Re: [PATCH v5 08/11] dm: Forbid requeue of writes to zones

[Mike Snitzer <snitzer@redhat.com>]

On Tue, May 25 2021 at  5:24P -0400,
Damien Le Moal <damien.lemoal@wdc.com> wrote:

> A target map method requesting the requeue of a bio with
> DM_MAPIO_REQUEUE or completing it with DM_ENDIO_REQUEUE can cause
> unaligned write errors if the bio is a write operation targeting a
> sequential zone. If a zoned target request such a requeue, warn about
> it and kill the IO.
> 
> The function dm_is_zone_write() is introduced to detect write operations
> to zoned targets.
> 
> This change does not affect the target drivers supporting zoned devices
> and exposing a zoned device, namely dm-crypt, dm-linear and dm-flakey as
> none of these targets ever request a requeue.
> 
> Signed-off-by: Damien Le Moal <damien.lemoal@wdc.com>
> Reviewed-by: Hannes Reinecke <hare@suse.de>
> Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
> ---
>  drivers/md/dm-zone.c | 17 +++++++++++++++++
>  drivers/md/dm.c      | 18 +++++++++++++++---
>  drivers/md/dm.h      |  5 +++++
>  3 files changed, 37 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/md/dm-zone.c b/drivers/md/dm-zone.c
> index b42474043249..edc3bbb45637 100644
> --- a/drivers/md/dm-zone.c
> +++ b/drivers/md/dm-zone.c
> @@ -104,6 +104,23 @@ int dm_report_zones(struct block_device *bdev, sector_t start, sector_t sector,
>  }
>  EXPORT_SYMBOL_GPL(dm_report_zones);
>  
> +bool dm_is_zone_write(struct mapped_device *md, struct bio *bio)
> +{
> +	struct request_queue *q = md->queue;
> +
> +	if (!blk_queue_is_zoned(q))
> +		return false;
> +
> +	switch (bio_op(bio)) {
> +	case REQ_OP_WRITE_ZEROES:
> +	case REQ_OP_WRITE_SAME:
> +	case REQ_OP_WRITE:
> +		return !op_is_flush(bio->bi_opf) && bio_sectors(bio);
> +	default:
> +		return false;
> +	}
> +}
> +
>  void dm_set_zones_restrictions(struct dm_table *t, struct request_queue *q)
>  {
>  	if (!blk_queue_is_zoned(q))
> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> index c49976cc4e44..ed8c5a8df2e5 100644
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -846,11 +846,15 @@ static void dec_pending(struct dm_io *io, blk_status_t error)
>  			 * Target requested pushing back the I/O.
>  			 */
>  			spin_lock_irqsave(&md->deferred_lock, flags);
> -			if (__noflush_suspending(md))
> +			if (__noflush_suspending(md) &&
> +			    !WARN_ON_ONCE(dm_is_zone_write(md, bio)))
>  				/* NOTE early return due to BLK_STS_DM_REQUEUE below */
>  				bio_list_add_head(&md->deferred, io->orig_bio);
>  			else
> -				/* noflush suspend was interrupted. */
> +				/*
> +				 * noflush suspend was interrupted or this is
> +				 * a write to a zoned target.
> +				 */
>  				io->status = BLK_STS_IOERR;
>  			spin_unlock_irqrestore(&md->deferred_lock, flags);
>  		}

So I now see this incremental fix:
https://patchwork.kernel.org/project/dm-devel/patch/20210604004703.408562-1-damien.lemoal@opensource.wdc.com/

And I've folded it in...

> @@ -947,7 +951,15 @@ static void clone_endio(struct bio *bio)
>  		int r = endio(tio->ti, bio, &error);
>  		switch (r) {
>  		case DM_ENDIO_REQUEUE:
> -			error = BLK_STS_DM_REQUEUE;
> +			/*
> +			 * Requeuing writes to a sequential zone of a zoned
> +			 * target will break the sequential write pattern:
> +			 * fail such IO.
> +			 */
> +			if (WARN_ON_ONCE(dm_is_zone_write(md, bio)))
> +				error = BLK_STS_IOERR;
> +			else
> +				error = BLK_STS_DM_REQUEUE;
>  			fallthrough;
>  		case DM_ENDIO_DONE:
>  			break;

But I'm left wondering why dec_pending, now dm_io_dec_pending, needs
to be modified to also check dm_is_zone_write() if clone_endio() is
already dealing with it?

Not that big a deal, just not loving how we're sprinkling special
zoned code around...

Mike