Re: [PATCH] block: don't set GD_NEED_PART_SCAN if scan partition failed

[Yu Kuai <yukuai1@huaweicloud.com>]

Hi, Ming

在 2023/03/22 15:58, Ming Lei 写道:
> On Wed, Mar 22, 2023 at 11:59:26AM +0800, Yu Kuai wrote:
>> From: Yu Kuai <yukuai3@huawei.com>
>>
>> Currently if disk_scan_partitions() failed, GD_NEED_PART_SCAN will still
>> set, and partition scan will be proceed again when blkdev_get_by_dev()
>> is called. However, this will cause a problem that re-assemble partitioned
>> raid device will creat partition for underlying disk.
>>
>> Test procedure:
>>
>> mdadm -CR /dev/md0 -l 1 -n 2 /dev/sda /dev/sdb -e 1.0
>> sgdisk -n 0:0:+100MiB /dev/md0
>> blockdev --rereadpt /dev/sda
>> blockdev --rereadpt /dev/sdb
>> mdadm -S /dev/md0
>> mdadm -A /dev/md0 /dev/sda /dev/sdb
>>
>> Test result: underlying disk partition and raid partition can be
>> observed at the same time
>>
>> Note that this can still happen in come corner cases that
>> GD_NEED_PART_SCAN can be set for underlying disk while re-assemble raid
>> device.
>>
>> Fixes: e5cfefa97bcc ("block: fix scan partition for exclusively open device again")
>> Signed-off-by: Yu Kuai <yukuai3@huawei.com>
> 
> The issue still can't be avoided completely, such as, after rebooting,
> /dev/sda1 & /dev/md0p1 can be observed at the same time. And this one
> should be underlying partitions scanned before re-assembling raid, I
> guess it may not be easy to avoid.

Yes, this is possible and I don't know how to fix this yet...
> 
> Also seems the following change added in e5cfefa97bcc isn't necessary:
> 
>                  /* Make sure the first partition scan will be proceed */
>                  if (get_capacity(disk) && !(disk->flags & GENHD_FL_NO_PART) &&
>                      !test_bit(GD_SUPPRESS_PART_SCAN, &disk->state))
>                          set_bit(GD_NEED_PART_SCAN, &disk->state);
> 
> since the following disk_scan_partitions() in device_add_disk() should cover
> partitions scan.

This can't be guaranteed if someone else open the device excl after
bdev_add and before disk_scan_partitions:

t1: 			t2:
device_add_disk
  bdev_add
   insert_inode_hash
			// open device excl
  disk_scan_partitions
  // will fail

However, this is just in theory, and it's unlikely to happen in
practice.

Thanks,
Kuai
> 
>> ---
>>   block/genhd.c | 8 +++++++-
>>   1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/block/genhd.c b/block/genhd.c
>> index 08bb1a9ec22c..a72e27d6779d 100644
>> --- a/block/genhd.c
>> +++ b/block/genhd.c
>> @@ -368,7 +368,6 @@ int disk_scan_partitions(struct gendisk *disk, fmode_t mode)
>>   	if (disk->open_partitions)
>>   		return -EBUSY;
>>   
>> -	set_bit(GD_NEED_PART_SCAN, &disk->state);
>>   	/*
>>   	 * If the device is opened exclusively by current thread already, it's
>>   	 * safe to scan partitons, otherwise, use bd_prepare_to_claim() to
>> @@ -381,12 +380,19 @@ int disk_scan_partitions(struct gendisk *disk, fmode_t mode)
>>   			return ret;
>>   	}
>>   
>> +	set_bit(GD_NEED_PART_SCAN, &disk->state);
>>   	bdev = blkdev_get_by_dev(disk_devt(disk), mode & ~FMODE_EXCL, NULL);
>>   	if (IS_ERR(bdev))
>>   		ret =  PTR_ERR(bdev);
>>   	else
>>   		blkdev_put(bdev, mode & ~FMODE_EXCL);
>>   
>> +	/*
>> +	 * If blkdev_get_by_dev() failed early, GD_NEED_PART_SCAN is still set,
>> +	 * and this will cause that re-assemble partitioned raid device will
>> +	 * creat partition for underlying disk.
>> +	 */
>> +	clear_bit(GD_NEED_PART_SCAN, &disk->state);
> 
> I feel GD_NEED_PART_SCAN becomes a bit hard to follow.
> 
> So far, it is only consumed by blkdev_get_whole(), and cleared in
> bdev_disk_changed(). That means partition scan can be retried
> if bdev_disk_changed() fails.
> 
> Another mess is that more drivers start to touch this flag, such as
> nbd/sd, probably it is better to change them into one API of
> blk_disk_need_partition_scan(), and hide implementation detail
> to drivers.
> 
> 
> thanks,
> Ming
> 
> .
>