Re: [PATCH 2/2] block: loop: delete partitions after clearing & changing fd

[Ming Lei <ming.lei@redhat.com>]

On Tue, Jul 07, 2020 at 07:53:12PM +0200, Christoph Hellwig wrote:
> On Tue, Jul 07, 2020 at 04:45:52PM +0800, Ming Lei wrote:
> > After clearing fd or changing fd, we have to delete old partitions,
> > otherwise they may become ghost partitions.
> > 
> > Fix this issue by clearing GENHD_FL_NO_PART_SCAN during calling
> > bdev_disk_changed() which won't drop old partitions if GENHD_FL_NO_PART_SCAN
> > isn't set.
> 
> I don't think messing with GENHD_FL_NO_PART_SCAN is a good idea, as
> that will also cause an actual partition scan.  But except for historic
> reasons I can't think of a good idea to even check for
> GENHD_FL_NO_PART_SCAN in blk_drop_partitions.

I think it is safe to not check it in blk_drop_partitions(), how about
the following patch?

From a20209464c367c338beee5555f2cb5c8e8ad9f78 Mon Sep 17 00:00:00 2001
From: Ming Lei <ming.lei@redhat.com>
Date: Wed, 8 Jul 2020 16:07:19 +0800
Subject: [PATCH] block: always remove partitions in blk_drop_partitions()

So far blk_drop_partitions() only removes partitions when
disk_part_scan_enabled() return true. This way can make ghost partition on
loop device after changing/clearing FD in case that PARTSCAN is disabled.

Fix this issue by always removing partitions in blk_drop_partitions(), and
this way is correct because:

1) only loop, mmc and GENHD_FL_HIDDEN disks(nvme multipath) may set
GENHD_FL_NO_PART_SCAN

2) GENHD_FL_HIDDEN disks doesn't expose disk to block device fs, and
bdev_disk_changed()/blk_drop_partitions() won't be called for this kind of
disk

3) for mmc, if GENHD_FL_NO_PART_SCAN is set, no any partitions can be added
for this kind of disk, so blk_drop_partitions() basically does nothing no
matter if GENHD_FL_NO_PART_SCAN is set or not because disk_max_parts(disk) <= 1

4) for loop, bdev_disk_changed() is called in two cases: one is set fd and set
status, when there shouldn't be any partitions; another is clearing/changing fd,
we need to remove old partitions and re-read new partitions if there are and
PART_SCAN is enabled.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/partitions/core.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/block/partitions/core.c b/block/partitions/core.c
index 78951e33b2d7..e62a98a8eeb7 100644
--- a/block/partitions/core.c
+++ b/block/partitions/core.c
@@ -619,8 +619,6 @@ int blk_drop_partitions(struct block_device *bdev)
 	struct disk_part_iter piter;
 	struct hd_struct *part;
 
-	if (!disk_part_scan_enabled(bdev->bd_disk))
-		return 0;
 	if (bdev->bd_part_count)
 		return -EBUSY;
 
-- 
2.25.2




thanks,
Ming