Re: [PATCH 4/8] xfs: async blkdev cache flush

["Darrick J. Wong" <djwong@kernel.org>]

On Tue, Feb 23, 2021 at 02:34:38PM +1100, 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 | 30 ++++++++++++++++++++++++++++++
>  fs/xfs/xfs_linux.h  |  1 +
>  2 files changed, 31 insertions(+)
> 
> diff --git a/fs/xfs/xfs_bio_io.c b/fs/xfs/xfs_bio_io.c
> index 5abf653a45d4..d55420bc72b5 100644
> --- a/fs/xfs/xfs_bio_io.c
> +++ b/fs/xfs/xfs_bio_io.c
> @@ -67,3 +67,33 @@ xfs_flush_bdev(
>  	blkdev_issue_flush(bdev, GFP_NOFS);
>  }
>  
> +void
> +xfs_flush_bdev_async_endio(
> +	struct bio	*bio)
> +{
> +	if (bio->bi_private)
> +		complete(bio->bi_private);
> +	bio_put(bio);
> +}
> +
> +/*
> + * Submit a request for an async cache flush to run. If the caller needs 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 block_device	*bdev,

Not sure why this isn't a buftarg function, since (AFAICT) this is the
only caller in the ~30 patches you've sent to the list.  Is there
something else coming down the pipeline such that you only have a raw
block_device pointer?

> +	struct completion	*done)
> +{
> +	struct bio *bio;
> +
> +	bio = bio_alloc(GFP_NOFS, 0);
> +	bio_set_dev(bio, bdev);
> +	bio->bi_opf = REQ_OP_WRITE | REQ_PREFLUSH | REQ_SYNC;
> +	bio->bi_private = done;
> +        bio->bi_end_io = xfs_flush_bdev_async_endio;

Weird indent here.

--D

> +
> +	submit_bio(bio);
> +}
> diff --git a/fs/xfs/xfs_linux.h b/fs/xfs/xfs_linux.h
> index e94a2aeefee8..293ff2355e80 100644
> --- a/fs/xfs/xfs_linux.h
> +++ b/fs/xfs/xfs_linux.h
> @@ -197,6 +197,7 @@ static inline uint64_t howmany_64(uint64_t x, uint32_t y)
>  int xfs_rw_bdev(struct block_device *bdev, sector_t sector, unsigned int count,
>  		char *data, unsigned int op);
>  void xfs_flush_bdev(struct block_device *bdev);
> +void xfs_flush_bdev_async(struct block_device *bdev, struct completion *done);
>  
>  #define ASSERT_ALWAYS(expr)	\
>  	(likely(expr) ? (void)0 : assfail(NULL, #expr, __FILE__, __LINE__))
> -- 
> 2.28.0
>