Re: [PATCH 4/4] Adds ioctl interface support for ext4 project

Dave Chinner
2014-09-26T00:18:39Z

On Thu, Sep 25, 2014 at 07:34:38PM +0800, lixi wrote:
> Hi Dave,
>
> I was mostly working on the semantics of inherit flag on these patches. And
> I didn’t realized that the interface differences would bother you so much. Sorry
> for that.

It's not the differences that bother me - it's the fact that I was
repeatedly ignored until someone else raised the same issue....

> I agree that we should choose a good interface. It would be good that it is
> general so that it works well for all file systems. I agree that adding an
> ext4 specific ioctl() is far from the best choice. I am willing to change it to
> any general interface. A general ioctl() sounds good to me. Extend setattr()
> and getattr() for project ID sounds even better, since project ID looks like
> UID/GID.

Ah, no, project ID is not a uid/gid. It's a completely independent
construct.

[ Which brings me to, once again, the issue of being ignored during
reviews: project IDs should not be mapped by user namespaces, nor be
accessible from anything other than the init namespace.

In XFS we've turned off access to project IDs within namespace
containers because they are used for container space management
(i.e. by the init namespace) to manage the amount of filesystem
space a container or set of containers can use. We do not want
project IDs to be manipulated from within such containers, and
therefore have to prevent access to them from within user
namespaces. ]

> And general xattr name is another choice. But it is might be a little
> bit confusing if we use xattr actually, since we are not saving project ID as
> extended attribute on Ext4. Any choice is fine with me, as long as the
> implementation won't introduce nasty codes or inconsistent design.

We can easily create another ioctl name if we have to. It just needs
sto be defined to the same value as the XFS ioctl names currently
are. We've done this before when making ioctls that originated in
XFS generic (e.g. with freeze/thaw ioctls)....

> However, the problem is, I do not quite understand why we should keep
> the interface exactly the same with XFS. It would be good if we can. But
> as far as I can see, it seems hard. XFS uses a lot interfaces which are
> not so standard and used by other file systems. For example, struct
> fsxattr is not used by other file systems at all except XFS.

Moving a existing structure definitions to a different header file
is too hard?

> I am not sure why we should introduce this into Ext4 if there are
> a lot of other better ways. I would be happy to change to XFS
> interfaces, if it is general. However, I don’t think it is
> general enough.

How is it not general enough? Examples, please, not handwaving:
which bit of the quota interface can't ext4 use because it's XFS
specific?

We already have a perfectly functional interface and a large body of
code that implements and tests it. You're saying "oh, it's too much
work for me to implement an existing interface" and ignoring the
fact that not implementing the existing interface forces a huge
amount of downstream work. e.g.

- we need completely new test infrastructure to replicate
existing tests.
- we need new tests to ensure the different APIs and
utilities provide the same functionality, and that the
work identically.
- administrators are going to have to learn how ext4 is
different to what they already know and understand.
- administrators that has tools written to manage project
quotas is going to have to rewrite them to support ext4.

It's an entirely selfish argument that ignores what already existing
out in userspace. i.e. you're saying that existing downstream users of
project quotas simple don't matter to you...

> I know xfstest is using the existing project quota interfaces of XFS. And
> maybe there are some applications which are using them too. But
> keeping the interfaces exactly the same with XFS would cost so much
> effort that I’d like to get enough reasons before start working on it. Is it
> really necessary? I am not so sure.

You have to have a stronger argument than that to justify creating a
new incompatible user interface. The XFS interfaces have been
available for more than 10 years and support all the functionality
ext4 requires. If it was any other userspace interface (e.g.
syscalls) or any filesystem other than ext4 there would be people
from all over telling you "use the existing interfaces!" and you'd
need very strong reasons for creating a new one.

i.e. you need to demonstrate that the existing interfaces are
inadequate for the intended purpose of the new functionality. That's
clearly not the case here so why should we allow you to create an
incompatible userspace API rather than use the existing, fully
functional API?

> It is so easy to change user space applications comparing to
> changing a weird interfaces.

The existing generic quota tools (i.e quotactl, repquota, etc)
already implement the XFS quota API to be able to query XFS
filesystems. There's no "changing to wierd interfaces" necessary
for userspace; it's already all there. Hence any work you do to add
project quota awareness to those generic userspace tools will need to
add the support to the XFS queries anyway.

IOWs, you're not making it any easier for yourself in userspace by
creating a new API for ext4 - it just doubles the amount of work you
have to in userspace to make existing tools project quota aware.

> For
> example, I think it won’t cost even more than a day to add xfstest
> support for new Ext4 project quota.

A day of whose time?

Ever thought about how much time it will take reviewers to look at
your tests and iterate over them to get it all right? If you're
introducing new userspace infrastructure that xfstests will need to
depend on and test for, then it's a lot more than just writing new
tests.

Indeed, I'm likely to want new project quota tests to be generic
(i.e. works and passes on any filesystem that supports project
quotas) with the introduction of ext4 project quota support. It's
the same functionality and so it should work the same just like user
and group quotas do across all filesystems.

> And since project quota is far from
> a widely used feature,

I don't think you realise quite how widespread it's use is on XFS.

> I don’t think there is much compatibility problems
> for existing applications. And If the new project interface are general
> enough, there won’t be any compatibility problems for new applications
> at all.

Again, you are ignoring the compatibility problems with existing
applications that are project quota aware. For them you are
*creating new compatibility problems* by implementing a new
interface. i.e. Existing applications will not work on ext4, and
new applications written to work on ext4 won't work on XFS.

That's the crux of the issue - we have existing applications using
the existing interface and so introducing a new interface introduces
compatibility problems. You can't just wave this problem away
because you don't think the existing interface matters.

"It's easier for me to create a new interface" is not a valid reason
for creating a new interface....

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com