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Subject: Re: [PATCH v3] block: check queue's limits.discard_granularity in
 __blkdev_issue_discard()
To:     "Darrick J. Wong" <darrick.wong@oracle.com>
Cc:     linux-block@vger.kernel.org, linux-bcache@vger.kernel.org,
        linux-kernel@vger.kernel.org, Ming Lei <ming.lei@redhat.com>,
        Bart Van Assche <bvanassche@acm.org>,
        Christoph Hellwig <hch@lst.de>,
        Enzo Matsumiya <ematsumiya@suse.com>,
        Evan Green <evgreen@chromium.org>,
        Hannes Reinecke <hare@suse.com>, Jens Axboe <axboe@kernel.dk>,
        "Martin K . Petersen" <martin.petersen@oracle.com>,
        Xiao Ni <xni@redhat.com>
References: <20200805063150.41037-1-colyli@suse.de>
 <20200805154843.GB191798@magnolia>
From:   Coly Li <colyli@suse.de>
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Message-ID: <f018ad35-9b1c-4a31-1499-3da16cd8001b@suse.de>
Date:   Thu, 6 Aug 2020 00:49:43 +0800
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On 2020/8/5 23:48, Darrick J. Wong wrote:
> On Wed, Aug 05, 2020 at 02:31:50PM +0800, Coly Li wrote:
>> If create a loop device with a backing NVMe SSD, current loop device
>> driver doesn't correctly set its  queue's limits.discard_granularity and
>> leaves it as 0. If a discard request at LBA 0 on this loop device, in
>> __blkdev_issue_discard() the calculated req_sects will be 0, and a zero
>> length discard request will trigger a BUG() panic in generic block layer
>> code at block/blk-mq.c:563.
>>
>> [  955.565006][   C39] ------------[ cut here ]------------
>> [  955.559660][   C39] invalid opcode: 0000 [#1] SMP NOPTI
>> [  955.622171][   C39] CPU: 39 PID: 248 Comm: ksoftirqd/39 Tainted: G            E     5.8.0-default+ #40
>> [  955.622171][   C39] Hardware name: Lenovo ThinkSystem SR650 -[7X05CTO1WW]-/-[7X05CTO1WW]-, BIOS -[IVE160M-2.70]- 07/17/2020
>> [  955.622175][   C39] RIP: 0010:blk_mq_end_request+0x107/0x110
>> [  955.622177][   C39] Code: 48 8b 03 e9 59 ff ff ff 48 89 df 5b 5d 41 5c e9 9f ed ff ff 48 8b 35 98 3c f4 00 48 83 c7 10 48 83 c6 19 e8 cb 56 c9 ff eb cb <0f> 0b 0f 1f 80 00 00 00 00 0f 1f 44 00 00 55 48 89 e5 41 56 41 54
>> [  955.622179][   C39] RSP: 0018:ffffb1288701fe28 EFLAGS: 00010202
>> [  955.749277][   C39] RAX: 0000000000000001 RBX: ffff956fffba5080 RCX: 0000000000004003
>> [  955.749278][   C39] RDX: 0000000000000003 RSI: 0000000000000000 RDI: 0000000000000000
>> [  955.749279][   C39] RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
>> [  955.749279][   C39] R10: ffffb1288701fd28 R11: 0000000000000001 R12: ffffffffa8e05160
>> [  955.749280][   C39] R13: 0000000000000004 R14: 0000000000000004 R15: ffffffffa7ad3a1e
>> [  955.749281][   C39] FS:  0000000000000000(0000) GS:ffff95bfbda00000(0000) knlGS:0000000000000000
>> [  955.749282][   C39] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [  955.749282][   C39] CR2: 00007f6f0ef766a8 CR3: 0000005a37012002 CR4: 00000000007606e0
>> [  955.749283][   C39] DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
>> [  955.749284][   C39] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
>> [  955.749284][   C39] PKRU: 55555554
>> [  955.749285][   C39] Call Trace:
>> [  955.749290][   C39]  blk_done_softirq+0x99/0xc0
>> [  957.550669][   C39]  __do_softirq+0xd3/0x45f
>> [  957.550677][   C39]  ? smpboot_thread_fn+0x2f/0x1e0
>> [  957.550679][   C39]  ? smpboot_thread_fn+0x74/0x1e0
>> [  957.550680][   C39]  ? smpboot_thread_fn+0x14e/0x1e0
>> [  957.550684][   C39]  run_ksoftirqd+0x30/0x60
>> [  957.550687][   C39]  smpboot_thread_fn+0x149/0x1e0
>> [  957.886225][   C39]  ? sort_range+0x20/0x20
>> [  957.886226][   C39]  kthread+0x137/0x160
>> [  957.886228][   C39]  ? kthread_park+0x90/0x90
>> [  957.886231][   C39]  ret_from_fork+0x22/0x30
>> [  959.117120][   C39] ---[ end trace 3dacdac97e2ed164 ]---
>>
>> 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
>>
>> This patch fixes the issue by checking q->limits.discard_granularity in
>> __blkdev_issue_discard() before composing the discard bio. If the value
>> is 0, then prints a warning oops information and returns -EOPNOTSUPP to
>> the caller to indicate that this buggy device driver doesn't support
>> discard request.
>>
>> Fixes: 9b15d109a6b2 ("block: improve discard bio alignment in __blkdev_issue_discard()")
>> Fixes: c52abf563049 ("loop: Better discard support for block devices")
>> Reported-and-suggested-by: Ming Lei <ming.lei@redhat.com>
>> Signed-off-by: Coly Li <colyli@suse.de>
>> Reviewed-by: Ming Lei <ming.lei@redhat.com>
>> Cc: Bart Van Assche <bvanassche@acm.org>
>> Cc: Christoph Hellwig <hch@lst.de>
>> Cc: Enzo Matsumiya <ematsumiya@suse.com>
>> Cc: Evan Green <evgreen@chromium.org>
>> Cc: Hannes Reinecke <hare@suse.com>
>> Cc: Jens Axboe <axboe@kernel.dk>
>> Cc: Martin K. Petersen <martin.petersen@oracle.com>
>> Cc: Xiao Ni <xni@redhat.com>
>> ---
>> Changelog:
>> v3: print device name assocated with the buggy driver.
>> v2: fix typo of the wrong return error code.
>> v1: first version.
>>
>>  block/blk-lib.c | 9 +++++++++
>>  1 file changed, 9 insertions(+)
>>
>> diff --git a/block/blk-lib.c b/block/blk-lib.c
>> index 019e09bb9c0e..d3bbb3d9fac3 100644
>> --- a/block/blk-lib.c
>> +++ b/block/blk-lib.c
>> @@ -47,6 +47,15 @@ int __blkdev_issue_discard(struct block_device *bdev, sector_t sector,
>>  		op = REQ_OP_DISCARD;
>>  	}
>>  
>> +	/* In case the discard granularity isn't set by buggy device driver */
>> +	if (WARN_ON_ONCE(!q->limits.discard_granularity)) {
>> +		char dev_name[BDEVNAME_SIZE];
>> +
>> +		bdevname(bdev, dev_name);
>> +		pr_err("%s: Error: discard_granularity is 0.\n", dev_name);
> 
> Hm, you might want to ratelimit this, before some buggy device +
> careless program flood dmesg.
> 

Sure, I will use pr_err_ratelimit() in next version.

> Also, why is it necessary to WARN_ON_ONCE /and/ pr_err the same
> condition?
> 

The WARN_ON_ONCE() just though the warning message, but does not point
out which device is buggy. Indeed this is a serious problem, so current
kernel message is a warning oops info and followed by "loop: Error:
discard_granularity is 0."

Thanks.

Coly Li