[PATCH v3 1/1] dm-stripe: adjust max_hw_discard_sectors to avoid unnecessary discard bio splitting

[PATCH v3 1/1] dm-stripe: adjust max_hw_discard_sectors to avoid unnecessary discard bio splitting

[Yongpeng Yang]

From: Yongpeng Yang <yangyongpeng@xiaomi.com>

Currently, the max_hw_discard_sectors of a stripe target is set to the
minimum max_hw_discard_sectors among all sub devices. When the discard
bio is larger than max_hw_discard_sectors, this may cause the stripe
device to split discard bios unnecessarily, because the value of
max_hw_discard_sectors affects max_discard_sectors, which equal to
min(max_hw_discard_sectors, max_user_discard_sectors).

For example:
root@vm:~# echo '0 33554432 striped 2 256 /dev/vdd 0 /dev/vde 0' | dmsetup create stripe_dev
root@vm:~# cat /sys/block/dm-1/queue/discard_max_bytes
536870912
root@vm:~# cat /sys/block/dm-1/slaves/vdd/queue/discard_max_bytes
536870912
root@vm:~# blkdiscard -o 0 -l 1073741824 -p 1073741824 /dev/mapper/stripe_dev

dm-1 is the stripe device, and its discard_max_bytes is equal to
each sub device’s discard_max_bytes. Since the requested discard
length exceeds discard_max_bytes, the block layer splits the discard bio:

block_bio_queue: 252,1 DS 0 + 2097152 [blkdiscard]
block_split: 252,1 DS 0 / 1048576 [blkdiscard]
block_rq_issue: 253,48 DS 268435456 () 0 + 524288 be,0,4 [blkdiscard]
block_bio_queue: 253,64 DS 524288 + 524288 [blkdiscard]

However, both vdd and vde can actually handle a discard bio of 536870912
bytes, so this split is not necessary.

This patch updates the stripe target’s q->limits.max_hw_discard_sectors
to be the minimum max_hw_discard_sectors of the sub devices multiplied
by the # of stripe devices, and max_hw_discard_sectors must round down to
chunk size multiply # of stripe devices to avoid issue discard bio to sub
devices which is larger than max_hw_discard_sectors.

This patch enables the stripe device to handle larger discard bios
without incurring unnecessary splitting.

Signed-off-by: Yongpeng Yang <yangyongpeng@xiaomi.com>
---
v3:
- Rounding stipe device's max_hw_discard_sectors down to a multiple of
chunk_size.
- Drop sc->max_discard_sectors code.
v2:
- When an overflow occurs, assign (UINT_MAX >> SECTOR_SHIFT) to
limits->max_hw_discard_sectors.
---
 drivers/md/dm-stripe.c | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/drivers/md/dm-stripe.c b/drivers/md/dm-stripe.c
index 1461dc740dae..8a872f5d633e 100644
--- a/drivers/md/dm-stripe.c
+++ b/drivers/md/dm-stripe.c
@@ -456,7 +456,7 @@ static void stripe_io_hints(struct dm_target *ti,
 			    struct queue_limits *limits)
 {
 	struct stripe_c *sc = ti->private;
-	unsigned int io_min, io_opt;
+	unsigned int io_min, io_opt, max_hw_discard_sectors = limits->max_hw_discard_sectors;
 
 	limits->chunk_sectors = sc->chunk_size;
 
@@ -465,6 +465,14 @@ static void stripe_io_hints(struct dm_target *ti,
 		limits->io_min = io_min;
 		limits->io_opt = io_opt;
 	}
+	if (max_hw_discard_sectors >= sc->chunk_size) {
+		if (!check_mul_overflow(max_hw_discard_sectors, sc->stripes, &max_hw_discard_sectors)) {
+			max_hw_discard_sectors = rounddown(max_hw_discard_sectors,
+					sc->chunk_size * sc->stripes);
+			limits->max_hw_discard_sectors = max_hw_discard_sectors;
+		} else
+			limits->max_hw_discard_sectors = UINT_MAX >> SECTOR_SHIFT;
+	}
 }
 
 static struct target_type stripe_target = {
-- 
2.43.0

Re: [PATCH v3 1/1] dm-stripe: adjust max_hw_discard_sectors to avoid unnecessary discard bio splitting

[Benjamin Marzinski]

On Fri, Dec 12, 2025 at 09:09:55PM +0800, Yongpeng Yang wrote:
> From: Yongpeng Yang <yangyongpeng@xiaomi.com>
> 
> Currently, the max_hw_discard_sectors of a stripe target is set to the
> minimum max_hw_discard_sectors among all sub devices. When the discard
> bio is larger than max_hw_discard_sectors, this may cause the stripe
> device to split discard bios unnecessarily, because the value of
> max_hw_discard_sectors affects max_discard_sectors, which equal to
> min(max_hw_discard_sectors, max_user_discard_sectors).
> 
> For example:
> root@vm:~# echo '0 33554432 striped 2 256 /dev/vdd 0 /dev/vde 0' | dmsetup create stripe_dev
> root@vm:~# cat /sys/block/dm-1/queue/discard_max_bytes
> 536870912
> root@vm:~# cat /sys/block/dm-1/slaves/vdd/queue/discard_max_bytes
> 536870912
> root@vm:~# blkdiscard -o 0 -l 1073741824 -p 1073741824 /dev/mapper/stripe_dev
> 
> dm-1 is the stripe device, and its discard_max_bytes is equal to
> each sub device’s discard_max_bytes. Since the requested discard
> length exceeds discard_max_bytes, the block layer splits the discard bio:
> 
> block_bio_queue: 252,1 DS 0 + 2097152 [blkdiscard]
> block_split: 252,1 DS 0 / 1048576 [blkdiscard]
> block_rq_issue: 253,48 DS 268435456 () 0 + 524288 be,0,4 [blkdiscard]
> block_bio_queue: 253,64 DS 524288 + 524288 [blkdiscard]
> 
> However, both vdd and vde can actually handle a discard bio of 536870912
> bytes, so this split is not necessary.
> 
> This patch updates the stripe target’s q->limits.max_hw_discard_sectors
> to be the minimum max_hw_discard_sectors of the sub devices multiplied
> by the # of stripe devices, and max_hw_discard_sectors must round down to
> chunk size multiply # of stripe devices to avoid issue discard bio to sub
> devices which is larger than max_hw_discard_sectors.
> 
> This patch enables the stripe device to handle larger discard bios
> without incurring unnecessary splitting.
> 
> Signed-off-by: Yongpeng Yang <yangyongpeng@xiaomi.com>

Reviewed-by: Benjamin Marzinski <bmarzins@redhat.com>