[RFC PATCH v1 1/7] fs: Add inode_get_ino() and implement get_ino() for NFS

[Jeff Layton]

On Thu, 2024-10-17 at 11:15 -0400, Paul Moore wrote:
> On Thu, Oct 17, 2024 at 10:58 AM Christoph Hellwig <hch at infradead.org> wrote:
> > On Thu, Oct 17, 2024 at 10:54:12AM -0400, Paul Moore wrote:
> > > Okay, good to know, but I was hoping that there we could come up with
> > > an explicit list of filesystems that maintain their own private inode
> > > numbers outside of inode-i_ino.
> > 
> > Anything using iget5_locked is a good start.  Add to that file systems
> > implementing their own inode cache (at least xfs and bcachefs).
> 
> Also good to know, thanks.  However, at this point the lack of a clear
> answer is making me wonder a bit more about inode numbers in the view
> of VFS developers; do you folks care about inode numbers?  I'm not
> asking to start an argument, it's a genuine question so I can get a
> better understanding about the durability and sustainability of
> inode->i_no.  If all of you (the VFS folks) aren't concerned about
> inode numbers, I suspect we are going to have similar issues in the
> future and we (the LSM folks) likely need to move away from reporting
> inode numbers as they aren't reliably maintained by the VFS layer.
> 

Like Christoph said, the kernel doesn't care much about inode numbers.

People care about them though, and sometimes we have things in the
kernel that report them in some fashion (tracepoints, procfiles, audit
events, etc.). Having those match what the userland stat() st_ino field
tells you is ideal, and for the most part that's the way it works.

The main exception is when people use 32-bit interfaces (somewhat rare
these days), or they have a 32-bit kernel with a filesystem that has a
64-bit inode number space (NFS being one of those). The NFS client has
basically hacked around this for years by tracking its own fileid field
in its inode. That's really a waste though. That could be converted
over to use i_ino instead if it were always wide enough.

It'd be better to stop with these sort of hacks and just fix this the
right way once and for all, by making i_ino 64 bits everywhere.

A lot of the changes can probably be automated via coccinelle. I'd
probably start by turning all of the direct i_ino accesses into static
inline wrapper function calls. The hard part will be parceling out that
work into digestable chunks. If you can avoid "flag day" changes, then
that's ideal.  You'd want a patch per subsystem so you can collect
ACKs. 

The hardest part will probably be the format string changes. I'm not
sure you can easily use coccinelle for that, so that may need to be
done by hand or scripted with python or something.
-- 
Jeff Layton <jlayton at kernel.org>