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 |
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.
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.
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).
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 |
>
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 |
On Thu, Apr 28, 2016 at 11:00:20AM +0200, Markus Pargmann wrote:
> 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.
Yes, from the kernel POV you're right.
However, at some point I agreed with Paul (your predecessor) that when
this happens due to an error condition (as opposed to it being due to an
explicit disconnect), the kernel would block all reads from or writes to
the device, and the client may try to reconnect *from the same
PID* (i.e., it may not fork()). If that succeeds, the next NBD_DO_IT is
assumed to be connected to the same server; if instead the process
exits, then the block device is assumed to be dead, will be reset, and
all pending reads or writes would error.
In principle, this allows for a proper reconnect from userspace if it
can be done. However, I'm not sure whether this ever worked well or
whether it was documented, so it's probably fine if you think it should
be replaced with something else.
(obviously, userspace reconnecting the device to a different device is
wrong and should not be done, but that's a case of "if you break it, you
get to keep both pieces)
At any rate, I think it makes sense for userspace to be given a chance
to *attempt* to reconnect a device when the connection drops
unexpectedly.
--
< ron> I mean, the main *practical* problem with C++, is there's like a dozen
people in the world who think they really understand all of its rules,
and pretty much all of them are just lying to themselves too.
-- #debian-devel, OFTC, 2016-02-12
Hi,
On Thursday 28 April 2016 09:57 PM, Wouter Verhelst wrote:
> On Thu, Apr 28, 2016 at 11:00:20AM +0200, Markus Pargmann wrote:
>> 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.
> Yes, from the kernel POV you're right.
>
> However, at some point I agreed with Paul (your predecessor) that when
> this happens due to an error condition (as opposed to it being due to an
> explicit disconnect), the kernel would block all reads from or writes to
> the device, and the client may try to reconnect *from the same
> PID* (i.e., it may not fork()). If that succeeds, the next NBD_DO_IT is
> assumed to be connected to the same server; if instead the process
> exits, then the block device is assumed to be dead, will be reset, and
> all pending reads or writes would error.
>
> In principle, this allows for a proper reconnect from userspace if it
> can be done. However, I'm not sure whether this ever worked well or
> whether it was documented, so it's probably fine if you think it should
> be replaced with something else.
>
> (obviously, userspace reconnecting the device to a different device is
> wrong and should not be done, but that's a case of "if you break it, you
> get to keep both pieces)
>
> At any rate, I think it makes sense for userspace to be given a chance
> to *attempt* to reconnect a device when the connection drops
> unexpectedly.
To summarize different options we have:
1. Wait for all the buffers to get cleaned:
a) ... at the time of disconnect:
Not possible, because we can neither force upper layers to do the required
(ext4 umount crash scenario), nor kill mapped buffers unconditionally.
b) ... after disconnect, fail next reconnect with EBUSY if bd_openers > 1:
This would prevent even harmless reconnects from same device along with
problematic different device reconnects.
2. Do not wait for all buffers to get cleaned:
a) This particular patch does this. Inconsistency with a different device
reconnect is its disadvantage.
We may want to give preference same device reconnects as it is not rare.
(For floppy/cdrom disk drivers also, changing disk with different disk in the
middle results in inconsistency, and they left this case as user responsibility
to not do it.)
-Ratna