Re: [PATCH/RFC] NFSD: handle BTRFS subvolumes better.

[Christoph Hellwig <hch@infradead.org>]

On Thu, Jul 15, 2021 at 10:09:37AM -0400, Josef Bacik wrote:
> I'm going to restate what I think the problem is you're having just so I'm
> sure we're on the same page.
> 
> 1. We export a btrfs volume via nfsd that has multiple subvolumes.
> 2. We run find, and when we stat a file, nfsd doesn't send along our bogus
> st_dev, it sends it's own thing (I assume?).  This confuses du/find because
> you get the same inode number with different parents.
> 
> Is this correct?  If that's the case then it' be relatively straightforward
> to add another callback into export_operations to grab this fsid right?
> Hell we could simply return the objectid of the root since that's unique
> across the entire file system.  We already do our magic FH encoding to make
> sure we keep all this straight for NFS, another callback to give that info
> isn't going to kill us.  Thanks,

Hell no.  btrfs is broken plain and simple, and we've been arguing about
this for years without progress.  btrfs needs to stop claiming different
st_dev inside the same mount, otherwise hell is going to break lose left
right and center, and this is just one of the many cases where it does.