Re: [PATCH 2/2 v3] nbd: fix possible page fault for nbd disk

Mike Christie
2019-09-02T21:30:25Z

On 08/29/2019 07:58 PM, Xiubo Li wrote:
> 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:

Did this happen because you race with

nbd_put->nbd_dev_remove->del_gendisk->device_del->sysfs_remove_dir

? If so, does that still happen after you moved

mutex_unlock(&nbd_index_mutex);

in nbd_put? It seems before that part of your patch was added we could
hit this race and got the duplicate sysfs entry trace below:

1. nbd_put -> idr_remove.
2. nbd_put drops mutex.
3. nbd_genl_connect takes mutex (index != -1 for this call).
4. nbd_genl_connect-> idr_find fails due to remove in #1.
5. nbd_genl_connect->nbd_dev_add is then able to try to add the device
to sysfs before the nbd_put->nbd_dev_remove path has deleted the device.

When you now do the idr and sysfs removal and idr addition/search and
sysfs addition all under the nbd_index_mutex it shouldn't happen anymore.

>
> 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
> [...]
>