Re: [PATCH 3/3] ovl: redirect on rename-dir

From: Konstantin Khlebnikov
Date: Mon Nov 07 2016 - 04:58:31 EST

On Mon, Nov 7, 2016 at 11:07 AM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
> On Sun, Nov 6, 2016 at 8:14 PM, Konstantin Khlebnikov <koct9i@xxxxxxxxx> wrote:
>> On Wed, Oct 26, 2016 at 2:12 PM, Miklos Szeredi <miklos@xxxxxxxxxx> wrote:
>>> On Tue, Oct 25, 2016 at 1:57 PM, Raphael Hertzog <hertzog@xxxxxxxxxx> wrote:
>>>> Do you plan to make it the default in the future when it has been
>>>> available for a while?
>>>> Barring any regression introduced by your patch, it seems that the feature
>>>> is best available by default since it allows legitimate operations to
>>>> succeed that are otherwise refused. I understand that it makes it
>>>> impossible to mount the overlay filesystem with an older kernel but is
>>>> that problem more widespread than the one we're fixing here? On my side,
>>>> overlayfs is only used in scenarios where the kernel is always the same
>>>> (or newer compared to what created the initial filesystem).
>>> I think it would be safe to make it the default if upperdir is empty.
>>> Nonempty implies that it was created with old kernel (or it was
>>> crafted by hand). But there should be a way to explicitly turn it
>>> off; either because of the need for backward compatibility or because
>>> the old format is simply easier to work with for humans.
>>> How about:
>>> - If upper is nonempty, then leave redirect feature alone except when
>>> mount option "-oredirect=on" is used to force enabling it.
>>> - If upper is empty, then enable redirect feature except when mount
>>> option "-oredirect=off" is used to force disabling it.
>> I don't like this empty-nonempty upper logic.
>> I think this feature should be off by default and be enabled
>> explicitly in mount option.
>> Available features could be listed in sysfs /sys/fs/overlay/..., like ext4 does.
>> Overlayfs mounting anyway is complicated operation.
>> User must know a lot about it and provide persistent state for each mount:
>> list layers in correct order, work and uppder directory on the same disk, etc.
>> Enabled features is a part of this state.
>> Probably this could be solved in userspace tool "mount.overlay" - it could load
>> features and layers from config file or xattr and set required mount
>> options automatically.
> It seems there are some conflicting opinions here. I have mine too,
> but I'll let this simmer and concentrate on actual features for now.

Ok =) let's try keep as simple as possible.

I've stumbled on somehow related problem - concurrent copy-ups are
strictly serialized by rename locks.
Obviously, file copying could be done in parallel: locks are required
only for final rename.
Because of that overlay slower that aufs for some workloads.