Re: [PATCH 1/6] fs: Add flag to file_system_type to indicate content is generated

[Amir Goldstein <amir73il@gmail.com>]

On Fri, Feb 12, 2021 at 9:49 AM Greg KH <gregkh@linuxfoundation.org> wrote:
>
> On Fri, Feb 12, 2021 at 12:44:00PM +0800, Nicolas Boichat wrote:
> > Filesystems such as procfs and sysfs generate their content at
> > runtime. This implies the file sizes do not usually match the
> > amount of data that can be read from the file, and that seeking
> > may not work as intended.
> >
> > This will be useful to disallow copy_file_range with input files
> > from such filesystems.
> >
> > Signed-off-by: Nicolas Boichat <drinkcat@chromium.org>
> > ---
> > I first thought of adding a new field to struct file_operations,
> > but that doesn't quite scale as every single file creation
> > operation would need to be modified.
>
> Even so, you missed a load of filesystems in the kernel with this patch
> series, what makes the ones you did mark here different from the
> "internal" filesystems that you did not?
>
> This feels wrong, why is userspace suddenly breaking?  What changed in
> the kernel that caused this?  Procfs has been around for a _very_ long
> time :)

That would be because of (v5.3):

5dae222a5ff0 vfs: allow copy_file_range to copy across devices

The intention of this change (series) was to allow server side copy
for nfs and cifs via copy_file_range().
This is mostly work by Dave Chinner that I picked up following requests
from the NFS folks.

But the above change also includes this generic change:

-       /* this could be relaxed once a method supports cross-fs copies */
-       if (file_inode(file_in)->i_sb != file_inode(file_out)->i_sb)
-               return -EXDEV;
-

The change of behavior was documented in the commit message.
It was also documented in:

88e75e2c5 copy_file_range.2: Kernel v5.3 updates

I think our rationale for the generic change was:
"Why not? What could go wrong? (TM)"
I am not sure if any workload really gained something from this
kernel cross-fs CFR.

In retrospect, I think it would have been safer to allow cross-fs CFR
only to the filesystems that implement ->{copy,remap}_file_range()...

Our option now are:
- Restore the cross-fs restriction into generic_copy_file_range()
- Explicitly opt-out of CFR per-fs and/or per-file as Nicolas' patch does

Preference anyone?

Thanks,
Amir.