linux-kernel - Re: [PATCH 5/5] btrfs: ioctl: Call btrfs_vol

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Date:   Fri, 25 Oct 2019 12:44:20 -0300
From:   Marcos Paulo de Souza <mpdesouza@...e.de>
To:     Graham Cobb <g.btrfs@...b.uk.net>,
        Marcos Paulo de Souza <marcos.souza.org@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     clm@...com, josef@...icpanda.com, dsterba@...e.com,
        linux-btrfs@...r.kernel.org, mpdesouza@...e.com
Subject: Re: [PATCH 5/5] btrfs: ioctl: Call btrfs_vol_uevent on subvolume
 deletion

On Fri, 2019-10-25 at 13:00 +0100, Graham Cobb wrote:
> On 24/10/2019 03:36, Marcos Paulo de Souza wrote:
> > From: Marcos Paulo de Souza <mpdesouza@...e.com>
> > 
> > Since the function btrfs_ioctl_snap_destroy is used for deleting
> both
> > subvolumes and snapshots it was needed call btrfs_is_snapshot,
> > which checks a giver btrfs_root and returns true if it's a
> snapshot.
> > The current code is interested in subvolumes only.
> 
> To me, as a user, a snapshot *is* a subvolume. I don't even know what
> "is a snapshot" means. Does it mean "was created using the btrfs
> subvolume snapshot command"? Does it matter whether the snapshot has
> been modified? Whether the originally snapshot subvolume still
> exists?
> Or what?
> 
> I note that the man page for "btrfs subvolume" says "A snapshot is a
> subvolume like any other, with given initial content.". And I
> certainly
> have some subvolumes which are being used as normal parts of the
> filesystem, which were originally created as snapshots (for various
> reasons, including reverting changes and going back to an earlier
> snapshot, or an easy way to make sure that large common files are
> actually sharing blocks).

Agreed.

> 
> I would expect this event would be generated for any subvolume
> deletion.
> If it is useful to distinguish subvolumes originally created as
> snapshots in some way then export another flag (named to make it
> clear
> what it really indicates, such as BTRFS_VOL_FROM_SNAPSHOT). I don't
> know
> your particular purpose, but my guess is that a more useful flag
> might
> actually be BTRFS_VOL_FROM_READONLY_SNAPSHOT.

As soon as these patches got merged I will send new ones to take care
of snapshoting. Same things: when a snapsoht is created or removed,
send a uevent.

I liked this idea to separate both snapshots and volumes at first to
make things simpler, and get reviews faster. Also, I think it's good to
separate subvolumes and snapshot events, makes easier for one to filter
such events.

  Marcos

> 
> Graham