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From:   Johannes Thumshirn <Johannes.Thumshirn@wdc.com>
To:     Coly Li <colyli@suse.de>,
        "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>
CC:     "linux-bcache@vger.kernel.org" <linux-bcache@vger.kernel.org>,
        "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>
Subject: Re: [PATCH] block: tolerate 0 byte discard_granularity in
 __blkdev_issue_discard()
Thread-Topic: [PATCH] block: tolerate 0 byte discard_granularity in
 __blkdev_issue_discard()
Thread-Index: AQHWamrhOskbhhn9f0O9q4CKi9C/MA==
Date:   Tue, 4 Aug 2020 14:39:56 +0000
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On 04/08/2020 16:37, Johannes Thumshirn wrote:=0A=
> On 04/08/2020 16:34, Coly Li wrote:=0A=
>> On 2020/8/4 22:31, Johannes Thumshirn wrote:=0A=
>>> On 04/08/2020 16:23, Coly Li wrote:=0A=
>>>> This is the procedure to reproduce the panic,=0A=
>>>>   # modprobe scsi_debug delay=3D0 dev_size_mb=3D2048 max_queue=3D1=0A=
>>>>   # losetup -f /dev/nvme0n1 --direct-io=3Don=0A=
>>>>   # blkdiscard /dev/loop0 -o 0 -l 0x200=0A=
>>>=0A=
>>> losetup -f /dev/sdX isn't it?=0A=
>>>=0A=
>>=0A=
>> In my case, I use a NVMe SSD as the backing device of the loop device.=
=0A=
>> Because I don't have a scsi lun.=0A=
>>=0A=
>> And loading scsi_debug module seems necessary, otherwise the discard=0A=
>> process just hang and I cannot see the kernel panic (I don't know why ye=
t).=0A=
> =0A=
> OK, now that's highly interesting. Does it also happen if you back loop w=
ith=0A=
> a file? loop_config_discard() has different cases for the different backi=
ng devices/files. S=0A=
> =0A=
=0A=
Damn I didn't want to hit sent....=0A=
=0A=
Does this (untested) change make a difference:=0A=
=0A=
diff --git a/drivers/block/loop.c b/drivers/block/loop.c=0A=
index 475e1a738560..8a07a89d702e 100644=0A=
--- a/drivers/block/loop.c=0A=
+++ b/drivers/block/loop.c=0A=
@@ -895,6 +895,9 @@ static void loop_config_discard(struct loop_device *lo)=
=0A=
                blk_queue_max_write_zeroes_sectors(q,=0A=
                        backingq->limits.max_write_zeroes_sectors);=0A=
 =0A=
+               q->limits.discard_granularity =3D=0A=
+                       backingq->limits.discard_granularity;=0A=
+=0A=
        /*=0A=
         * We use punch hole to reclaim the free space used by the=0A=
         * image a.k.a. discard. However we do not support discard if=0A=
=0A=