Hi, On Saturday 23 April 2016 07:47:21 Ratna Manoj wrote: > Thanks for the review. > > Atleast for ext4 this crash happens on a sys_umount() call, timing of > which is not in control of block driver. Block driver cannot force the > filesystems to be unmounted, and the file system does not expect > buffers to get unmapped under it.
Yes the block driver can't force a clean umount. > > Ext4 can be fixed with the this patch: > http://www.spinics.net/lists/linux-ext4/msg51112.html > It did not make to the kernel. It checks the state of the buffer head > before committing. > > When we consider diskett/CD as user space thread that called NBD_DO_IT, > this problem is analogous to changing disk with another or the same > disk suddenly when the file system is still mounted. > > If we completely kill the block device we would loss some writes when > same thread is reconnected. I am not so sure about your exact use-case here. If the NBD_DO_IT thread returns I am considering the connection and block device as dead and disconnected. Securing any data afterwards with a new connection is potentially dangerous as it may be a different server. > > if we do not completely kill or if we only invalidate clean buffers, > we will have inconsistency on re-attach with a different thread > (analogous to replacing disk with different disk suddenly). Yes exactly. That's why I suggested that NBD_DO_IT waits until all blockdevice users are gone. This would avoid any issues with writing/reading data to a wrong server. Best Regards, Markus > > Ratna. > > > On Wed, Apr 20, 2016 at 4:36 PM, Markus Pargmann <[email protected]> wrote: > > > Hi, > > > > On Thursday 24 March 2016 07:04:10 Ratna Manoj wrote: > > > From: Ratna Manoj Bolla <[email protected]> > > > > > > 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 <[email protected]> > > > --- > > > 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/[email protected]/msg02388.html > > > > Thanks, please CC [email protected], > > [email protected] 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 */ > > > > > > > > > > > > > -- > > Pengutronix e.K. | | > > Industrial Linux Solutions | http://www.pengutronix.de/ | > > Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | > > Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 | > > > > > -- Pengutronix e.K. | | Industrial Linux Solutions | http://www.pengutronix.de/ | Peiner Str. 6-8, 31137 Hildesheim, Germany | Phone: +49-5121-206917-0 | Amtsgericht Hildesheim, HRA 2686 | Fax: +49-5121-206917-5555 |
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