dm-stripe: adjust max_hw_discard_sectors to avoid unnecessary discard bio splitting

dm-stripe: adjust max_hw_discard_sectors to avoid unnecessary discard bio splitting

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>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>