Re: btrfs regression since 4.X kernel NULL pointer dereference

[Jeff Mahoney <jeffm@suse.com>]

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On 8/31/15 8:06 PM, Chris Mason wrote:
> On Mon, Aug 31, 2015 at 07:32:09PM +0200, Stefan Priebe -
> Profihost AG wrote:
>>> Am 25.08.2015 um 15:51 schrieb Chris Mason <clm@fb.com>:
>>> 
>>>> On Tue, Aug 25, 2015 at 11:00:30AM +0200, Christoph Hellwig 
>>>> wrote: I think this is btrfs using a struct block_device
>>>> that doesn't have a valid queue pointer in it's gendisk for 
>>>> ->s_bdev.  And there are some fishy looking ->s_bdev 
>>>> assignments in the code which I suspect are related to it:
>>>> 
>>>> fs/btrfs/dev-replace.c: if (fs_info->sb->s_bdev == 
>>>> src_device->bdev) fs/btrfs/dev-replace.c: fs_info->sb->s_bdev
>>>> = tgt_device->bdev; fs/btrfs/volumes.c: if (device->bdev ==
>>>> root->fs_info->sb->s_bdev) fs/btrfs/volumes.c:
>>>> root->fs_info->sb->s_bdev = next_device->bdev;
>>>> fs/btrfs/volumes.c:     if (tgtdev->bdev ==
>>>> fs_info->sb->s_bdev) fs/btrfs/volumes.c: fs_info->sb->s_bdev
>>>> = next_device->bdev;
>>> 
>>> We've had trouble with this in the past, I'll take a look.
>> 
>> Any news?
> 
> Haven't been able to reproduce yet, I'll try again in the morning.

I'm unable to reproduce it as well and I'm hearing about people
hitting it in different forums.

Can you describe your storage configuration?  There's a gendisk
without a queue sneaking in somehow.  There are some spots in dm where
the mddev gets a new queue assigned to it but the old queue associated
with the gendisk is left behind so that's not it.  If you're able to
reproduce it reliably with this configuration, that's certainly
interesting.  I'm wondering if the case is that vgcfgbackup is
iterating over devices and causing something that might not have been
previously scanned to be scanned and added to the btrfs device list.
Are there any messages above the oops indicating when a device is
added?  Could you post your complete dmesg somewhere?

Thanks,

- -Jeff

- -- 
Jeff Mahoney
SUSE Labs
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