Re: [resend PATCH 3/3] writeback: fix false positive WARN in __mark_inode_dirty

[Dave Chinner <david@fromorbit.com>]

On Tue, Jan 05, 2016 at 11:59:41AM -0800, Dan Williams wrote:
> On Mon, Jan 4, 2016 at 8:23 PM, Dave Chinner <david@fromorbit.com> wrote:
> > On Mon, Jan 04, 2016 at 10:20:16AM -0800, Dan Williams wrote:
> >> This warning was added as a debugging aid way back in commit
> >> 500b067c5e6c "writeback: check for registered bdi in flusher add and
> >> inode dirty" when we were switching over to per-bdi writeback.
> >>
> >> Once the block device has been torn down it's no longer useful to
> >> complain about unregistered bdi's.  Clear the writeback capability under
> >> the the wb->list_lock(), so that __mark_inode_dirty has no opportunity
> >> to race bdi_unregister() to this WARN() condition.
> >>
> >> Alternatively we could just delete the warning...
> >
> > The warning is correct - the filesytem is trying to mark an inode
> > dirty on a device that can't do writeback anymore. Seems to me like
> > it is functioning as it should.
> >
> >> Found this while testing block device remove from underneath an active
> >> fs triggering traces like:
> >>
> >>  WARNING: CPU: 6 PID: 2129 at fs/fs-writeback.c:2065 __mark_inode_dirty+0x261/0x350()
> >>  bdi-block not registered
> >>  [..]
> >>  Call Trace:
> >>   [<ffffffff81459f02>] dump_stack+0x44/0x62
> >>   [<ffffffff810a1f32>] warn_slowpath_common+0x82/0xc0
> >>   [<ffffffff810a1fcc>] warn_slowpath_fmt+0x5c/0x80
> >>   [<ffffffff812830b1>] __mark_inode_dirty+0x261/0x350
> >>   [<ffffffff8126d019>] generic_update_time+0x79/0xd0
> >>   [<ffffffff8126d19d>] file_update_time+0xbd/0x110
> >>   [<ffffffff812e4ab8>] ext4_dax_fault+0x68/0x110
> >>   [<ffffffff811f7f3e>] __do_fault+0x4e/0xf0
> >
> > This seems like the problem to me - you haven't implemented a
> > shutdown hook for ext4, and so it continues to allow page faults to
> > make progress after the device has been removed. The DAX fault
> > should have been failed before the filesystem gets to the point of
> > marking the inode dirty....
> 
> xfs doesn't check that the fs is still alive before calling
> file_update_time().  Also, I don't think we need/want to sprinkle "is
> fs alive?" checks to address this when the block device shutdown path
> can simply turn off writeback.

That's because the timestamp update is aborted in XFS during
transaction commit because xfs_trans_commit has a "is the filesystem
shutdown" check to prevent situations like this occurring. i.e. XFS
has shutdown checks sprinkled all through it in strategic places to
ensure that shutdown does what it is supposed to and does not
trigger warnings in generic/VFS code.

If you've removed a device, those inodes can *never* be written
back, and hence marking new inodes dirty for writeback after a
shutdown is completely wrong. e.g. if ext4 has had some other fatal
corruption error, it will continue to allow timestamp updates and
inode writeback through this mechanism. That's a bug in the
filesystem error handling, not a bug in the writeback code.

Cheers,

Dave.
-- 
Dave Chinner
david@fromorbit.com