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Subject: Re: [PATCH 2/2 v3] nbd: fix possible page fault for nbd disk
To:     Mike Christie <mchristi@redhat.com>, josef@toxicpanda.com,
        axboe@kernel.dk
Cc:     linux-block@vger.kernel.org, nbd@other.debian.org,
        linux-kernel@vger.kernel.org
References: <20190822075923.11996-1-xiubli@redhat.com>
 <20190822075923.11996-3-xiubli@redhat.com> <5D686498.5090602@redhat.com>
From:   Xiubo Li <xiubli@redhat.com>
Message-ID: <78d16d10-1d06-6ce1-7c51-64c42e51f549@redhat.com>
Date:   Fri, 30 Aug 2019 08:58:42 +0800
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On 2019/8/30 7:49, Mike Christie wrote:
> On 08/22/2019 02:59 AM, xiubli@redhat.com wrote:
>> From: Xiubo Li <xiubli@redhat.com>
>>
>> When the NBD_CFLAG_DESTROY_ON_DISCONNECT flag is set and at the same
>> time when the socket is closed due to the server daemon is restarted,
>> just before the last DISCONNET is totally done if we start a new connection
>> by using the old nbd_index, there will be crashing randomly, like:
>>
>> <3>[  110.151949] block nbd1: Receive control failed (result -32)
>> <1>[  110.152024] BUG: unable to handle page fault for address: 0000058000000840
>> <1>[  110.152063] #PF: supervisor read access in kernel mode
>> <1>[  110.152083] #PF: error_code(0x0000) - not-present page
>> <6>[  110.152094] PGD 0 P4D 0
>> <4>[  110.152106] Oops: 0000 [#1] SMP PTI
>> <4>[  110.152120] CPU: 0 PID: 6698 Comm: kworker/u5:1 Kdump: loaded Not tainted 5.3.0-rc4+ #2
>> <4>[  110.152136] Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
>> <4>[  110.152166] Workqueue: knbd-recv recv_work [nbd]
>> <4>[  110.152187] RIP: 0010:__dev_printk+0xd/0x67
>> <4>[  110.152206] Code: 10 e8 c5 fd ff ff 48 8b 4c 24 18 65 48 33 0c 25 28 00 [...]
>> <4>[  110.152244] RSP: 0018:ffffa41581f13d18 EFLAGS: 00010206
>> <4>[  110.152256] RAX: ffffa41581f13d30 RBX: ffff96dd7374e900 RCX: 0000000000000000
>> <4>[  110.152271] RDX: ffffa41581f13d20 RSI: 00000580000007f0 RDI: ffffffff970ec24f
>> <4>[  110.152285] RBP: ffffa41581f13d80 R08: ffff96dd7fc17908 R09: 0000000000002e56
>> <4>[  110.152299] R10: ffffffff970ec24f R11: 0000000000000003 R12: ffff96dd7374e900
>> <4>[  110.152313] R13: 0000000000000000 R14: ffff96dd7374e9d8 R15: ffff96dd6e3b02c8
>> <4>[  110.152329] FS:  0000000000000000(0000) GS:ffff96dd7fc00000(0000) knlGS:0000000000000000
>> <4>[  110.152362] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
>> <4>[  110.152383] CR2: 0000058000000840 CR3: 0000000067cc6002 CR4: 00000000001606f0
>> <4>[  110.152401] Call Trace:
>> <4>[  110.152422]  _dev_err+0x6c/0x83
>> <4>[  110.152435]  nbd_read_stat.cold+0xda/0x578 [nbd]
>> <4>[  110.152448]  ? __switch_to_asm+0x34/0x70
>> <4>[  110.152468]  ? __switch_to_asm+0x40/0x70
>> <4>[  110.152478]  ? __switch_to_asm+0x34/0x70
>> <4>[  110.152491]  ? __switch_to_asm+0x40/0x70
>> <4>[  110.152501]  ? __switch_to_asm+0x34/0x70
>> <4>[  110.152511]  ? __switch_to_asm+0x40/0x70
>> <4>[  110.152522]  ? __switch_to_asm+0x34/0x70
>> <4>[  110.152533]  recv_work+0x35/0x9e [nbd]
>> <4>[  110.152547]  process_one_work+0x19d/0x340
>> <4>[  110.152558]  worker_thread+0x50/0x3b0
>> <4>[  110.152568]  kthread+0xfb/0x130
>> <4>[  110.152577]  ? process_one_work+0x340/0x340
>> <4>[  110.152609]  ? kthread_park+0x80/0x80
>> <4>[  110.152637]  ret_from_fork+0x35/0x40
>>
>> This is very easy to reproduce by running the nbd-runner.
>>
>> Signed-off-by: Xiubo Li <xiubli@redhat.com>
>> ---
>>   drivers/block/nbd.c | 36 +++++++++++++++++++++++++++++++++++-
>>   1 file changed, 35 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
>> index 8c2f17b99224..a25b59725c6e 100644
>> --- a/drivers/block/nbd.c
>> +++ b/drivers/block/nbd.c
>> @@ -26,6 +26,7 @@
>>   #include <linux/ioctl.h>
>>   #include <linux/mutex.h>
>>   #include <linux/compiler.h>
>> +#include <linux/completion.h>
>>   #include <linux/err.h>
>>   #include <linux/kernel.h>
>>   #include <linux/slab.h>
>> @@ -80,6 +81,9 @@ struct link_dead_args {
>>   #define NBD_RT_DESTROY_ON_DISCONNECT	6
>>   #define NBD_RT_DISCONNECT_ON_CLOSE	7
>>   
>> +#define NBD_DESTROY_ON_DISCONNECT	0
>> +#define NBD_DISCONNECT_REQUESTED	1
>> +
>>   struct nbd_config {
>>   	u32 flags;
>>   	unsigned long runtime_flags;
>> @@ -112,6 +116,9 @@ struct nbd_device {
>>   	struct list_head list;
>>   	struct task_struct *task_recv;
>>   	struct task_struct *task_setup;
>> +
>> +	struct completion destroy_complete;
>> +	unsigned long flags;
>>   };
>>   
>>   #define NBD_CMD_REQUEUED	1
>> @@ -222,6 +229,16 @@ static void nbd_dev_remove(struct nbd_device *nbd)
>>   		disk->private_data = NULL;
>>   		put_disk(disk);
>>   	}
>> +
>> +	/*
>> +	 * Place this in the last just before the nbd is freed to
>> +	 * make sure that the disk and the related kobject are also
>> +	 * totally removed to avoid duplicate creation of the same
>> +	 * one.
>> +	 */
>> +	if (test_bit(NBD_DESTROY_ON_DISCONNECT, &nbd->flags))
>> +		complete(&nbd->destroy_complete);
>> +
>>   	kfree(nbd);
>>   }
>>   
>> @@ -230,8 +247,8 @@ static void nbd_put(struct nbd_device *nbd)
>>   	if (refcount_dec_and_mutex_lock(&nbd->refs,
>>   					&nbd_index_mutex)) {
>>   		idr_remove(&nbd_index_idr, nbd->index);
>> -		mutex_unlock(&nbd_index_mutex);
>>   		nbd_dev_remove(nbd);
>> +		mutex_unlock(&nbd_index_mutex);
>>   	}
>>   }
>>   
>> @@ -1103,6 +1120,7 @@ static int nbd_disconnect(struct nbd_device *nbd)
>>   
>>   	dev_info(disk_to_dev(nbd->disk), "NBD_DISCONNECT\n");
>>   	set_bit(NBD_RT_DISCONNECT_REQUESTED, &config->runtime_flags);
>> +	set_bit(NBD_DISCONNECT_REQUESTED, &nbd->flags);
>>   	send_disconnects(nbd);
>>   	return 0;
>>   }
>> @@ -1596,6 +1614,7 @@ static int nbd_dev_add(int index)
>>   	nbd->tag_set.flags = BLK_MQ_F_SHOULD_MERGE |
>>   		BLK_MQ_F_BLOCKING;
>>   	nbd->tag_set.driver_data = nbd;
>> +	init_completion(&nbd->destroy_complete);
>>   
>>   	err = blk_mq_alloc_tag_set(&nbd->tag_set);
>>   	if (err)
>> @@ -1761,6 +1780,16 @@ static int nbd_genl_connect(struct sk_buff *skb, struct genl_info *info)
>>   		mutex_unlock(&nbd_index_mutex);
>>   		return -EINVAL;
>>   	}
>> +
>> +	if (test_bit(NBD_DESTROY_ON_DISCONNECT, &nbd->flags) &&
> Why does this have to be set? If this is not set would you end up
> hitting the config_refs check:
>
> if (refcount_read(&nbd->config_refs)) {
>
> and possibly returning failure?

Yeah, this is a good question. Before I have also tried to fix it with 
this, but it still won't work for me.

 From my test cases almost more than 50% times, the crash will be hit in 
the gap just after the nbd->config already been released, and before the 
nbd itself not yet, so the nbd->config_refs will be 0.


>
> If you moved the complete() to nbd_config_put would it work if this bit
> was set or not?

Tried it already, it still won't work.

There is one case that when disconnecting the nbd device, the userspace 
service will do the open()/release()? things, please see [1], and the 
sequence is not the same every time, if the 
NBD_CFLAG_DESTROY_ON_DISCONNECT bit is set the crash still exists.

So sometimes when the nbd_put() called from the nbd_config_put(), the 
&nbd->refs in nbd_put won't be 0, it could be 1. And it will be 0 just 
after the release() is triggered later.

So I just place the complete() before "free(nbd);", or there will be 
another Call trace will be seen very often:

 ?? 2489 Aug 20 18:10:04 lxbfd2 kernel: sysfs: cannot create duplicate 
filename '/devices/virtual/block/nbd0'
 ?? 2490 Aug 20 18:10:04 lxbfd2 kernel: CPU: 0 PID: 8635 Comm: nbd-clid 
Kdump: loaded Tainted: G????? D 5.1.18-300.fc30.x86_64 #1
 ?? 2491 Aug 20 18:10:04 lxbfd2 kernel: Hardware name: Red Hat KVM, BIOS 
0.5.1 01/01/2011
 ?? 2492 Aug 20 18:10:04 lxbfd2 kernel: Call Trace:
 ?? 2493 Aug 20 18:10:04 lxbfd2 kernel: dump_stack+0x5c/0x80
 ?? 2494 Aug 20 18:10:04 lxbfd2 kernel: sysfs_warn_dup.cold+0x17/0x2d
 ?? 2495 Aug 20 18:10:04 lxbfd2 kernel: sysfs_create_dir_ns+0xb6/0xd0
 ?? 2496 Aug 20 18:10:04 lxbfd2 kernel: kobject_add_internal+0xb7/0x280
 ?? 2497 Aug 20 18:10:04 lxbfd2 kernel: kobject_add+0x7e/0xb0
 ?? 2498 Aug 20 18:10:04 lxbfd2 kernel: ? _cond_resched+0x15/0x30
 ?? 2499 Aug 20 18:10:04 lxbfd2 kernel: device_add+0x12b/0x690
 ?? 2500 Aug 20 18:10:04 lxbfd2 kernel: __device_add_disk+0x1b5/0x470
 ?? 2501 Aug 20 18:10:04 lxbfd2 kernel: nbd_dev_add+0x21d/0x2b0 [nbd]
 ?? 2502 Aug 20 18:10:04 lxbfd2 kernel: nbd_genl_connect+0x16e/0x630 [nbd]
 ?? 2503 Aug 20 18:10:04 lxbfd2 kernel: genl_family_rcv_msg+0x1a9/0x3b0
 ?? 2504 Aug 20 18:10:04 lxbfd2 kernel: ? __switch_to_asm+0x35/0x70
 ?? 2505 Aug 20 18:10:04 lxbfd2 kernel: ? __switch_to_asm+0x41/0x70
 ?? 2506 Aug 20 18:10:04 lxbfd2 kernel: ? __switch_to+0x11f/0x4c0
 ?? 2507 Aug 20 18:10:04 lxbfd2 kernel: ? __switch_to_asm+0x41/0x70
 ?? [...]


[1] https://lkml.org/lkml/2019/5/29/125


>
>> +	    test_bit(NBD_DISCONNECT_REQUESTED, &nbd->flags)) {
>> +		mutex_unlock(&nbd_index_mutex);
>> +
>> +		/* Wait untill the recv_work exit */
> If that is all you need we could do a flush_workqueue(nbd->recv_workq)
> (you would need Jens's for next branch which has some work queue changes
> in nbd).

Yeah, this makes sense.

> I think that might be too messy with how we do the puts for the
> nbd_device and config and the locking though.
>
>
>> +		wait_for_completion(&nbd->destroy_complete);
> The completion is allocated as part of the nbd device struct. Right
> after the other thread does a complete() we will free the nbd device
> struct, and we could still access the destroy_complete completion in
> this thread in wait_for_completion.
>
> You would want to do DECLARE_COMPLETION_ONSTACK in this function, make
> destroy_complete a pointer and set the destroy_complete pointer to the
> completion declared in this function.
>
> In nbd_dev_remove you would then just check if destroy_complete is set
> to a non-NULL pointer.

Hmm, good catch.

Will fix it.

Thanks Mike.
BRs
Xiubo


>
>
>
>> +		goto again;
>> +	}
>> +
>>   	if (!refcount_inc_not_zero(&nbd->refs)) {
>>   		mutex_unlock(&nbd_index_mutex);
>>   		if (index == -1)
>> @@ -1817,7 +1846,10 @@ static int nbd_genl_connect(struct sk_buff *skb, struct genl_info *info)
>>   		if (flags & NBD_CFLAG_DESTROY_ON_DISCONNECT) {
>>   			set_bit(NBD_RT_DESTROY_ON_DISCONNECT,
>>   				&config->runtime_flags);
>> +			set_bit(NBD_DESTROY_ON_DISCONNECT, &nbd->flags);
>>   			put_dev = true;
>> +		} else {
>> +			clear_bit(NBD_DESTROY_ON_DISCONNECT, &nbd->flags);
>>   		}
>>   		if (flags & NBD_CFLAG_DISCONNECT_ON_CLOSE) {
>>   			set_bit(NBD_RT_DISCONNECT_ON_CLOSE,
>> @@ -1987,10 +2019,12 @@ static int nbd_genl_reconfigure(struct sk_buff *skb, struct genl_info *info)
>>   			if (!test_and_set_bit(NBD_RT_DESTROY_ON_DISCONNECT,
>>   					      &config->runtime_flags))
>>   				put_dev = true;
>> +			set_bit(NBD_DESTROY_ON_DISCONNECT, &nbd->flags);
>>   		} else {
>>   			if (test_and_clear_bit(NBD_RT_DESTROY_ON_DISCONNECT,
>>   					       &config->runtime_flags))
>>   				refcount_inc(&nbd->refs);
>> +			clear_bit(NBD_DESTROY_ON_DISCONNECT, &nbd->flags);
>>   		}
>>   
>>   		if (flags & NBD_CFLAG_DISCONNECT_ON_CLOSE) {
>>