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

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

[Matteo Croce]

From: Matteo Croce <mcroce@microsoft.com>

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(-)

-- 
2.29.2

[PATCH 1/5] block: add disk sequence number

[Matteo Croce]

From: Matteo Croce <mcroce@microsoft.com>

Add a sequence number to the disk devices. This number is put in the
uevent so userspace can correlate events when a driver reuses a device,
like the loop one.

Signed-off-by: Matteo Croce <mcroce@microsoft.com>
---
 block/genhd.c         | 19 +++++++++++++++++++
 include/linux/genhd.h |  2 ++
 2 files changed, 21 insertions(+)

diff --git a/block/genhd.c b/block/genhd.c
index 9e741a4f351b..4dbf589e1610 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -1425,8 +1425,17 @@ static void disk_release(struct device *dev)
 		blk_put_queue(disk->queue);
 	kfree(disk);
 }
+
+static int block_uevent(struct device *dev, struct kobj_uevent_env *env)
+{
+	struct gendisk *disk = dev_to_disk(dev);
+
+	return add_uevent_var(env, "DISKSEQ=%llu", disk->diskseq);
+}
+
 struct class block_class = {
 	.name		= "block",
+	.dev_uevent	= block_uevent,
 };
 
 static char *block_devnode(struct device *dev, umode_t *mode,
@@ -1601,6 +1610,8 @@ struct gendisk *__alloc_disk_node(int minors, int node_id)
 	disk_to_dev(disk)->class = &block_class;
 	disk_to_dev(disk)->type = &disk_type;
 	device_initialize(disk_to_dev(disk));
+	inc_diskseq(disk);
+
 	return disk;
 
 out_bdput:
@@ -2149,3 +2160,11 @@ static void disk_release_events(struct gendisk *disk)
 	WARN_ON_ONCE(disk->ev && disk->ev->block != 1);
 	kfree(disk->ev);
 }
+
+void inc_diskseq(struct gendisk *disk)
+{
+	static atomic64_t diskseq;
+
+	disk->diskseq = atomic64_inc_return(&diskseq);
+}
+EXPORT_SYMBOL_GPL(inc_diskseq);
diff --git a/include/linux/genhd.h b/include/linux/genhd.h
index 809aaa32d53c..2e5a0b8893db 100644
--- a/include/linux/genhd.h
+++ b/include/linux/genhd.h
@@ -177,6 +177,7 @@ struct gendisk {
 	int node_id;
 	struct badblocks *bb;
 	struct lockdep_map lockdep_map;
+	u64 diskseq;
 };
 
 /*
@@ -335,6 +336,7 @@ static inline void bd_unlink_disk_holder(struct block_device *bdev,
 #endif /* CONFIG_SYSFS */
 
 extern struct rw_semaphore bdev_lookup_sem;
+extern void inc_diskseq(struct gendisk *disk);
 
 dev_t blk_lookup_devt(const char *name, int partno);
 void blk_request_module(dev_t devt);
-- 
2.29.2

[PATCH 2/5] block: add ioctl to read the disk sequence number

[Matteo Croce]

From: Matteo Croce <mcroce@microsoft.com>

Add a new BLKGETDISKSEQ ioctl which retrieves the disk sequence number
from the genhd structure.

Signed-off-by: Matteo Croce <mcroce@microsoft.com>
---
 block/ioctl.c           | 2 ++
 include/uapi/linux/fs.h | 1 +
 2 files changed, 3 insertions(+)

diff --git a/block/ioctl.c b/block/ioctl.c
index d61d652078f4..037fec888098 100644
--- a/block/ioctl.c
+++ b/block/ioctl.c
@@ -460,6 +460,8 @@ static int blkdev_common_ioctl(struct block_device *bdev, fmode_t mode,
 				BLKDEV_DISCARD_SECURE);
 	case BLKZEROOUT:
 		return blk_ioctl_zeroout(bdev, mode, arg);
+	case BLKGETDISKSEQ:
+		return put_u64(argp, bdev->bd_disk->diskseq);
 	case BLKREPORTZONE:
 		return blkdev_report_zones_ioctl(bdev, mode, cmd, arg);
 	case BLKRESETZONE:
diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h
index f44eb0a04afd..5dc72bbdd9b7 100644
--- a/include/uapi/linux/fs.h
+++ b/include/uapi/linux/fs.h
@@ -184,6 +184,7 @@ struct fsxattr {
 #define BLKSECDISCARD _IO(0x12,125)
 #define BLKROTATIONAL _IO(0x12,126)
 #define BLKZEROOUT _IO(0x12,127)
+#define BLKGETDISKSEQ _IOR(0x12,128,__u64)
 /*
  * A jump here: 130-131 are reserved for zoned block devices
  * (see uapi/linux/blkzoned.h)
-- 
2.29.2

[PATCH 3/5] block: refactor sysfs code

[Matteo Croce]

From: Matteo Croce <mcroce@microsoft.com>

Move the sysfs register code from a function named disk_add_events() to
a new function named disk_add_sysfs(). Also, rename the attribute list
with a more generic name than disk_events_attrs.

Signed-off-by: Matteo Croce <mcroce@microsoft.com>
---
 block/genhd.c | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/block/genhd.c b/block/genhd.c
index 4dbf589e1610..a59a35cf452c 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -41,6 +41,7 @@ static void disk_alloc_events(struct gendisk *disk);
 static void disk_add_events(struct gendisk *disk);
 static void disk_del_events(struct gendisk *disk);
 static void disk_release_events(struct gendisk *disk);
+static void disk_add_sysfs(struct gendisk *disk);
 
 void set_capacity(struct gendisk *disk, sector_t sectors)
 {
@@ -760,6 +761,7 @@ static void __device_add_disk(struct device *parent, struct gendisk *disk,
 	 */
 	WARN_ON_ONCE(!blk_get_queue(disk->queue));
 
+	disk_add_sysfs(disk);
 	disk_add_events(disk);
 	blk_integrity_add(disk);
 }
@@ -2049,7 +2051,7 @@ static const DEVICE_ATTR(events_poll_msecs, 0644,
 			 disk_events_poll_msecs_show,
 			 disk_events_poll_msecs_store);
 
-static const struct attribute *disk_events_attrs[] = {
+static const struct attribute *disk_sysfs_attrs[] = {
 	&dev_attr_events.attr,
 	&dev_attr_events_async.attr,
 	&dev_attr_events_poll_msecs.attr,
@@ -2120,13 +2122,16 @@ static void disk_alloc_events(struct gendisk *disk)
 	disk->ev = ev;
 }
 
-static void disk_add_events(struct gendisk *disk)
+static void disk_add_sysfs(struct gendisk *disk)
 {
 	/* FIXME: error handling */
-	if (sysfs_create_files(&disk_to_dev(disk)->kobj, disk_events_attrs) < 0)
+	if (sysfs_create_files(&disk_to_dev(disk)->kobj, disk_sysfs_attrs) < 0)
 		pr_warn("%s: failed to create sysfs files for events\n",
 			disk->disk_name);
+}
 
+static void disk_add_events(struct gendisk *disk)
+{
 	if (!disk->ev)
 		return;
 
@@ -2151,7 +2156,7 @@ static void disk_del_events(struct gendisk *disk)
 		mutex_unlock(&disk_events_mutex);
 	}
 
-	sysfs_remove_files(&disk_to_dev(disk)->kobj, disk_events_attrs);
+	sysfs_remove_files(&disk_to_dev(disk)->kobj, disk_sysfs_attrs);
 }
 
 static void disk_release_events(struct gendisk *disk)
-- 
2.29.2

[PATCH 4/5] block: export diskseq in sysfs

[Matteo Croce]

From: Matteo Croce <mcroce@microsoft.com>

Add a new sysfs handle to export the new diskseq value.
Place it in <sysfs>/block/<disk>/diskseq and document it.

Signed-off-by: Matteo Croce <mcroce@microsoft.com>
---
 Documentation/ABI/testing/sysfs-block | 12 ++++++++++++
 block/genhd.c                         | 11 +++++++++++
 2 files changed, 23 insertions(+)

diff --git a/Documentation/ABI/testing/sysfs-block b/Documentation/ABI/testing/sysfs-block
index e34cdeeeb9d4..a0ed87386639 100644
--- a/Documentation/ABI/testing/sysfs-block
+++ b/Documentation/ABI/testing/sysfs-block
@@ -28,6 +28,18 @@ Description:
 		For more details refer Documentation/admin-guide/iostats.rst
 
 
+What:		/sys/block/<disk>/diskseq
+Date:		February 2021
+Contact:	Matteo Croce <mcroce@microsoft.com>
+Description:
+		The /sys/block/<disk>/diskseq files reports the disk
+		sequence number, which is a monotonically increasing
+		number assigned to every drive.
+		Some devices, like the loop device, refresh such number
+		every time the backing file is changed.
+		The value type is 64 bit unsigned.
+
+
 What:		/sys/block/<disk>/<part>/stat
 Date:		February 2008
 Contact:	Jerome Marchand <jmarchan@redhat.com>
diff --git a/block/genhd.c b/block/genhd.c
index a59a35cf452c..1aedd4fab6f3 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -1975,6 +1975,7 @@ static void disk_check_events(struct disk_events *ev,
  * events_async		: list of events which can be detected w/o polling
  *			  (always empty, only for backwards compatibility)
  * events_poll_msecs	: polling interval, 0: disable, -1: system default
+ * diskseq		: disk sequence number, since boot
  */
 static ssize_t __disk_events_show(unsigned int events, char *buf)
 {
@@ -2045,16 +2046,26 @@ static ssize_t disk_events_poll_msecs_store(struct device *dev,
 	return count;
 }
 
+static ssize_t diskseq_show(struct device *dev,
+			    struct device_attribute *attr, char *buf)
+{
+	struct gendisk *disk = dev_to_disk(dev);
+
+	return sprintf(buf, "%llu\n", disk->diskseq);
+}
+
 static const DEVICE_ATTR(events, 0444, disk_events_show, NULL);
 static const DEVICE_ATTR(events_async, 0444, disk_events_async_show, NULL);
 static const DEVICE_ATTR(events_poll_msecs, 0644,
 			 disk_events_poll_msecs_show,
 			 disk_events_poll_msecs_store);
+static const DEVICE_ATTR(diskseq, 0444, diskseq_show, NULL);
 
 static const struct attribute *disk_sysfs_attrs[] = {
 	&dev_attr_events.attr,
 	&dev_attr_events_async.attr,
 	&dev_attr_events_poll_msecs.attr,
+	&dev_attr_diskseq.attr,
 	NULL,
 };
 
-- 
2.29.2

[PATCH 5/5] loop: increment sequence number

[Matteo Croce]

From: Matteo Croce <mcroce@microsoft.com>

On a very loaded system, if there are many events queued up from multiple
attach/detach cycles, it's impossible to match them up with the
LOOP_CONFIGURE or LOOP_SET_FD call, since we don't know where the position
of our own association in the queue is[1].
Not even an empty uevent queue is a reliable indication that we already
received the uevent we were waiting for, since with multi-partition block
devices each partition's event is queued asynchronously and might be
delivered later.

Increment the disk sequence number when setting or changing the backing
file, so the userspace knows which backing file generated the event.

[1] https://github.com/systemd/systemd/issues/17469#issuecomment-762919781

Signed-off-by: Matteo Croce <mcroce@microsoft.com>
---
 drivers/block/loop.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index e5ff328f0917..c12b3faae3ab 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -734,6 +734,7 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
 		goto out_err;
 
 	/* and ... switch */
+	inc_diskseq(lo->lo_disk);
 	blk_mq_freeze_queue(lo->lo_queue);
 	mapping_set_gfp_mask(old_file->f_mapping, lo->old_gfp_mask);
 	lo->lo_backing_file = file;
@@ -1122,6 +1123,8 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
 	if (error)
 		goto out_unlock;
 
+	inc_diskseq(lo->lo_disk);
+
 	if (!(file->f_mode & FMODE_WRITE) || !(mode & FMODE_WRITE) ||
 	    !file->f_op->write_iter)
 		lo->lo_flags |= LO_FLAGS_READ_ONLY;
-- 
2.29.2

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

[Lennart Poettering]

On Sa, 06.02.21 01:08, Matteo Croce (mcroce@linux.microsoft.com) wrote:

> From: Matteo Croce <mcroce@microsoft.com>
>
> 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.

Patch set looks excellent to me. This would be great to have for the
systems project, as it would allow us to fix some major races around
loop device allocation, that are relatively easily triggered on loaded
systems.

Lennart

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

[Matteo Croce]

On Sat, Feb 6, 2021 at 1:09 AM Matteo Croce <mcroce@linux.microsoft.com> wrote:
>
> From: Matteo Croce <mcroce@microsoft.com>
>
> 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(-)
>
> --
> 2.29.2
>

Hi,

Did anyone have a chance to look at this series?

Ideas or suggestions?

Regards,


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