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To:     Johannes Thumshirn <Johannes.Thumshirn@wdc.com>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>
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        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Ming Lei <ming.lei@redhat.com>,
        Hannes Reinecke <hare@suse.com>, Xiao Ni <xni@redhat.com>,
        Bart Van Assche <bvanassche@acm.org>,
        Christoph Hellwig <hch@lst.de>,
        Enzo Matsumiya <ematsumiya@suse.com>,
        Jens Axboe <axboe@kernel.dk>, Evan Green <evgreen@chromium.org>
References: <20200804142332.29961-1-colyli@suse.de>
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From:   Coly Li <colyli@suse.de>
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Subject: Re: [PATCH] block: tolerate 0 byte discard_granularity in
 __blkdev_issue_discard()
Message-ID: <2f70177e-4964-0fce-14bf-506b0821128b@suse.de>
Date:   Tue, 4 Aug 2020 22:43:53 +0800
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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