[PATCH 0/5] block: add a sequence number to disks

Matteo Croce
2021-02-06T03:03:36Z

With this series a monotonically increasing number is added to disks,
precisely in the genhd struct, and it's exported in sysfs and uevent.

This helps the userspace correlate events for devices that reuse the
same device, like loop.

The first patch is the core one, the 2..4 expose the information in
different ways, while the last one increase the sequence number for
loop devices at every attach.

# udevadm monitor -kp |grep -e ^DEVNAME -e ^DISKSEQ &
[1] 523
# losetup -fP 3part
[ 3698.615848] loop0: detected capacity change from 16384 to 0
DEVNAME=/dev/loop0
DISKSEQ=13
[ 3698.647189] loop0: p1 p2 p3
DEVNAME=/dev/loop0
DISKSEQ=13
DEVNAME=/dev/loop0p1
DISKSEQ=13
DEVNAME=/dev/loop0p2
DISKSEQ=13
DEVNAME=/dev/loop0p3
DISKSEQ=13
# losetup -fP 2part
[ 3705.170766] loop1: detected capacity change from 40960 to 0
DEVNAME=/dev/loop1
DISKSEQ=14
[ 3705.247280] loop1: p1 p2
DEVNAME=/dev/loop1
DISKSEQ=14
DEVNAME=/dev/loop1p1
DISKSEQ=14
DEVNAME=/dev/loop1p2
DISKSEQ=14
# ./getdiskseq /dev/loop*
/dev/loop0: 13
/dev/loop0p1: 13
/dev/loop0p2: 13
/dev/loop0p3: 13
/dev/loop1: 14
/dev/loop1p1: 14
/dev/loop1p2: 14
/dev/loop2: 5
/dev/loop3: 6
/dev/loop-control: Function not implemented
# grep . /sys/class/block/*/diskseq
/sys/class/block/loop0/diskseq:13
/sys/class/block/loop1/diskseq:14
/sys/class/block/loop2/diskseq:5
/sys/class/block/loop3/diskseq:6
/sys/class/block/ram0/diskseq:1
/sys/class/block/ram1/diskseq:2
/sys/class/block/vda/diskseq:7

If merged, this feature will immediately used by the userspace:
https://github.com/systemd/systemd/issues/17469#issuecomment-762919781

Matteo Croce (5):
block: add disk sequence number
block: add ioctl to read the disk sequence number
block: refactor sysfs code
block: export diskseq in sysfs
loop: increment sequence number

Documentation/ABI/testing/sysfs-block | 12 ++++++++
block/genhd.c | 43 ++++++++++++++++++++++++---
block/ioctl.c | 2 ++
drivers/block/loop.c | 3 ++
include/linux/genhd.h | 2 ++
include/uapi/linux/fs.h | 1 +
6 files changed, 59 insertions(+), 4 deletions(-)

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2.29.2