linux-kernel - Re: [PATCH] block: tolerate 0 byte discard_granularity in __blkdev_issue

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Message-ID: <06f2bb53-8917-82d6-3e0c-76270cd80e06@suse.de>
Date:   Tue, 4 Aug 2020 22:45:19 +0800
From:   Coly Li <colyli@...e.de>
To:     Johannes Thumshirn <Johannes.Thumshirn@....com>,
        "linux-block@...r.kernel.org" <linux-block@...r.kernel.org>
Cc:     "linux-bcache@...r.kernel.org" <linux-bcache@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Ming Lei <ming.lei@...hat.com>,
        Hannes Reinecke <hare@...e.com>, Xiao Ni <xni@...hat.com>,
        Bart Van Assche <bvanassche@....org>,
        Christoph Hellwig <hch@....de>,
        Enzo Matsumiya <ematsumiya@...e.com>,
        Jens Axboe <axboe@...nel.dk>, Evan Green <evgreen@...omium.org>
Subject: Re: [PATCH] block: tolerate 0 byte discard_granularity in
 __blkdev_issue_discard()

On 2020/8/4 22:39, Johannes Thumshirn wrote:
> On 04/08/2020 16:37, 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
>>
> 
> Damn I didn't want to hit sent....
> 
> Does this (untested) change make a difference:
> 
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 475e1a738560..8a07a89d702e 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -895,6 +895,9 @@ static void loop_config_discard(struct loop_device *lo)
>                 blk_queue_max_write_zeroes_sectors(q,
>                         backingq->limits.max_write_zeroes_sectors);
>  
> +               q->limits.discard_granularity =
> +                       backingq->limits.discard_granularity;
> +
>         /*
>          * We use punch hole to reclaim the free space used by the
>          * image a.k.a. discard. However we do not support discard if
> 

Yes, Ming just posts a patch with a very similar change to loop device
driver.

Coly Li