[PATCH] block, loop: support partitions without scanning

[Stefan Bader]

From: Christoph Hellwig <hch at lst.de>

BugLink: https://bugs.launchpad.net/bugs/2056143

Historically we did distinguish between a flag that surpressed partition
scanning, and a combinations of the minors variable and another flag if
any partitions were supported.  This was generally confusing and doesn't
make much sense, but some corner case uses of the loop driver actually
do want to support manually added partitions on a device that does not
actively scan for partitions.  To make things worsee the loop driver
also wants to dynamically toggle the scanning for partitions on a live
gendisk, which makes the disk->flags updates non-atomic.

Introduce a new GD_SUPPRESS_PART_SCAN bit in disk->state that disables
just scanning for partitions, and toggle that instead of GENHD_FL_NO_PART
in the loop driver.

Fixes: 1ebe2e5f9d68 ("block: remove GENHD_FL_EXT_DEVT")
Reported-by: Ming Lei <ming.lei at redhat.com>
Signed-off-by: Christoph Hellwig <hch at lst.de>
Reviewed-by: Ming Lei <ming.lei at redhat.com>
Link: https://lore.kernel.org/r/20220527055806.1972352-1-hch@lst.de
Signed-off-by: Jens Axboe <axboe at kernel.dk>

(backported from commit b9684a71fca793213378dd410cd11675d973eaa1)
[smb: Flag and test for partition scan in genhd.h instead]
Signed-off-by: Stefan Bader <stefan.bader at canonical.com>
---
 drivers/block/loop.c  | 8 ++++----
 include/linux/genhd.h | 3 +++
 2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 714c87900c54..e82497ae24a9 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1364,7 +1364,7 @@ static int loop_configure(struct loop_device *lo, fmode_t mode,
 		lo->lo_flags |= LO_FLAGS_PARTSCAN;
 	partscan = lo->lo_flags & LO_FLAGS_PARTSCAN;
 	if (partscan)
-		lo->lo_disk->flags &= ~GENHD_FL_NO_PART;
+		clear_bit(GD_SUPPRESS_PART_SCAN, &lo->lo_disk->state);
 
 	/* enable and uncork uevent now that we are done */
 	dev_set_uevent_suppress(disk_to_dev(lo->lo_disk), 0);
@@ -1517,7 +1517,7 @@ static int __loop_clr_fd(struct loop_device *lo, bool release)
 	mutex_lock(&lo->lo_mutex);
 	lo->lo_flags = 0;
 	if (!part_shift)
-		lo->lo_disk->flags |= GENHD_FL_NO_PART;
+		set_bit(GD_SUPPRESS_PART_SCAN, &lo->lo_disk->state);
 	lo->lo_state = Lo_unbound;
 	mutex_unlock(&lo->lo_mutex);
 
@@ -1636,7 +1636,7 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
 
 	if (!err && (lo->lo_flags & LO_FLAGS_PARTSCAN) &&
 	     !(prev_lo_flags & LO_FLAGS_PARTSCAN)) {
-		lo->lo_disk->flags &= ~GENHD_FL_NO_PART;
+		clear_bit(GD_SUPPRESS_PART_SCAN, &lo->lo_disk->state);
 		partscan = true;
 	}
 out_unlock:
@@ -2489,7 +2489,7 @@ static int loop_add(int i)
 	 * userspace tools. Parameters like this in general should be avoided.
 	 */
 	if (!part_shift)
-		disk->flags |= GENHD_FL_NO_PART;
+		set_bit(GD_SUPPRESS_PART_SCAN, &disk->state);
 	disk->flags |= GENHD_FL_EXT_DEVT;
 	atomic_set(&lo->lo_refcnt, 0);
 	mutex_init(&lo->lo_mutex);
diff --git a/include/linux/genhd.h b/include/linux/genhd.h
index 690b7f7996d1..fc69ab1751c2 100644
--- a/include/linux/genhd.h
+++ b/include/linux/genhd.h
@@ -132,6 +132,7 @@ struct gendisk {
 #define GD_READ_ONLY			1
 #define GD_DEAD				2
 #define GD_NATIVE_CAPACITY		3
+#define GD_SUPPRESS_PART_SCAN		5
 
 	struct mutex open_mutex;	/* open/close mutex */
 	unsigned open_partitions;	/* number of open partitions */
@@ -185,6 +186,8 @@ static inline int disk_max_parts(struct gendisk *disk)
 
 static inline bool disk_part_scan_enabled(struct gendisk *disk)
 {
+	if (test_bit(GD_SUPPRESS_PART_SCAN, &disk->state))
+		return 0;
 	return disk_max_parts(disk) > 1 && !(disk->flags & GENHD_FL_NO_PART);
 }
 
-- 
2.34.1