Re: [PATCH 12/13] vfs: count unlinked inodes

[Christoph Hellwig <hch@infradead.org>]

On Fri, Oct 28, 2011 at 09:53:53AM +0200, Miklos Szeredi wrote:
> On Fri, 2011-10-28 at 02:08 -0400, Christoph Hellwig wrote:
> > This one gets me a repeated spew of WARN_ONs during XFS log recover,
> > repeating the following pattern:
> 
> The assumption here is that set_nlink() is called with a non-zero count.
> The point of introducing set_nlink() was to verify this assumption,
> otherwise pending delete accounting will be screwed up.
> 
> If setting i_nlink to zero is not a bug in xfs we can do two things:
> 
>  1) do a conditional clear_nlink() in xfs_setup_inode() to document that
> nlink can indeed be zero
> 
>  2) remove the warning from set_nlink().  That may hide some cases where
> i_nlink was cleared without the intention of the filesystem but it will
> work just fine wrt. the pending delete accounting. 

I suspect 2 might be the better option.  The pattern we see here in
XFS is pretty typical for transactional filesytems - when we mount a
filesystem after an unclean shutdown we need to drop all inodes that
were open but unlinked when the system crashed, and that typically
means reading inodes from disk that have a zero i_nlink.  Maybe some
filesystems never instanciate a VFS inode for it, but I suspect many
do.

For XFS we will actually see it during regularly testing as we have
an ioctl that simulates a shutdown and can thus trigger log recovery
easily, while for others like ext4 you'd actually have to do a real
reset of your (physical or virtual) machine.