Re: [PATCH 5/8] iomap: optionally use ioends for direct I/O

[Brian Foster <bfoster@redhat.com>]

On Wed, Dec 11, 2024 at 09:53:45AM +0100, Christoph Hellwig wrote:
> struct iomap_ioend currently tracks outstanding buffered writes and has
> some really nice code in core iomap and XFS to merge contiguous I/Os
> an defer them to userspace for completion in a very efficient way.
> 
> For zoned writes we'll also need a per-bio user context completion to
> record the written blocks, and the infrastructure for that would look
> basically like the ioend handling for buffered I/O.
> 
> So instead of reinventing the wheel, reuse the existing infrastructure.
> 
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
>  fs/iomap/buffered-io.c |  3 +++
>  fs/iomap/direct-io.c   | 50 +++++++++++++++++++++++++++++++++++++++++-
>  fs/iomap/internal.h    |  7 ++++++
>  include/linux/iomap.h  |  4 +++-
>  4 files changed, 62 insertions(+), 2 deletions(-)
>  create mode 100644 fs/iomap/internal.h
> 
...
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index b521eb15759e..b5466361cafe 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
...
> @@ -163,6 +166,51 @@ static inline void iomap_dio_set_error(struct iomap_dio *dio, int ret)
>  	cmpxchg(&dio->error, 0, ret);
>  }
>  
> +u32 iomap_finish_ioend_direct(struct iomap_ioend *ioend)
> +{
> +	struct iomap_dio *dio = ioend->io_bio.bi_private;
> +	bool should_dirty = (dio->flags & IOMAP_DIO_DIRTY);
> +	struct kiocb *iocb = dio->iocb;
> +	u32 vec_count = ioend->io_bio.bi_vcnt;
> +
> +	if (ioend->io_error)
> +		iomap_dio_set_error(dio, ioend->io_error);
> +
> +	if (atomic_dec_and_test(&dio->ref)) {
> +		struct inode *inode = file_inode(iocb->ki_filp);
> +
> +		if (dio->wait_for_completion) {
> +			struct task_struct *waiter = dio->submit.waiter;
> +
> +			WRITE_ONCE(dio->submit.waiter, NULL);
> +			blk_wake_io_task(waiter);
> +		} else if (!inode->i_mapping->nrpages) {
> +			WRITE_ONCE(iocb->private, NULL);
> +
> +			/*
> +			 * We must never invalidate pages from this thread to
> +			 * avoid deadlocks with buffered I/O completions.
> +			 * Tough luck if you hit the tiny race with someone
> +			 * dirtying the range now.
> +			 */
> +			dio->flags |= IOMAP_DIO_NO_INVALIDATE;
> +			iomap_dio_complete_work(&dio->aio.work);
> +		} else {
> +			INIT_WORK(&dio->aio.work, iomap_dio_complete_work);
> +			queue_work(inode->i_sb->s_dio_done_wq, &dio->aio.work);
> +		}
> +	}
> +
> +	if (should_dirty) {
> +		bio_check_pages_dirty(&ioend->io_bio);
> +	} else {
> +		bio_release_pages(&ioend->io_bio, false);
> +		bio_put(&ioend->io_bio);
> +	}
> +

Not that it matters all that much, but I'm a little curious about the
reasoning for using vec_count here. AFAICS this correlates to per-folio
writeback completions for buffered I/O, but that doesn't seem to apply
to direct I/O. Is there a reason to have the caller throttle based on
vec_counts, or are we just pulling some non-zero value for consistency
sake?

Brian

> +	return vec_count;
> +}
> +
>  void iomap_dio_bio_end_io(struct bio *bio)
>  {
>  	struct iomap_dio *dio = bio->bi_private;
> diff --git a/fs/iomap/internal.h b/fs/iomap/internal.h
> new file mode 100644
> index 000000000000..20cccfc3bb13
> --- /dev/null
> +++ b/fs/iomap/internal.h
> @@ -0,0 +1,7 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _IOMAP_INTERNAL_H
> +#define _IOMAP_INTERNAL_H 1
> +
> +u32 iomap_finish_ioend_direct(struct iomap_ioend *ioend);
> +
> +#endif /* _IOMAP_INTERNAL_H */
> diff --git a/include/linux/iomap.h b/include/linux/iomap.h
> index eaa8cb9083eb..f6943c80e5fd 100644
> --- a/include/linux/iomap.h
> +++ b/include/linux/iomap.h
> @@ -343,9 +343,11 @@ sector_t iomap_bmap(struct address_space *mapping, sector_t bno,
>  #define IOMAP_IOEND_UNWRITTEN		(1U << 1)
>  /* don't merge into previous ioend */
>  #define IOMAP_IOEND_BOUNDARY		(1U << 2)
> +/* is direct I/O */
> +#define IOMAP_IOEND_DIRECT		(1U << 3)
>  
>  #define IOMAP_IOEND_NOMERGE_FLAGS \
> -	(IOMAP_IOEND_SHARED | IOMAP_IOEND_UNWRITTEN)
> +	(IOMAP_IOEND_SHARED | IOMAP_IOEND_UNWRITTEN | IOMAP_IOEND_DIRECT)
>  
>  /*
>   * Structure for writeback I/O completions.
> -- 
> 2.45.2
> 
>