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Date:   Wed, 5 Aug 2020 09:54:00 +0800
From:   Coly Li <colyli@...e.de>
To:     Ming Lei <ming.lei@...hat.com>
Cc:     linux-block@...r.kernel.org, linux-bcache@...r.kernel.org,
        linux-kernel@...r.kernel.org, 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 2020/8/5 07:58, Ming Lei wrote:
> On Tue, Aug 04, 2020 at 10:23:32PM +0800, Coly Li wrote:
>> When some buggy driver doesn't set its queue->limits.discard_granularity
>> (e.g. current loop device driver), discard at LBA 0 on such device will
>> trigger a kernel BUG() panic from 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 is how the BUG() panic triggered by __blkdev_issue_discard(),
>> - For a NVMe SSD backing loop device, the driver does not initialize
>>   its queue->limits.discard_granularity and leaves it to 0.
>> - When discard on LBA 0 of the loop device, __blkdev_issue_discard()
>>   is called before loop device driver code.
>> - Inside __blkdev_issue_discard(), when calculating value of
>>   granularity_aligned_lba by
>> 	granularity_aligned_lba = round_up(sector_mapped,
>> 			q->limits.discard_granularity >> SECTOR_SHIFT);
>>   because sector_mapped is 0 (at LBA 0 and no partition offset), and
>>   q->limits.discard_granularity is 0 (by the buggy loop driver), the
>>   calculated granularity_aligned_lba is 0.
>> - The inline function bio_aligned_discard_max_sectors() is defined as
>> 	return round_down(UINT_MAX, q->limits.discard_granularity) >>
>> 			SECTOR_SHIFT;
>>    when q->limits.discard_granularity is 0 from loop device driver, the
>>    above calculation returns value 0.
>> - Now granularity_aligned_lba and sctor_mapped are 0, req_sectors is
>>   calculated by the following lines in __blkdev_issue_discard(),
>> 	if (granularity_aligned_lba == sector_mapped)
>> 		req_sects = min_t(sector_t, nr_sects,
>> 				  bio_aligned_discard_max_sectors(q));
>>   because bio_aligned_discard_max_sectors(q) returns 0, req_sects is
>>   calculated as 0.
>> - Now a discard bio is mistakenly initialized as a 0 byte bio by,
>> 	bio->bi_iter.bi_size = req_sects << 9;
>>   and sent to loop device driver.
>> - This discard request is handled by loop device driver by following
>>   code path,
>>     loop_handle_cmd => do_req_filebacked => lo_fallocate =>
>>     file->f_op->fallocate => blkdev_fallocate => blkdev_issue_zeroout =>
>>     __blkdev_issue_write_zeroes
>> - Inside __blkdev_issue_write_zeroes(), a 0 byte length discard bio is
>>   composed and sent to the backing device of the loop device.
>> - In the I/O completion code path, in my case it is,
>>     blk_done_softirq => nrq->q->mq_ops->complete => nvme_pci_complete_rq
>>     => nvme_complete_rq => blk_mq_end_request
>>   inside blk_mq_end_request(), blk_update_request() is called and due to
>>   req->bio is NULL in previous step, blk_update_request() returns false
>>   then the BUG() panic in blk_mq_end_request() is triggered.
>>
>> Although the above panic can be fixed in loop device driver, the generic
>> __blkdev_issue_discard() should also be fixed to tolerate the incorrect
>> 0 value from queue->limits.discard_granularity, in case some other buggy
>> driver makes such mistake again.
>>
>> This patch checks whether q->limits.discard_granularity is 0 in
>> __blkdev_issue_discard() and bio_aligned_discard_max_sectors(). If it is
>> 0 from some buggy driver queue, prints a warning oops information and
>> set queue_logical_block_size(q) to a local variable discard_granularity.
>> This local variable is used in round_up() and round_down() calculation,
>> now req_sects won't be 0  and no empty discard request is generated.
>>
>> Fixes: 9b15d109a6b2 ("block: improve discard bio alignment in __blkdev_issue_discard()")
>> Fixes: c52abf563049 ("loop: Better discard support for block devices")
>> Reported-by: Ming Lei <ming.lei@...hat.com>
>> Signed-off-by: Coly Li <colyli@...e.de>
>> Cc: Hannes Reinecke <hare@...e.com>
>> Cc: Ming Lei <ming.lei@...hat.com>
>> Cc: Xiao Ni <xni@...hat.com>
>> Cc: Bart Van Assche <bvanassche@....org>
>> Cc: Christoph Hellwig <hch@....de>
>> Cc: Enzo Matsumiya <ematsumiya@...e.com>
>> Cc: Jens Axboe <axboe@...nel.dk>
>> Cc: Evan Green <evgreen@...omium.org>
>> ---
>>  block/blk-lib.c | 8 +++++++-
>>  block/blk.h     | 9 +++++++--
>>  2 files changed, 14 insertions(+), 3 deletions(-)
>>
>> diff --git a/block/blk-lib.c b/block/blk-lib.c
>> index 019e09bb9c0e..3017e4cba923 100644
>> --- a/block/blk-lib.c
>> +++ b/block/blk-lib.c
>> @@ -30,6 +30,7 @@ int __blkdev_issue_discard(struct block_device *bdev, sector_t sector,
>>  	struct bio *bio = *biop;
>>  	unsigned int op;
>>  	sector_t bs_mask, part_offset = 0;
>> +	sector_t discard_granularity;
>>  
>>  	if (!q)
>>  		return -ENXIO;
>> @@ -54,6 +55,11 @@ int __blkdev_issue_discard(struct block_device *bdev, sector_t sector,
>>  	if (!nr_sects)
>>  		return -EINVAL;
>>  
>> +	discard_granularity = q->limits.discard_granularity;
>> +	/* In case some buggy driver does not set limits.discard_granularity */
>> +	if (WARN_ON_ONCE(discard_granularity == 0))
>> +		discard_granularity = queue_logical_block_size(q);
> 
> This code path is supposed to not run in case of zero q->limits.discard_granularity,
> and looks it is fine to just warn and return -EINVAL in this case,
> see Documentation/ABI/testing/sysfs-block:
> 
> 	What:       /sys/block/<disk>/queue/discard_granularity
> 	Date:       May 2011
> 	Contact:    Martin K. Petersen <martin.petersen@...cle.com>
> 	Description:
> 	        Devices that support discard functionality may
> 	        internally allocate space using units that are bigger
> 	        than the logical block size. The discard_granularity
> 	        parameter indicates the size of the internal allocation
> 	        unit in bytes if reported by the device. Otherwise the
> 	        discard_granularity will be set to match the device's
> 	        physical block size. A discard_granularity of 0 means
> 	        that the device does not support discard functionality.
> 
> What we need to fix is loop driver, if it claims to support discard,
> q->limits.discard_granularity has to be one valid value.

Yes your suggestion is much simpler, let me do it :-)

Thanks.

Coly Li

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