Re: Re: Re: [PATCH v2 2/4] fs: add FS_IOC_FSSETXATTRAT and FS_IOC_FSGETXATTRAT

Andrey Albershteyn
2024-06-10T11:50:48Z

On 2024-06-10 12:19:50, Amir Goldstein wrote:
> On Mon, Jun 10, 2024 at 11:17 AM Andrey Albershteyn <aalbersh@redhat.com> wrote:
> >
> > On 2024-06-06 12:27:38, Dave Chinner wrote:
> ...
> > >
> > > The only reason XFS returns -EXDEV to rename across project IDs is
> > > because nobody wanted to spend the time to work out how to do the
> > > quota accounting of the metadata changed in the rename operation
> > > accurately. So for that rare case (not something that would happen
> > > on the NAS product) we returned -EXDEV to trigger the mv command to
> > > copy the file to the destination and then unlink the source instead,
> > > thereby handling all the quota accounting correctly.
> > >
> > > IOWs, this whole "-EXDEV on rename across parent project quota
> > > boundaries" is an implementation detail and nothing more.
> > > Filesystems that implement project quotas and the directory tree
> > > sub-variant don't need to behave like this if they can accurately
> > > account for the quota ID changes during an atomic rename operation.
> > > If that's too hard, then the fallback is to return -EXDEV and let
> > > userspace do it the slow way which will always acocunt the resource
> > > usage correctly to the individual projects.
> > >
> > > Hence I think we should just fix the XFS kernel behaviour to do the
> > > right thing in this special file case rather than return -EXDEV and
> > > then forget about the rest of it.
> >
> > I see, I will look into that, this should solve the original issue.
>
> I see that you already got Darrick's RVB on the original patch:
> https://lore.kernel.org/linux-xfs/20240315024826.GA1927156@frogsfrogsfrogs/
>
> What is missing then?
> A similar patch for rename() that allows rename of zero projid special
> file as long as (target_dp->i_projid == src_dp->i_projid)?
>
> In theory, it would have been nice to fix the zero projid during the
> above link() and rename() operations, but it would be more challenging
> and I see no reason to do that if all the other files remain with zero
> projid after initial project setup (i.e. if not implementing the syscalls).

I think Dave suggests to get rid of this if-guard and allow
link()/rename() for special files but with correct quota calculation.

>
> >
> > But those special file's inodes still will not be accounted by the
> > quota during initial project setup (xfs_quota will skip them), would
> > it worth it adding new syscalls anyway?
> >
>
> Is it worth it to you?
>
> Adding those new syscalls means adding tests and documentation
> and handle all the bugs later.
>
> If nobody cared about accounting of special files inodes so far,
> there is no proof that anyone will care that you put in all this work.

I already have patch and some simple man-pages prepared, I'm
wondering if this would be useful for any other usecases which would
require setting extended attributes on spec indodes.

>
> Thanks,
> Amir.
>

--
- Andrey