Re: [PATCH v2 10/11] iomap: Add support for zone append writes

[Dave Chinner <david@fromorbit.com>]

On Wed, Mar 25, 2020 at 12:24:53AM +0900, Johannes Thumshirn wrote:
> Use REQ_OP_ZONE_APPEND for direct I/O write BIOs, instead of REQ_OP_WRITE
> if the file-system requests it. The file system can make this request
> by setting the new flag IOCB_ZONE_APPEND for a direct I/O kiocb before
> calling iompa_dio_rw(). Using this information, this function propagates
> the zone append flag using IOMAP_ZONE_APPEND to the file system
> iomap_begin() method. The BIOs submitted for the zone append DIO will be
> set to use the REQ_OP_ZONE_APPEND operation.
> 
> Since zone append operations cannot be split, the iomap_apply() and
> iomap_dio_rw() internal loops are executed only once, which may result
> in short writes.
> 
> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> ---
>  fs/iomap/direct-io.c  | 80 ++++++++++++++++++++++++++++++++++++-------
>  include/linux/fs.h    |  1 +
>  include/linux/iomap.h | 22 ++++++------
>  3 files changed, 79 insertions(+), 24 deletions(-)
> 
> diff --git a/fs/iomap/direct-io.c b/fs/iomap/direct-io.c
> index 23837926c0c5..b3e2aadce72f 100644
> --- a/fs/iomap/direct-io.c
> +++ b/fs/iomap/direct-io.c
> @@ -17,6 +17,7 @@
>   * Private flags for iomap_dio, must not overlap with the public ones in
>   * iomap.h:
>   */
> +#define IOMAP_DIO_ZONE_APPEND	(1 << 27)
>  #define IOMAP_DIO_WRITE_FUA	(1 << 28)
>  #define IOMAP_DIO_NEED_SYNC	(1 << 29)
>  #define IOMAP_DIO_WRITE		(1 << 30)
> @@ -39,6 +40,7 @@ struct iomap_dio {
>  			struct task_struct	*waiter;
>  			struct request_queue	*last_queue;
>  			blk_qc_t		cookie;
> +			sector_t		sector;
>  		} submit;
>  
>  		/* used for aio completion: */
> @@ -151,6 +153,9 @@ static void iomap_dio_bio_end_io(struct bio *bio)
>  	if (bio->bi_status)
>  		iomap_dio_set_error(dio, blk_status_to_errno(bio->bi_status));
>  
> +	if (dio->flags & IOMAP_DIO_ZONE_APPEND)
> +		dio->submit.sector = bio->bi_iter.bi_sector;
> +
>  	if (atomic_dec_and_test(&dio->ref)) {
>  		if (dio->wait_for_completion) {
>  			struct task_struct *waiter = dio->submit.waiter;
> @@ -194,6 +199,21 @@ iomap_dio_zero(struct iomap_dio *dio, struct iomap *iomap, loff_t pos,
>  	iomap_dio_submit_bio(dio, iomap, bio);
>  }
>  
> +static sector_t
> +iomap_dio_bio_sector(struct iomap_dio *dio, struct iomap *iomap, loff_t pos)
> +{
> +	sector_t sector = iomap_sector(iomap, pos);
> +
> +	/*
> +	 * For zone append writes, the BIO needs to point at the start of the
> +	 * zone to append to.
> +	 */
> +	if (dio->flags & IOMAP_DIO_ZONE_APPEND)
> +		sector = ALIGN_DOWN(sector, bdev_zone_sectors(iomap->bdev));
> +
> +	return sector;
> +}

This seems to me like it should be done by the ->iomap_begin
implementation when mapping the IO. I don't see why this needs to be
specially handled by the iomap dio code.

> +
>  static loff_t
>  iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
>  		struct iomap_dio *dio, struct iomap *iomap)
> @@ -204,6 +224,7 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
>  	struct bio *bio;
>  	bool need_zeroout = false;
>  	bool use_fua = false;
> +	bool zone_append = false;
>  	int nr_pages, ret = 0;
>  	size_t copied = 0;
>  	size_t orig_count;
> @@ -235,6 +256,9 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
>  			use_fua = true;
>  	}
>  
> +	if (dio->flags & IOMAP_DIO_ZONE_APPEND)
> +		zone_append = true;
> +
>  	/*
>  	 * Save the original count and trim the iter to just the extent we
>  	 * are operating on right now.  The iter will be re-expanded once
> @@ -266,12 +290,28 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
>  
>  		bio = bio_alloc(GFP_KERNEL, nr_pages);
>  		bio_set_dev(bio, iomap->bdev);
> -		bio->bi_iter.bi_sector = iomap_sector(iomap, pos);
> +		bio->bi_iter.bi_sector = iomap_dio_bio_sector(dio, iomap, pos);
>  		bio->bi_write_hint = dio->iocb->ki_hint;
>  		bio->bi_ioprio = dio->iocb->ki_ioprio;
>  		bio->bi_private = dio;
>  		bio->bi_end_io = iomap_dio_bio_end_io;
>  
> +		if (dio->flags & IOMAP_DIO_WRITE) {
> +			bio->bi_opf = REQ_SYNC | REQ_IDLE;
> +			if (zone_append)
> +				bio->bi_opf |= REQ_OP_ZONE_APPEND;
> +			else
> +				bio->bi_opf |= REQ_OP_WRITE;
> +			if (use_fua)
> +				bio->bi_opf |= REQ_FUA;
> +			else
> +				dio->flags &= ~IOMAP_DIO_WRITE_FUA;
> +		} else {
> +			bio->bi_opf = REQ_OP_READ;
> +			if (dio->flags & IOMAP_DIO_DIRTY)
> +				bio_set_pages_dirty(bio);
> +		}

Why move all this code? If it's needed, please split it into a
separate patchi to separate it from the new functionality...

> +
>  		ret = bio_iov_iter_get_pages(bio, dio->submit.iter);
>  		if (unlikely(ret)) {
>  			/*
> @@ -284,19 +324,10 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
>  			goto zero_tail;
>  		}
>  
> -		n = bio->bi_iter.bi_size;
> -		if (dio->flags & IOMAP_DIO_WRITE) {
> -			bio->bi_opf = REQ_OP_WRITE | REQ_SYNC | REQ_IDLE;
> -			if (use_fua)
> -				bio->bi_opf |= REQ_FUA;
> -			else
> -				dio->flags &= ~IOMAP_DIO_WRITE_FUA;
> +		if (dio->flags & IOMAP_DIO_WRITE)
>  			task_io_account_write(n);
> -		} else {
> -			bio->bi_opf = REQ_OP_READ;
> -			if (dio->flags & IOMAP_DIO_DIRTY)
> -				bio_set_pages_dirty(bio);
> -		}
> +
> +		n = bio->bi_iter.bi_size;
>  
>  		dio->size += n;
>  		pos += n;
> @@ -304,6 +335,15 @@ iomap_dio_bio_actor(struct inode *inode, loff_t pos, loff_t length,
>  
>  		nr_pages = iov_iter_npages(dio->submit.iter, BIO_MAX_PAGES);
>  		iomap_dio_submit_bio(dio, iomap, bio);
> +
> +		/*
> +		 * Issuing multiple BIOs for a large zone append write can
> +		 * result in reordering of the write fragments and to data
> +		 * corruption. So always stop after the first BIO is issued.
> +		 */
> +		if (zone_append)
> +			break;

I don't think this sort of functionality should be tied to "zone
append". If there is a need for "issue a single (short) bio only" it
should be a flag to iomap_dio_rw() set by the filesystem, which can
then handle the short read/write that is returned.

> +		/*
> +		 * Zone append writes cannot be split and be shorted. Break
> +		 * here to let the user know instead of sending more IOs which
> +		 * could get reordered and corrupt the written data.
> +		 */
> +		if (flags & IOMAP_ZONE_APPEND)
> +			break;

ditto.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com