block: add a partscan sysfs attribute for disks

Userspace had been unknowingly relying on a non-stable interface of
kernel internals to determine if partition scanning is enabled for a
given disk. Provide a stable interface for this purpose instead.

Cc: stable@vger.kernel.org # 6.3+
Depends-on: 140ce28d ("block: add a disk_has_partscan helper")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/linux-block/ZhQJf8mzq_wipkBH@gardel-login/
Link: https://lore.kernel.org/r/20240502130033.1958492-3-hch@lst.de
[axboe: add links and commit message from Keith]
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Documentation/ABI/stable/sysfs-block

@@ -101,6 +101,16 @@ Description:
 		devices that support receiving integrity metadata.
 
 
+What:		/sys/block/<disk>/partscan
+Date:		May 2024
+Contact:	Christoph Hellwig <hch@lst.de>
+Description:
+		The /sys/block/<disk>/partscan files reports if partition
+		scanning is enabled for the disk.  It returns "1" if partition
+		scanning is enabled, or "0" if not.  The value type is a 32-bit
+		unsigned integer, but only "0" and "1" are valid values.
+
+
 What:		/sys/block/<disk>/<partition>/alignment_offset
 Date:		April 2009
 Contact:	Martin K. Petersen <martin.petersen@oracle.com>

block/genhd.c

@@ -1044,6 +1044,12 @@ static ssize_t diskseq_show(struct device *dev,
 	return sprintf(buf, "%llu\n", disk->diskseq);
 }
 
+static ssize_t partscan_show(struct device *dev,
+		struct device_attribute *attr, char *buf)
+{
+	return sprintf(buf, "%u\n", disk_has_partscan(dev_to_disk(dev)));
+}
+
 static DEVICE_ATTR(range, 0444, disk_range_show, NULL);
 static DEVICE_ATTR(ext_range, 0444, disk_ext_range_show, NULL);
 static DEVICE_ATTR(removable, 0444, disk_removable_show, NULL);
@@ -1057,6 +1063,7 @@ static DEVICE_ATTR(stat, 0444, part_stat_show, NULL);
 static DEVICE_ATTR(inflight, 0444, part_inflight_show, NULL);
 static DEVICE_ATTR(badblocks, 0644, disk_badblocks_show, disk_badblocks_store);
 static DEVICE_ATTR(diskseq, 0444, diskseq_show, NULL);
+static DEVICE_ATTR(partscan, 0444, partscan_show, NULL);
 
 #ifdef CONFIG_FAIL_MAKE_REQUEST
 ssize_t part_fail_show(struct device *dev,
@@ -1103,6 +1110,7 @@ static struct attribute *disk_attrs[] = {
 	&dev_attr_events_async.attr,
 	&dev_attr_events_poll_msecs.attr,
 	&dev_attr_diskseq.attr,
+	&dev_attr_partscan.attr,
 #ifdef CONFIG_FAIL_MAKE_REQUEST
 	&dev_attr_fail.attr,
 #endif