Re: [PATCH] block: tolerate 0 byte discard_granularity in __blkdev_issue_discard()

[Coly Li]

On 2020/8/4 22:37, Johannes Thumshirn wrote:
> On 04/08/2020 16:34, Coly Li wrote:
>> On 2020/8/4 22:31, Johannes Thumshirn wrote:
>>> On 04/08/2020 16:23, Coly Li wrote:
>>>> This is the procedure to reproduce the panic,
>>>>   # modprobe scsi_debug delay=0 dev_size_mb=2048 max_queue=1
>>>>   # losetup -f /dev/nvme0n1 --direct-io=on
>>>>   # blkdiscard /dev/loop0 -o 0 -l 0x200
>>>
>>> losetup -f /dev/sdX isn't it?
>>>
>>
>> In my case, I use a NVMe SSD as the backing device of the loop device.
>> Because I don't have a scsi lun.
>>
>> And loading scsi_debug module seems necessary, otherwise the discard
>> process just hang and I cannot see the kernel panic (I don't know why yet).
> 
> OK, now that's highly interesting. Does it also happen if you back loop with
> a file? loop_config_discard() has different cases for the different backing devices/files. S
> 
No, for a file backing, q->limits.discard_granularity is set to
inode->i_sb->s_blocksize. And the encrypted loop device does not support
discard.

Such issue just only happens on a device backing loop device which
announces supporting discard. Without Ming's fix to loop device driver,
discard on LBA 0 will trigger the BUG() panic in my setup (Maybe it is
more easier to trigger this BUG() panic with scsi lun).

Coly Li