[dm-devel] [RFC PATCH] dm: fix IO splitting [was: Re: [PATCH v2] block: use gcd() to fix chunk_sectors limit stacking]

[Mike Snitzer]

On Fri, Dec 04 2020 at 12:32P -0500,
Mike Snitzer <snitzer at redhat.com> wrote:

> On Fri, Dec 04 2020 at 11:47P -0500,
> Mike Snitzer <snitzer at redhat.com> wrote:
> 
> > On Thu, Dec 03 2020 at 10:59pm -0500,
> > Ming Lei <ming.lei at redhat.com> wrote:
> > 
> > > On Thu, Dec 03, 2020 at 09:03:43PM -0500, Mike Snitzer wrote:
> > > > Stacking chunk_sectors seems ill-conceived.  One size-fits-all splitting
> > > > is too rigid.
> > > 
> > > DM/VDO knows exactly it is one hard chunk_sectors limit, and DM shouldn't play
> > > the stacking trick on VDO's chunk_sectors limit, should it?
> > 
> > Feel like I already answered this in detail but... correct, DM cannot
> > and should not use stacked chunk_sectors as basis for splitting.
> > 
> > Up until 5.9, where I changed DM core to set and then use chunk_sectors
> > for splitting via blk_max_size_offset(), DM only used its own per-target
> > ti->max_io_len in drivers/md/dm.c:max_io_len().
> > 
> > But I reverted back to DM's pre-5.9 splitting in this stable@ fix that
> > I'll be sending to Linus today for 5.10-rcX:
> > https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-5.10-rcX&id=6bb38bcc33bf3093c08bd1b71e4f20c82bb60dd1
> > 
> > DM is now back to pre-5.9 behavior where it doesn't even consider
> > chunk_sectors for splitting (NOTE: dm-zoned sets ti->max_io_len though
> > so it is effectively achieves the same boundary splits via max_io_len).
> 
> Last question for all, I'd be fine with the following fix instead of
> the above referenced commit 6bb38bcc33. It'd allow DM to continue to
> use blk_max_size_offset(), any opinions?
> 
> From: Mike Snitzer <snitzer at redhat.com>
> Date: Fri, 4 Dec 2020 12:03:25 -0500
> Subject: [RFC PATCH] dm: fix IO splitting
> 
> FIXME: add proper header
> Add chunk_sectors override to blk_max_size_offset().
> 
> Signed-off-by: Mike Snitzer <snitzer at redhat.com>
> ---
>  block/blk-merge.c      |  2 +-
>  drivers/md/dm-table.c  |  5 -----
>  drivers/md/dm.c        | 19 +++++++++++--------
>  include/linux/blkdev.h |  9 +++++----
>  4 files changed, 17 insertions(+), 18 deletions(-)
> 
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index bcf5e4580603..97b7c2821565 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -144,7 +144,7 @@ static struct bio *blk_bio_write_same_split(struct request_queue *q,
>  static inline unsigned get_max_io_size(struct request_queue *q,
>  				       struct bio *bio)
>  {
> -	unsigned sectors = blk_max_size_offset(q, bio->bi_iter.bi_sector);
> +	unsigned sectors = blk_max_size_offset(q, bio->bi_iter.bi_sector, 0);
>  	unsigned max_sectors = sectors;
>  	unsigned pbs = queue_physical_block_size(q) >> SECTOR_SHIFT;
>  	unsigned lbs = queue_logical_block_size(q) >> SECTOR_SHIFT;
> diff --git a/drivers/md/dm-table.c b/drivers/md/dm-table.c
> index 2073ee8d18f4..7eeb7c4169c9 100644
> --- a/drivers/md/dm-table.c
> +++ b/drivers/md/dm-table.c
> @@ -18,7 +18,6 @@
>  #include <linux/mutex.h>
>  #include <linux/delay.h>
>  #include <linux/atomic.h>
> -#include <linux/lcm.h>
>  #include <linux/blk-mq.h>
>  #include <linux/mount.h>
>  #include <linux/dax.h>
> @@ -1449,10 +1448,6 @@ int dm_calculate_queue_limits(struct dm_table *table,
>  			zone_sectors = ti_limits.chunk_sectors;
>  		}
>  
> -		/* Stack chunk_sectors if target-specific splitting is required */
> -		if (ti->max_io_len)
> -			ti_limits.chunk_sectors = lcm_not_zero(ti->max_io_len,
> -							       ti_limits.chunk_sectors);
>  		/* Set I/O hints portion of queue limits */
>  		if (ti->type->io_hints)
>  			ti->type->io_hints(ti, &ti_limits);
> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> index 98866e725f25..f7eb3d2964f3 100644
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -1039,15 +1039,18 @@ static sector_t max_io_len(struct dm_target *ti, sector_t sector)
>  	sector_t max_len;
>  
>  	/*
> -	 * Does the target need to split even further?
> -	 * - q->limits.chunk_sectors reflects ti->max_io_len so
> -	 *   blk_max_size_offset() provides required splitting.
> -	 * - blk_max_size_offset() also respects q->limits.max_sectors
> +	 * Does the target need to split IO even further?
> +	 * - varied (per target) IO splitting is a tenet of DM; this
> +	 *   explains why stacked chunk_sectors based splitting via
> +	 *   blk_max_size_offset() isn't possible here. So pass in
> +	 *   ti->max_io_len to override stacked chunk_sectors.
>  	 */
> -	max_len = blk_max_size_offset(ti->table->md->queue,
> -				      target_offset);
> -	if (len > max_len)
> -		len = max_len;
> +	if (ti->max_io_len) {
> +		max_len = blk_max_size_offset(ti->table->md->queue,
> +					      target_offset, ti->max_io_len);
> +		if (len > max_len)
> +			len = max_len;
> +	}
>  
>  	return len;
>  }
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 639cae2c158b..f56dc5497e67 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -1073,11 +1073,12 @@ static inline unsigned int blk_queue_get_max_sectors(struct request_queue *q,
>   * file system requests.
>   */
>  static inline unsigned int blk_max_size_offset(struct request_queue *q,
> -					       sector_t offset)
> +					       sector_t offset,
> +					       unsigned int chunk_sectors)
>  {
> -	unsigned int chunk_sectors = q->limits.chunk_sectors;
> -
> -	if (!chunk_sectors)
> +	if (!chunk_sectors && q->limits.chunk_sectors)
> +		chunk_sectors = q->limits.chunk_sectors;
> +	else
>  		return q->limits.max_sectors;
>  
>  	if (likely(is_power_of_2(chunk_sectors)))

FYI, above blkdev.h diff missed this hunk:

@@ -1101,7 +1102,7 @@ static inline unsigned int blk_rq_get_max_sectors(struct request *rq,
 	    req_op(rq) == REQ_OP_SECURE_ERASE)
 		return blk_queue_get_max_sectors(q, req_op(rq));
 
-	return min(blk_max_size_offset(q, offset),
+	return min(blk_max_size_offset(q, offset, 0),
 			blk_queue_get_max_sectors(q, req_op(rq)));
 }