 0)
> +			break;
> +
> +		/* Don't need to wait on completed, clean iclogs */
> +		if (prev->ic_state == XLOG_STATE_DIRTY ||
> +		    prev->ic_state == XLOG_STATE_ACTIVE) {
> +			continue;
> +		}
> +
> +		/* wait for completion on this iclog */
> +		xlog_wait(&prev->ic_force_wait, &log->l_icloglock);
> +		return 0;
> +	}
> +
> +out_unlock:
> +	spin_unlock(&log->l_icloglock);
> +	return error;
> +}
> +
>  /*
>   * Write out an unmount record using the ticket provided. We have to account for
>   * the data space used in the unmount ticket as this write is not done from a
> diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> index b0ef071b3cb5..c5cc1b7ad25e 100644
> --- a/fs/xfs/xfs_log_cil.c
> +++ b/fs/xfs/xfs_log_cil.c
> @@ -870,6 +870,13 @@ 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)
> +		xlog_wait_on_iclog_lsn(commit_iclog, ctx->start_lsn);
> +
>  	/* 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..a7ac85aaff4e 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_lsn(struct xlog_in_core *iclog, xfs_lsn_t start_lsn);
> +
>  /*
>   * 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


-- 
chandan