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In-Reply-To: <162788285645.32159.12666247391785546590@noble.neil.brown.name>
From:   Amir Goldstein <amir73il@gmail.com>
Date:   Mon, 2 Aug 2021 10:54:22 +0300
Message-ID: <CAOQ4uxgnGWMUvtyJ0MMxMzHFwiyR68FHorDNmLSva0CdpVNNcQ@mail.gmail.com>
Subject: Re: A Third perspective on BTRFS nfsd subvol dev/inode number issues.
To:     NeilBrown <neilb@suse.de>
Cc:     Al Viro <viro@zeniv.linux.org.uk>,
        Miklos Szeredi <miklos@szeredi.hu>,
        Christoph Hellwig <hch@infradead.org>,
        Josef Bacik <josef@toxicpanda.com>,
        "J. Bruce Fields" <bfields@fieldses.org>,
        Chuck Lever <chuck.lever@oracle.com>, Chris Mason <clm@fb.com>,
        David Sterba <dsterba@suse.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Linux NFS list <linux-nfs@vger.kernel.org>,
        Btrfs BTRFS <linux-btrfs@vger.kernel.org>
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On Mon, Aug 2, 2021 at 8:41 AM NeilBrown <neilb@suse.de> wrote:
>
> On Mon, 02 Aug 2021, Al Viro wrote:
> > On Mon, Aug 02, 2021 at 02:18:29PM +1000, NeilBrown wrote:
> >
> > > It think we need to bite-the-bullet and decide that 64bits is not
> > > enough, and in fact no number of bits will ever be enough.  overlayfs
> > > makes this clear.
> >
> > Sure - let's go for broke and use XML.  Oh, wait - it's 8 months too
> > early...
> >
> > > So I think we need to strongly encourage user-space to start using
> > > name_to_handle_at() whenever there is a need to test if two things are
> > > the same.
> >
> > ... and forgetting the inconvenient facts, such as that two different
> > fhandles may correspond to the same object.
>
> Can they?  They certainly can if the "connectable" flag is passed.
> name_to_handle_at() cannot set that flag.
> nfsd can, so using name_to_handle_at() on an NFS filesystem isn't quite
> perfect.  However it is the best that can be done over NFS.
>
> Or is there some other situation where two different filehandles can be
> reported for the same inode?
>
> Do you have a better suggestion?
>

Neil,

I think the plan of "changing the world" is not very realistic.
Sure, *some* tools can be changed, but all of them?

I went back to read your initial cover letter to understand the
problem and what I mostly found there was that the view of
/proc/x/mountinfo was hiding information that is important for
some tools to understand what is going on with btrfs subvols.

Well I am not a UNIX history expert, but I suppose that
/proc/PID/mountinfo was created because /proc/mounts and
/proc/PID/mounts no longer provided tool with all the information
about Linux mounts.

Maybe it's time for a new interface to query the more advanced
sb/mount topology? fsinfo() maybe? With mount2 compatible API for
traversing mounts that is not limited to reporting all entries inside
a single page. I suppose we could go for some hierarchical view
under /proc/PID/mounttree. I don't know - new API is hard.

In any case, instead of changing st_dev and st_ino or changing the
world to work with file handles, why not add inode generation (and
maybe subvol id) to statx().
filesystem that care enough will provide this information and tools that
care enough will use it.

Thanks,
Amir.