linux-kernel - Re: [PATCH] NBD: replace kill_bdev() with __invalidate

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Date:	Wed, 20 Apr 2016 13:06:34 +0200
From:	Markus Pargmann <mpa@...gutronix.de>
To:	Ratna Manoj <manoj.br@...il.com>
Cc:	pbonzini@...hat.com, jack@...e.cz, grao@...tworx.com,
	jv@...tworx.com, nbd-general@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] NBD: replace kill_bdev() with __invalidate_device()

Hi,

On Thursday 24 March 2016 07:04:10 Ratna Manoj wrote:
> From: Ratna Manoj Bolla <manoj.br@...il.com>
> 
> When a filesystem is mounted on a nbd device and on a disconnect, because 
> of kill_bdev(), and resetting bdev size to zero, buffer_head mappings are 
> getting destroyed under mounted filesystem.
> 
> After a bdev size reset(i.e bdev->bd_inode->i_size = 0) on a disconnect,
> followed by a sys_umount(),
>         generic_shutdown_super()->...
>         ->__sync_blockdev()->...
>         -> blkdev_writepages()->...
>         ->do_invalidatepage()->...
>         -> discard_buffer()   is discarding superblock buffer_head assumed
> to be in mapped state by ext4_commit_super().
> 
> 
> 
> Signed-off-by: Ratna Manoj Bolla <manoj.br@...il.com>
> ---
> This script reproduces both the kernel panic scenarios:
>  
> $ qemu-img create -f qcow2 f.img 1G
> $ mkfs.ext4 f.img
> $ qemu-nbd -c /dev/nbd0 f.img
> $ mount /dev/nbd0 dir
> $ killall -KILL qemu-nbd
> $ sleep 1
> $ ls dir
> $ umount dir
> 
> Bug reports:
> http://www.kernelhub.org/?p=2&msg=361407
> https://www.mail-archive.com/nbd-general@lists.sourceforge.net/msg02388.html

Thanks, please CC nbd-general@...ts.sourceforge.net,
linux-kernel@...r.kernel.org as well.

So this patch simply does not cleanup the blockdevice to avoid any
errors on the filesystem side. The userspace thread that called
NBD_DO_IT will exit immediately before the filesystem decided to release
the blockdevice. The nbd driver assumes that the shutdown was done and
accepts new clients setting up sockets and so on. Couldn't this lead to
a lot of problems?

Currently NBD_DO_IT returns when it is save to use the NBD device again.
This patch changes this as the blockdevice may still be in use when
NBD_DO_IT returns. I think it would be better to delay NBD_DO_IT until
everything is cleaned up and all filesystems are closed.

Best Regards,

Markus

> 
> diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
> index f6b51d7..6e77b3a 100644
> --- a/drivers/block/nbd.c
> +++ b/drivers/block/nbd.c
> @@ -119,7 +119,8 @@ static const char *nbdcmd_to_ascii(int cmd)
>  
>  static int nbd_size_clear(struct nbd_device *nbd, struct block_device *bdev)
>  {
> -	bdev->bd_inode->i_size = 0;
> +	if (bdev->bd_openers <= 1)
> +		bdev->bd_inode->i_size = 0;
>  	set_capacity(nbd->disk, 0);
>  	kobject_uevent(&nbd_to_dev(nbd)->kobj, KOBJ_CHANGE);
>  
> @@ -678,6 +679,9 @@ static void nbd_reset(struct nbd_device *nbd)
>  
>  static void nbd_bdev_reset(struct block_device *bdev)
>  {
> +	if (bdev->bd_openers > 1)
> +		return;
> +
>  	set_device_ro(bdev, false);
>  	bdev->bd_inode->i_size = 0;
>  	if (max_part > 0) {
> @@ -735,7 +739,7 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
>  		nbd_clear_que(nbd);
>  		BUG_ON(!list_empty(&nbd->queue_head));
>  		BUG_ON(!list_empty(&nbd->waiting_queue));
> -		kill_bdev(bdev);
> +		__invalidate_device(bdev, true);
>  		return 0;
>  
>  	case NBD_SET_SOCK: {
> @@ -809,7 +813,7 @@ static int __nbd_ioctl(struct block_device *bdev, struct nbd_device *nbd,
>  
>  		sock_shutdown(nbd);
>  		nbd_clear_que(nbd);
> -		kill_bdev(bdev);
> +		__invalidate_device(bdev, true);
>  		nbd_bdev_reset(bdev);
>  
>  		if (nbd->disconnect) /* user requested, ignore socket errors */
> 
> 
> 

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