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@xxxxxxxxxxxxx -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
