Date:	Thu, 18 Jun 2015 15:28:21 -0400
From:	Mike Snitzer <snitzer@...hat.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	dm-devel@...hat.com, Andi Kleen <andi@...stfloor.org>,
	linux-kernel@...r.kernel.org, Christoph Hellwig <hch@....de>
Subject: Re: WARNING: at fs/block_dev.c:5 when removing LV on removed device

On Thu, Jun 18 2015 at  3:08pm -0400,
Vivek Goyal <vgoyal@...hat.com> wrote:

> On Thu, Jun 18, 2015 at 02:16:19PM -0400, Mike Snitzer wrote:
> > On Thu, Jun 18 2015 at  2:08pm -0400,
> > Andi Kleen <andi@...stfloor.org> wrote:
> > 
> > > > Hmm, so you have a filesystem active on it too?
> > > 
> > > I unmounted it before.
> > > 
> > > >  
> > > > > Also the VG removal did not work of course.
> > > > 
> > > > Once you resolve the filesystem piece, from vgremove man page:
> > > > 
> > > > "vgremove allows you to remove one or more volume groups.  If one or
> > > > more physical volumes in the volume group are lost, consider vgreduce
> > > > --removemissing to make the volume group metadata consistent again."
> > > 
> > > Well in any case there should not be WARN()s.
> > 
> > Yes well I don't even know what WARN_ON you're hitting.  You're running
> > a 4.0.4 fedora kernel.  Which WARN_ON() is triggering?  The
> > WARN_ON_ONCE() in bdev_write_inode()? -- likely since the only caller of
> > bdev_write_inode is __blkdev_put...
> > 
> > /**
> >  * write_inode_now      -       write an inode to disk
> >  * @inode: inode to write to disk
> >  * @sync: whether the write should be synchronous or not
> >  *
> >  * This function commits an inode to disk immediately if it is dirty. This is
> >  * primarily needed by knfsd.
> >  *
> >  * The caller must either have a ref on the inode or must have set I_WILL_FREE.
> >  */
> > 
> > So I have no idea why bdev_write_inode() is using WARN_ON_ONCE.. makes
> > since that write_inode_now() will fail if the disk no longer exists.  SO
> > the WARN_ON_ONCE seems misplaced.
> > 
> > Git blame shows its all hch's fault:
> > 
> > 564f00f6c (Christoph Hellwig  2015-01-14 10:42:33 +0100   57) WARN_ON_ONCE(write_inode_now(inode, true));
> > 
> > 564f00f6c block_dev: only write bdev inode on close
> 
> I can reproduce it too in a Fedora 22 virtual machine. I just have to do
> "umount <mnt>" after hot unplugging the virtIO disk.
> 
> But it does not happen without lvm. If I create ext4 fs directly on
> /dev/vda1, then when disk goes way, fs automatically gets unmounted.
> 
> But same does not happen when /dev/vda1 is added to a volume group and
> I carve out a logical volume and create and mount fs.
> 
> In that case if I do umount after device has gone away, I can see above
> WARN(). And it does seem to be coming from.

write_inode_now() should fail.. the device is no longer there.  No idea
how virtio-blk avoids it, devil is in the blkdev refcount details I'm
sure.

> WARN_ON_ONCE(write_inode_now(inode, true))
> 
> If we failed to write back inode, then warning about it sounds right?

A warning is fine.. not a WARN_ON().  Pretty alarming backtrace spew but
maybe I'm missing something and DM's blkdev refcount mgmt couldn't
trigger this WARN_ON()?  I fail to see how to avoid it given the device
isn't thre so write_inode_now() fails.

> What's wrong with that? Should it be just a kernel log of level KERN_WARN
> instead?

Ideally, but I honestly don't have all the details paged in my head to
say definitively.  First need to answer how vitrio-blk isn't hitting
this (and DM is).  Could it be that __blkdev_put isn't getting called
for virtio-blk!?
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