Re: [PATCH 05/45] xfs: async blkdev cache flush

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

On 05 Mar 2021 at 10:41, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> The new checkpoint caceh flush mechanism requires us to issue an
> unconditional cache flush before we start a new checkpoint. We don't
> want to block for this if we can help it, and we have a fair chunk
> of CPU work to do between starting the checkpoint and issuing the
> first journal IO.
>
> Hence it makes sense to amortise the latency cost of the cache flush
> by issuing it asynchronously and then waiting for it only when we
> need to issue the first IO in the transaction.
>
> TO do this, we need async cache flush primitives to submit the cache
> flush bio and to wait on it. THe block layer has no such primitives
> for filesystems, so roll our own for the moment.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
>  fs/xfs/xfs_bio_io.c | 36 ++++++++++++++++++++++++++++++++++++
>  fs/xfs/xfs_linux.h  |  2 ++
>  2 files changed, 38 insertions(+)
>
> diff --git a/fs/xfs/xfs_bio_io.c b/fs/xfs/xfs_bio_io.c
> index 17f36db2f792..668f8bd27b4a 100644
> --- a/fs/xfs/xfs_bio_io.c
> +++ b/fs/xfs/xfs_bio_io.c
> @@ -9,6 +9,42 @@ static inline unsigned int bio_max_vecs(unsigned int count)
>  	return bio_max_segs(howmany(count, PAGE_SIZE));
>  }
>  
> +void
> +xfs_flush_bdev_async_endio(
> +	struct bio	*bio)
> +{
> +	if (bio->bi_private)
> +		complete(bio->bi_private);
> +}
> +
> +/*
> + * Submit a request for an async cache flush to run. If the request queue does
> + * not require flush operations, just skip it altogether. If the caller needsi
> + * to wait for the flush completion at a later point in time, they must supply a
> + * valid completion. This will be signalled when the flush completes.  The
> + * caller never sees the bio that is issued here.
> + */
> +void
> +xfs_flush_bdev_async(
> +	struct bio		*bio,
> +	struct block_device	*bdev,
> +	struct completion	*done)
> +{
> +	struct request_queue	*q = bdev->bd_disk->queue;
> +
> +	if (!test_bit(QUEUE_FLAG_WC, &q->queue_flags)) {
> +		complete(done);

complete() should be invoked only when "done" has a non-NULL value.

> +		return;
> +	}

-- 
chandan